EPA-600/1-77-040
September 1977
Environmental Health Effects Research Series
PLATINUM-GROUP METALS
Health Effects Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL HEALTH EFFECTS RE-
SEARCH series. This series describes projects and studies relating to the toler-
ances of man for unhealthful substances or conditions. This work is generally
assessed from a medical viewpoint, including physiological or psychological
studies. In addition to toxicology and other medical specialities, study areas in-
clude biomedical instrumentation and health research techniques utilizing ani-
mals — but always with intended application to human health measures.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/1-77-040
September 1977
PLATINUM-GROUP METALS
by
Subcommittee of Platinum-Group Metals
Committee on Medical and Biologic Effects of
Environmental Pollutants
National Research Council
National Academy of Sciences
Washington, D.C.
Contract No. 68-02-1226
Project Officer
Orin Stopinski
Criteria and Special Studies Office
Health Effects Research Laboratory
Research Triangle Park, N.C. 27711
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
HEALTH EFFECTS RESEARCH LABORATORY
RESEARCH TRIANGLE PARK, N.C. 27711
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DISCLAIMER
This report has been reviewed by the Health Effects Research
Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection
Agency, nor does mention of trade names or commercial products
constitute endorsement or recommendation for ua*.
NOTICE
The project that is the subject of this report was approved by the-
Governing Board of the National Research Council, whose members are
drawn from the Councils of the National Academy of Sciences, the National
Academy of Engineering, and the Institute of Medicine. The members of
the Committee responsible for the report were chosed for their special
competences and with regard for apropriate balance.
This report has been reviewed by a group other than the authors
according to procedures approved by a Report Review Committee consisting
of members of the National Academy of Sciences, the National Academy of
Engineering, and the Institute of Medicine.
ii
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FOREWORD
The many benefits of our modem, developing, industrial society are
accompanied by certain hazards. Careful assessment of the relative risk
of existing and new man-made environmental hazards is necessary for the
establishment of sound regulatory policy. These regulations serve to
enhance the quality of our environment in order to promote the public
health and welfare and the productive capacity of our Nation's population.
The Health Effects Research Laboratory, Research Triangle Park,
conducts a coordinated environmental health research program in toxicology,
epidemiology, and clinical studies using human volunteer subjects. These
studies address problems in air pollution, non-ionizing radiation,
environmental carcinogenesis and the toxicology of pesticides as well as
other chemical pollutants. The Laboratory develops and revises air quality
criteria documents on pollutants for which national ambient air quality
standards exist or are proposed, provides the data for registration of new
pesticides or proposed suspension of those already in use, conducts research
on hazardous and toxic materials, and is preparing the health basis for
non-ionizing radiation standards. Direct support to the regulatory function
of the Agency is provided in the form of expert testimony and preparation of
affidavits as well as expert advice to the Administrator to assure the
adequacy of health care and surveillance of persons having suffered imminent
and substantial endangerment of their health.
«*
To aid the Health Effects Research Laboratory to fulfill the functions
listed above, the National Academy of Sciences (NAS). under EPA Contract
No. 68-02-1226 prepares evaluative reports of current knowledge of selected
atmospheric pollutants. These documents serve as background material for
the preparation or revision of criteria documents, scientific and technical
assessment reports, partial bases for EPA decisions and recommendations
for research needs. "Platinum-group Metals" is one of these reports.
John H, Knelson, M.D.
Director
Health Effects Research Laboratory
iii
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SUBCOMMITTEE ON PLATINUM-GROUP METALS
JOE W. 11ICHTOWER, Rice University, Houston, Texas, Chairman
RICHARD H. ADAHSON. National Inatltutea of llealtli, Bothpsda, Maryland
JOHN L. BEAR, University of Houston, Houston, Texas
HENRY FREISBR, University of Arizona, Tucson, Arizona
VLADIMIR HARHSEL, Universal Oil Products Company, Des Plalnea, Illinois
WILLIAM A. B. McBRYDE, University of Waterloo, Waterloo, Ontario, Canada
BARNETT ROSENBERG, Michigan State University, East Lansing, Michigan
J. PBPYS, Cardlothoraclc Institute, London, England, Consultant
JAMRS A. FRAZIER, National Research Council, Washington, D.C., Staff
Officer
COMMITTEE ON MEDICAL AND BIOLOGIC EFFECTS OP ENVIRONMENTAL POLLUTANTS
HERSCHEL E. GRIFFIN, University of Pittsburgh, Pittsburgh, Pennsylvania,
Chairman
MARTIN ALEXANDER, Cornell University, Ithaca, New York
ANDREW A. BENSON, University-'.of California, La Jolla, California
RONALD F. COBURN, University of Pennsylvania School of M.-
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PREFACE
Since the spring of 1970, the Division of Medical Sciences, National Academy
of Sciences—National tesearch Council, has produced several documents on the
medical, biologic, and environmental effects of selected pollutants. These docu-
ments have been prepared for the Environmental Protection Agency to establish a
broad background of information, to evaluate that information, and to recommend
studies aimed at remedying information inadequacies or gaps. The documents are
prepared by subcoimittees of the Committee on Medical and Biologic Effects of
Environmental Pollutants. This report is the result of the work of the Sub-
committee on Platinum-Group Metals.
The purpose of this document is to assemble, organize, and evaluate all
pertinent information (up to April 1976) about the effects on man and his en-
vironment that result either directly or indirectly from pollution by platinum-
group metals: iridium, osmium, palladium, platinum, rhodium, and ruthenium.
*•
The document describes physical and chemical properties, sources, measurement,
and effects on plants, animals, and humans. The information presented is sup-
ported by references to the scientific literature, and the summary, conclusions,
recommendations represent a consensus of the members of the Subcommittee.
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CONTENTS
1 INTRODUCTION
2 SOURCES AND PRODUCTION
3 USES
4 PHYSICAL AND CHEMICAL PROPERTIES
5 ANALYSIS AND DETERMINATION
6 TOXICOLOGY AND PHARMACOLOGY
7 ALLERGY TO PLATINUM COMPOUNDS
8 ENVIRONMENTAL CONSIDERATIONS
9 SUMMARY
10 CONCLUSIONS
11 RECOMMENDATIONS
APPENDIX
REFERENCES
VI
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CHAPTER 1
INTRODUCTION
Six elements of group VIII (in the periodic table) have been collec-
tively designated the "platinum-group metals." Included in this group are
platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir),
and osmium (Os). Sometimes called "noble metals" (with gold, silver, etc.),
because of their resistance to oxidation, these "precious metals" are present
in the earth's crust in very low concentrations. In spite of their limited
availability, these metals (and chemical compounds containing them) are ex-
tremely useful as catalysts in the chemical and petroleum industries, as con-
ductors in the electric industry, in extrusion devices, in dental and medical
prostheses, and in jewelry.
In their traditional applications, the platinum-group metals have been
considered relatively innocuous, with respect to direct environmental impact.
However, some new and more extensive uses of these materials may have both
direct and indirect impact on human health. It has been known for years that
physiologic activity is associated with some platinum-group metal compounds.
Some of the complex salts produce allergic reactions (e.g., platinosis) in
humans, and a few of the volatile oxides are very toxic. The purposes of this
report are to present information about the health effects of the platinum-
group metals and their compounds and to identify the hazards associated with
them.
Since the beginning of the 1975 model year, most new automobiles sold in
the United States have been equipped with catalytic converters whose purpose
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is the chemical removal of seme polluting substances from the exhaust. The
amount of platinum-group metals required in these devices is about equal to the
total amount in all other U.S. uses combined. Concern has been expressed about
the possibility that harmful compounds of these metals will be directly emitted
from the exhaust systems. In addition, the catalytic converters are responsible
for emission of sulfuric acid, and it is possible that the acid vail reach harm-
ful concentrations in the vicinity of heavily traveled highways. Both direct
metal emission and indirect sulfuric acid emission will be considered in this
report.
Another possible application of platinun-group metals is the use of some
complexes as anti-tutor agents in cancer chemotherapy. Recent successes in this
field have stimulated intensive research to find compounds that are highly ef-
fective either by themselves or in combination with other drugs in retarding
cancer but that have minimal harmful side effects to such organs as the kidneys.
An unusual mode of entry of some of the platinum-group metals into the
environment is through waste effluent from nuclear-fuel reprocessing plants.
In assessing the environmental impact of the platinum-group metals and
their compounds, this report considers the sources and uses of these materials,
their physicochemical properties and associated analytic methods, their toxicol-
ogy and pharmacology, and problems involved in the refinement and preparation
of end products containing them. For further information on the chemistry of
these metals, the reader is referred to a number of important books. '
161,177,227,249,254,255,467
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CHAPTER 2
SOURCES AND PRODUCTION
OCCURRENCE110'471
In most of their natural occurrences, the platinum-group metals are either
uncoribined or "native" ; only a few WBll-characterized mineral compounds are
known. For instance, native platinum generally contains small concentrations
of the other platinum metals (except osmium), base metals (such as iron, copper,
and nickel), or silver. Platinum is also a major or minor component of the other
native alloys of the platinum metals listed in Table 2-1.
The most familiar mineral species of these metals are sperrylite, PtAs2;
cooperite, (Pt,Pd)S; braggite, (Pt,Pd,Ni)S; potarite, PdHg or Pd3Hg2; stibio-
palladinite, Pd^Sb; and laurite, (Ru,Os)S2. In recent years, a number of other
mineral species have been identified, mainly as a result of the application of
new physical techniques, such as x-ray spectroscopy and electron-probe analysis.
^
The abundance of the six platinum metals in the earth's crust appears to
be very low (see Table 2-2); in fact, they are between seventy-first and eightieth
among all the elements. However, they are much more abundant in meteorites,
particularly meteorites or phases of meteorites that are rich in metals, notably
iron and nickel, as opposed to the "stony" meteorites. In geochemical terminol-
ogy, the platinum metals are said to be strongly siderophile (in contrast with
chalcophile). This has led geochemists to the supposition that the platinum
metals have concentrated during the earth's formation mainly in the iron-nickel
core and that this accounts for their relatively low abundance in the lithosphere,
or rocky crust of the earth.154 A similar preferential distribution occurs during
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TABLE 2-1
Native Platinum Metals and Alloysa
Name
Platinum
Platiniridiuro
Palladiplatinum
Palladium
Iridosmine
Osrairidium
Crystal Form
Cubic
Cubic
Cubic
Cubic
Hexagonal
Hexagonal
Cubic
Principal Constituents
Pt
Pt, Ir
Pd, Pt
Pd
Pd
Ir, Os
Os, Ir
at)ata from Wright and Fleischer471 and Crockett.110
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TABLE 2-2
Abundance of Platinum Metals in Earth's Crust13
Metal Concentration in Crust, ppn
Palladium 0.01
Platinun 0.005
Rhodixm 0.001
Iridium 0.001
Ruthenium 0.001
Osmium 0.001
from Mason.260
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the smelting of ores and results in a concentration of platinum metals when
conditions are chosen so as to produce fused base metals (e.g., iron, nickel,
and copper) and liquid slag; the precious metals are found almost entirely in
the metallic phase.
The platinum-group metals have been found in minerals in the earth's crust
at concentrations up to 20 ppm (1 ppm = 10~4 wt %)—but usually much lower—
and most commonly in sulfides, selenides, tellurides, and arsenides. Similar
abundances have been noted in some oxide minerals, especially chromite. Un-
fortunately, at such low concentrations, it has not yet proved possible to
state in what chemical forms (e.g., as distinct mineral species or in solid
solutions) the platinum metals are present. Current data reveal that, when
these metals are present in rocks, there is a marked preferential association
with ultrabasic, rather than silicic, species. The former are believed to have
crystallized early in the sequence of roagmatic separation, and it has been pro-
posed-*42 that the comparatively unreactive and high-melting-point platinum
metals would have been deposited early and become associated with the early
fractions crystallizing from the magma. Accordingly, these metals appear in
concentrations up to 1 ppm in dunites, pyroxinites, and serpentines, but in
very low (Concentrations in rocks that contain more silica and that are regarded
as having crystallized later.
SOURCES FOR COMMERCIAL PRODUCTION
Platinum metals available for industry and commerce are either extracted
from newly mined mineral (as described in the next section) or obtained by re-
fining used scrap metal. Because of the high intrinsic value of the metals,
coupled with their chemical inertness and physical durability, used metal can
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profitably be recycled; a significant fraction of annual production includes
such metal. Reclaimed metals are often referred to as having been "toll-
refined."*
The economically significant sources of platinum metals are in the
Republic of South Africa, Canada, and the U.S.S.R. They are all primary
deposits usually associated with ultrabasic rock formations and often with
copper and nickel sulfide deposits. The South African sources are the most
concentrated, but even they contain the platinum metals at only 4-10 ppm.
These sources are in the Bushveld Igneous Complex in the Transvaal district
just north of Pretoria. ^^ in Canada, the platinum metals occur mainly in
copper-nickel sulfide ores in the Sudbury area of Ontario and in the Thompson-
Wabowden area of Manitoba. ^°'^8^ In the Sudbury deposits, the platinum-metal
content has been estimated at less than 1 ppm, but the metals are concentrated
to profitable values during the refining of copper and nickel. In the U.S.S.R.,
the richest sources are in the Noril'sk region of Siberia and in the Kola
*•
Peninsula near Petsamo. The abundance (concentration) of platinum metals in the
Russian deposits has not been disclosed, but is believed to be between those
found in Canada and South Africa.
Almost 90% of the platinum metals in these sources consists of platinum
and palladium. Table 2-3 shows the relative abundance by weight of the various
metals in the three principal sources.
*Because of the specialized technology involved, users and some producers
of the platinum metals send their material to refiners for separation,
purification, etc., for which service a fee or toll is charged; hence,
"toll-refining."
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TABLE 2-3
Estimated Cotposition of Platinum-Group Metals fron Different Sources*2
Metal
Platinum
Palladium
Iridium
Rhodium
Ruthenium
Osmium
Percentage
Canada
43.4
42.9
2.2
3.0
8.5
0
by Weight
U.S.S.R.
30
60
2
2
6
0
South Africa
64.02
25.61
0.64
3.20
6.40
0.13
Approximate
ratio 1:1 1:2 2.5:1
Pt:Pd
aData from U.S. Department of the Interior270 and Platinum Metals Review.22
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The first platinum to be identified came from South America and was brought
to Europe in the seventeenth and eighteenth centuries as an almost undesirable
adjunct of gold from the New World. In fact, the name "platina," first applied
to this material by the Spaniards, signifies "little silver"—a debased form,
perhaps, of silver. It was from placer deposits,* some of which in Colombia
are still being mined. Smaller quantities of placer platinum are also being
obtained from western Alaska, Ethiopia, and the Philippines. The total amount
of new metal from all these sources is relatively small. In the nineteenth
century, extensive placer deposits in the Ural Mountains region in Russia con-
stituted the chief world source of platinum metals, and a small amount is still
obtained from that location. The metal in these placer deposits is present as
native alloys of varied composition.
The most recent available information on worldwide production of new
platinum metals is incorporated in Table 2-4.
Of platinum metals refined in the United States in 1973 (the latest avail-
able data), new metal, either placer or byproduct from gold- and copper-
refining, accounted for 8,218 troy oz (platinum, 43%; palladium, 42%), whereas
secondary metal recovered by recycling amounted to 232,276 troy oz (platinum,
36.8%; palladium, 55.4%). These numbers, especially those for new metal, tend
to fluctuate somewhat from year to year. Another statistic pertaining to re-
fining of these metals in the United States is the amount handled "on toll,"
a total of 1,361,723 troy oz in 1972, of which 84,219 oz were of crude or matte
from Colombia, Canada, and South Africa.
*The term "placer" signifies a deposit of sand or gravel of alluvial or
glacial origin and containing particles of gold or other precious minerals.
Normally, the metal, being heavier, is separated by washing away the sand
or gravel ("panning").
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TABLE 2-4
Approximate World Production of Platinum-Group Metals, 1971-1975a
Quantity, troy oz^
N)
00
Country and Metals
Australia:
Pallfv^ivin metal T>ntent (nickel ore)
Platinum metal content (nickel ore)
Canada: platinum and platinum-group
metals (nickel ore)
Colombia: placer platinum
Ethiopia: pJLacer platinum
Finland: platinum-group metals recovered
(copper ore)
Japan:
Palladium from nickel and copper
refineries ~~
Platinum from nickel and copper
refineries
Philippines :
>L "-JCT n T^ ' * r •
Palladium metal
Platinum metal
Republic of South Africa:
Platinum-group metals from platinum ores
Osmium-indium from gold ores
U.S.S.R. : placer platinum and platinum-group
metals from p]^tinum-nickel-copper
1971
—
475,169
25,610
217
600
5,375
3,451
1,756
703
1,250,000
3,200
1972
—
406,048
24,111
248
650
5,659
4,240
4,810
2,712
1,450,000
3,000
1973
750
225
354,223
26,358
235
725
5,834
4,363
4,180
2,476
2,360,000
2,800
1974
860
260
384,618
21,094
230
650
11,104
4,101
2,315
1,350
2,832,000
2,500
1975
1,400?
42CP
430,000
22,114
162
600
13,981
5,482
836
579
2,620,000
2,400
ores
United States: crude placer platinum and
byproduct from gold or copper
refining
Total
2,300,000 2,350,000 2,450,000 2,500,000 2,650,000
18,029
17,112
19,980
12,657
18,920
4,084,110 4,268,590 5,232,149 5,773,739 5,766,894
aQata from Butterman.68'68a
"precious metals in conmerce and production are for traditional reasons measured in troy ounces; 1 troy ounce
1.097 avoirdupois ounce = 0.0311 kg.
'Estimated.
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It is impossible to determine the exact amounts of the platinum metals that
can be recovered by mining. Estimates of the known world reserves of platinum
and palladium are shown in Table 2-5.
The Johns-Manville Corporation223'224 recently reported finding a large de-
posit of ore rich in platinum and palladium in the Stillwater Complex area,
Sweetgrass County, Montana. Although complete information is not available,
estimates of the recoverable metals run as high as 500 million troy ounces—
comparable with the total world reserves previously thought to exist. The ore
is highly concentrated, in the range of 10-12 pennyweight/ton,* or about 10 ppm.
The ore is rich in palladium, the platinumrpalladium ratio being about 1:3.5.
This is the most extensive deposit and most concentrated ore yet reported within
the United States, and studies are under way to determine the economic feasibility
of mining it commercially.
PRODUCTION AND
There are two principal stages in the isolation of reasonably pure platinum
metals from raw materials. One is extraction of a concentrate of precious metals
from a large body of ore. The other is the refining of the precious metals,
which involves their separation from the concentrate and from each other and
ultimately their purification. As indicated earlier, refining applies not only
to native sources of new metal, but also to a large quantity of scrap and other
used metal that is recycled. The details of the first of these operations de-
pend on the source of the raw material and its composition.
One pennyweight = 0.05 troy oz or 1.555 g.
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TABLE 2-5
Estimated World Reserves of Platinum and Palladi\ma
Country
South Africa
U.S.S.R.
Canada
Coldhbia
United States
Total
Reserves,
Platinum
142,400
60,000
6,940
5,000
950
215,290
1,000 troy oz
Palladium
50,200
120,000
6,860
0
1,960
179,020
Total
192,600
180,000
13,800
5,000
2,910
394,310
^Excluding recent find in Montana. Data from U.S. Department of the
Interior.
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Rather complete descriptions of the metallurgic operations of the Inter-
national Nickel Company in Sudbury, Ontario,119 and of the Rustenberg Platinum
Mines Limited in South Africa36 are available, and what follows is only a brief
synopsis.
In the International Nickel process, most of the platinum-group metal is
separated from the bulk of the copper and nickel during slow cooling of a
Bessemer matte. During this cooling, the oxidation of sulfur Is regulated so as
to produce small amounts of metallic nickel and copper. The latter serve as
collectors of the precious metals from the original ore, and separation of the
metallic phase is facilitated, because this phase is magnetic. The separated
material can be concentrated to an even richer alloy, the electrolytic refining
of which yields a rich concentrate in the anodic slimes. Smaller amounts of the
precious metals are also recovered during refining of nickel either electrolyt-
Ically or by the Mond carbonyl process. The electrolytic refining operations of
International Nickel are carried out at Port Colbome in the Niagara Peninsula
in Ontario. Most of the osmium In the anodic sludges (or slime) is recovered and
refined in the course of acid treatment of the roasted sludges. The remainder Is
sent to the refineries of the Mond Nickel Company, Acton (London), England.
Another company, Palconbridge Nickel Mines, Limited, operating in the
Sudbury, Canada, region, follows a somewhat different treatment of the ore
that results in a high-grade sulfide matte, rich in nickel and copper, which
is shipped to Kristiansand, Norway, for further treatment. Electrolytic re-
fining of the nickel in Norway also produces anodic sludges that can be worked
up to a rich concentrate of precious metals, which is partially returned to
North America for refining by Engelhard Minerals and Chemicals Corp. in New
Jersey.
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The South African ore is so rich in platinum metals that as much as two-
thirds can be recovered by gravity concentration alone. The remainder can be
obtained after smelting of the tailings, which contain appreciable amounts of
copper and nickel. The process results in recovery of these metals; in the
course of nickel-refining, platinum metals again accumulate in the electrolytic
process. About three-fourths of the various South African concentrates of
precious metals are shipped to England and refined at Brimsdown or at Boyston,
near London, by Johnson-Matthey Limited. The remaining one-fourth is processed
by the Impala Company in South Africa.
The electrolytic anodic slimes and other rich concentrates are subjected
to roasting and then acid-leaching to extract copper, nickel, or other base
metals. Under such conditions, the osmium may form a volatile tetroxide (see
Chapter 4) and be all or partially lost. Recently, the International Nickel
Company gave details of a process for the easy recovery of most of the
available osmium at this stage.217 in this process, treatment of the dried
anodic sludges with sulfuric acid is Tirol t-pd to temperatures not in excess of
200° C. This renders copper and nickel soluble, but leaves the platinum metals
insoluble, except for a small amount (less than 5%) of osmium that is volatilized.
This insoluble residue is dried and then brought to ignition at 800-900° C; this
volatilizes about 85% of the osmium as the tetroxide, OsO4. The tetroxide is
absorbed in alkaline scrubbing solution and retained for further purification.
The ignited acid-soluble residue is freed of sulfur, selenium, and arsenic
by this treatanent.34 It is then treated with aqua regia, which dissolves most
of the platinum, palladium, and gold. From the resulting solution, gold is
precipitated by addition of a ferrous salt. After separation of the gold, ammonia
chloride is added to precipitate the yellow salt ammonium hexachloroplatinate,
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(NH4)2 PtC£6; this is separated and ignited to yield the metal. For further
purification of the platinum, it is redissolved in aqua regia, and the solution
is evaporated gently with sodium chloride and hydrochloric acid to destroy oxides
of nitrogen and nitrosyl compounds. This solution of sodium hexachloroplatinate
(IV) Na2PtCJl6, is then treated with sodium bromate and its pH carefully raised
to cause precipitation of the hydrous oxides of any rhodium, iridium, or palladium
that was carried down in the initial precipitation of platinum. After removal of
solids by filtration, the solution is boiled with hydrochloric acid to destroy
bromate, and then treated with ammonium chloride for a second precipitation of
platinum as aronanium hexachloroplatinate (IV). This is filtered off, dried, and
brought slowly to ignition at 1,000° C. The resulting product is a gray sponge
of metal in a pure (over 99.9%) form.
The filtrate after removal of gold and platinum contains palladium as
chloropalladous acid, H^PdCJ^. Addition of aqueous ammonia to this solution
causes precipitation of the yellow complex dianmine dichloride, Pd (NH^) 2Ci2'
which redissolves in excess ammonia through formation of the complex ion
2+
PdtNH^)^ . From the araraoniacal solution, the insoluble dianmine dichloride is
reprecipitated by the addition of hydrochloric acid. Palladium is isolated and
further purified by successive precipitation and redissolution of this compound
in this way. Ignition of the complex salt yields palladium metal in spongy form;
the ignited sponge is usually cooled in an oxygen-free atmosphere to avoid super-
ficial oxidation.
The residue not already dissolved must then be treated to recover silver,
rhodium, iridium, ruthenium, and osmium. The precise details of the practices
followed by different refiners in handling this material are not always disclosed
and are certainly not uniform. In one account,97 the insoluble residue is heated
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with fluxes, much as in the classic fire assay, to produce a lead alley contain-
ing the precious metals. This is treated with nitric acid to dissolve the lead
and silver. The insoluble residue is then fused with sodium bisulfate; this
treatment selectively converts rhodium to a water-soluble sulfate. The solution
produced is made alkaline to precipitate rhodium hydroxide, Kh(OH)3, which is
separated and dissolved in hydrochloric acid. Brpurities are carried through
this step with the rhodium, but may be separated later by hydrolytic precipi-
tation in the presence of nitrite. The nitrite complex of rhodium, Rh(N02)g ,
is very stable over a wide range of pH and remains in solution under conditions
suitable for precipitation of many metal hydroxides. The metal is later pre-
cipitated as ammonium hexanitrorhodate (III), (NHjJRhtNO-^. Still further
purification is achieved by converting the latter compound to the chlororhodite,
RhCfcg3" through digestion with hydrochloric acid; the solution containing
chlororhodite is passed through a cation-exchange column to remove traces of
base metals. Rhodium is finally precipitated from this purified solution with
formic acid, dried, and ignited under hydrogen to a residue of very pure metal lie
sponge.
The remaining metallic material is heated with sodium peroxide at 5QCP C
to convert ruthenium and osmium (if this has not previously been recovered)
to the water-soluble salts sodium ruthenate, Na2RuO^, and sodium osmate,
Na^OsO,. Solutions of these compounds are then treated in such a way as to
distill the volatile tetroxides out of these two metals. The treatment varies
somewhat in accordance with the proportions of the two elements, but they are
separated at this stage by taking advantage of the fact that osmium tetroxide,
OsO4, can be distilled from nitric acid solutions while ruthenium is retained
in the pot as nitrosyl complexes. Distilled 0364 is collected in a solution
2-14
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of sodium hydroxide, usually containing alcohol. The absorbate is then digested
with ammonium chloride, during which process osrm/ltetrammine chloride,
Os02(NH3)4CZ2r precipitates. This compound is dried and ignited in hydrogen
to form a sponge of the pure metal. Other absorbing solutions and precipitation
forms for osmium are also used. If osmium is absent or present at a low concen-
tration, the solution extracted from the alkaline fusion is treated with chlorine
gas and heated. Ruthenium distills as tetroxide under these conditions and is
then absorbed in hydrochloric acid. Any osmium present at this stage may be
distilled out by boiling with nitric acid. The remaining solution is boiled
with hydrochloric acid to destroy nitrosyl complexes, and then the ruthenium
is precipitated by the addition of ammonium chloride as ammonium hexachloro-
ruthenate (III) , (NH^-RuCAg. This may be heated in an inert atmosphere to
yield the metal. If larger amounts of osmium are present, the amount of nitric
acid required for its removal as tetroxide may result in an alternative precipi-
tation form for ruthenium, namely, pentachloronitrosylruthenic acid (III) ,
The previously mentioned fusion with sodium peroxide converts iridium to
its dioxide, which is insoluble in water. This may be brought into solution by
treatment with aqua regia; from this solution, ammonium hexachloroiridate,
(NH4)2IrCJl6, is precipitated by the addition of ammonium chloride and nitric
acid. This salt may be further purified by dissolving it in ammonium sulfide
solution, in which iridium remains soluble while impurities are precipitated.
The latter are separated by filtration; ammonium chloroiridate is reprecipitated
as before, then ignited in an atmosphere of hydrogen to yield a pure sponge of
the metal.
2-15
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A sunmary of the extraction and refining of the platinum metals from the
principal South African source has been published elsewhere.157 Mare recently,
solvent extraction and ion-exchange techniques1273 for the recovery of the
platinum-group metals have been tested to seme extent, although their develop-
ment has apparently reached only the pilot-plant stage.
2-16
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CHAPTER 3
USES
In general, the uses of the platinum-group metals and their compounds
derive from the special properties of these substances. The metals, alone or
in alloys, have long been known to have extensive catalytic properties, the
first such observations having been coiraunicated by Sir Humphry Davy to the
Royal Society in 1817. In recent years, a number of compounds of the platinum-
group elements have also been introduced as catalysts in synthetic organic chem-
istry. Other properties that lead to the usefulness of these metals include
their resistance to oxidation, even at high temperatures; resistance to corro-
sion; high melting point; high mechanical strength; and, at least for platinum
and palladium, good ductility.
It may be broadly indicative of the nature and scale of the uses of the
platinum-group metals to record their sales to consuming industries. Table 3-1
gives data for 1973 according to the American Bureau of Metal Statistics.14
Tables 3-2 and 3-3 give data for platinum and palladium, according to the Bureau
of Mines,205'269'270'456 spanning a period of several years, from which any
fluctuations and trends may be discerned. All data refer to the United States
only.
CATALYTIC USES
The number of catalytic applications, their variety, and their selectivity
for specific reactions make it impossible to do justice to the full range of
these applications in such a publication as this. In the survey that follows,
a number of key references are cited, and these should be consulted for more
details.
3-1
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TABLE 3-1
Sales of Platinim-Group Metals to Consulting Industries. United States, 1973*2
Industrial
Category
Chemical
Petroleum
Glass
Electric
Dental and
medical
Jewelry and
decorative
Miscellaneous
Totals
Quantity
Platinum
238,809
119,875
72,533
100,607
18,103
20,366
53,479
623,772
Sold, troy oz
Palladium
239,394
3,803
1,439
458,652
62,776
21,658
61,254
848,976
Others
72,708
15,284
16,822
15,230
1,153
13,698
11,095
145,990
Total
550,911
138,962
90,794
574,489
82,032
55,722
125^828
1,618,738
Data from American Bureau of Metal Statistics,
14
3-2
-------
TABLE 3-2
c
Sales of Platinum to Consuming Industries, United States, 1968-1973
ial
Chemical
Etetroleum
.ectric
Cental and
medical
and
decorative
Lscellanaous
Totals
1973
238,809
U9f875
72,533
100,607
18,103
20,366
53,479
1972
225,895
98,847
26,970
92,381
30,462
20,655
50,089
1971
125,112
137,396
40,703
51,940
23,097
18,577
19,859
1970
147,029
181,014
34,577
88,146
19,794
30,093
15,355
1969
175,436
58,602
63,350
112,589
22,266
36,161
47,174
1968
157,677
161,050
47,935
117,256
24,903
40,184
31,150
623,772 545,299 416,684 516,008 515,578 580,155
ata from American Bureau of Metal Statistics and U.S. Bureau of Mines.205'269'270'456
3-3
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TABLE 3-3
Sales of Palladium to Consuming Industries, United States. 1968-1973°
Industrial
Category
Chemical
Petroleum
Glass
Electric
Dental and
medical
Jewelry and
decorative
1973
239,394
3,803
1,439
458,652
62,776
21,658
61,254
1972
292,710
14,499
2,250
425,505
94,274
19,375
27,835
1971
218,651
2,916
237
431,505
61,594
18,752
26,451
1970
186,001
15,494
21,147
419,089
54,426
17,507
22,842
1969
214,508
1,337
3,891
430,258
52,326
21,837
34,581
1968
228,318
22,683
10
329,012
61,636
17,797
62,023
Totals 848,976 876,448 760,106 736,506 758,738 721,479
^Data from American Bureau of Metal Statistics14 and U.S. Bureau of
Mines.205,269,270,456
3-4
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Naphtha-Reforming
The use of platinum in naphtha^reforming,, introduced in 1949, has grown
rapidly and today is one of its major industrial uses. The metal is dispersed
on small pellets of alumina or silica^aluminaf which, being porous themselves,
expose an enormous specific surface area of the platinum. The petroleum frac-
tion fed to the reactors comprises hydrocarbons, which boil at roughly 100-200° C,
and hydrogen generated in the process. The reactions increase the octane rating
of the gasoline fraction and produce large amounts of aromatic hydrocarbons that
may be separated from the product and used for purposes other than fuel. Briefly,
the reactions that take place during petroleum reformation include:
(1) conversion of naphthenes to aromatic hydrocarbons;
(2) isomerization or cracking of paraffin hydrocarbons and
conversion of some of them to aromaticsj
(3) conversion of any sulfur compounds to hydrogen sulfide
and the corresponding hydrocarbons; and
(4) saturation of olefinic hydrocarbons, followed by
reaction 2, 3, or both.112'117A38,151,167,306/327,351
In recent years, the trend in this application has been away from the use
of platinum alone toward the use of bimetallic catalysts, including mixtures of
platinum with rhenium or iridium, and possibly also germanium, indium, or gold.
The newer catalysts were reported to permit operation at lower pressures and
consequently to result in longer catalyst life and greater octane yield. This
application has led to a sharp increase in the demand for iridium.67,289,328
Until the introduction of catalytic converters for controlling pollutants
emitted from automobile engines, reforming was the major platinum^consuming
process. Because of its importance, a more comprehensive discussion of catalytic
reforming is given in Appendix A.
3-5
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Exhaust-Gas Control
For some time, ceramic honeycomb materials impregnated with platinum or other
platinum metals have been produced and used for exhaust-^as purification. These
materials can oxidize a wide array of substances that may be emitted from industrial
plants or vehicles. The first applications appear to have been mainly in stationary
sources of oxidizable gaseous effluents. Typical of catalysts for this applica-
tion are a preparation known as Qxycat203 and another called THT.2*3 These achieve
removal of objectionable volatile substances given off during the curing of a
resinous binder, removal of carbon-black fines and other combustible substances
given off in the manufacture of carbon black, and oxidation of exhaust gases
from petroleum-reforming operations or from the production of phthalic anhydride.
On a more limited scale, these materials can be used to remove objectionable
fumes from paint-baking ovens, self "-cleaning cooking ovens, electric incinerators,
etc.145'426
The same catalytic devices have been used to remove toxic or offensive fumes
from diesel-engine exhaust, so that the engines may be operated in confined areas—
for instance, on power fork-lift loaders in factories or warehouses and in the
locomotives in mines. 1*390 similar catalytic devices based on palladium have
also been described.
The next logical step has been to apply catalytic units of the foregoing type
to reduce hydrocarbon and carbon monoxide emission in automobile exhaust.4'5
These are based on platinum metals supported on either porous ceramic pebbles or
ceramic honeycomb. A suitable combination of these metals has been reported to
contain platinum and palladium in a ratio of 5:2,321 possibly with a small amount
of ruthenium or other platinum metal. The use of such catalytic afterburners
necessarily involves the use of unleaded fuels, because the lead halides emitted
3-6
-------
from gasoline that contains lead tetraethyl rapidly poison the catalyst bed.
An expanded description of this application will be given in Chapter 8,
The same type of honeycomb catalyst units have been applied to the reduction
of oxides of nitrogen in exhaust gas. This application was developed to reduce
the visible and objectionable emission of nitrogen dioxide from the absorbing
towers of nitric acid plants. The +a-n gas is blended with a reducing fuel gas,
such as hydrogen or arnnonia, and passed through the catalytic unit, whereupon the
nitrogen dioxide is reduced either to nitrous oxide or to elemental nitrogen,
according to the concentration of fuel gas added.211/388 The same principle
has been proposed for inclusion in the exhaust train of automobiles. By this
it is hoped to remove the nitric oxide, which has been linked to the formation
of photochemical smog. 4f 394 The catalyst for this purpose generally contains
some ruthenium. The technology applying to this particular device does not
appear to have reached the point where it can be applied to production automobiles.
*•
Ammonia Oxidation
The manufacture of nitric acid by the Ostwald process entails oxidation of
annnnia with air to form nitric oxide. This is accomplished by passing the syn-
thesis gases through gauze beds of an alloy of platinum and 10% rhodium. The
9 39 198
operating pressures are about 8 atm (811 kN/hr) . ' There is some small loss
of platinum from the catalyst gauze, and this and other changes of the platinum
metals have been the object of considerable investigation. 30,48,101,102,173, 315, 337, 383
Although some of the catalyst loss has been attributed to the formation of volatile
oxides of the metals at the high operating temperature some recovery of platinum
has been achieved by the use of gold-palladium catchment gauze supported on
stainless-steel mesh below the converter catalyst
3-7
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Sulfur Dioxide Oxidation
Although not a current use of platinum, sulfur dioxide oxidation is included
here as a reminder that the burning of fuels containing sulfur followed fay passage
of the combustion products over a platinum catalyst is quite capable of producing
sulfuric acid. In fact, the use of platinum catalysts for this reaction resulted
358
in the first patent ever issued in the field of catalysis (cited in Robertson""0).
When used industrially for the production of sulfur trioxide, the early catalysts
took the form of either platinized asbestos or finely dispersed platinum on mag-
nesium sulfate. The oxidation of sulfur dioxide is exothermic, so the yield of
product fans as the operating temperature increases. The optimal range of
temperature to maintain rapid oxidation and a good yield is about 400-450° C at
atmospheric pressure.305
Hydrogen Cyanide Manufacture
The Andrussow process for the manufacture of hydrogen cyanide involves the
passage of a mixture of air, ammonia, and methane through gauze of an alloy of
platinum and 10% rhodium.318'323 The essential reaction is
CH4 + MH3 + 3/2 02 -»• HCN + 31^0
and is exothermic. The yield is approximately two-thirds, on the basis of the
consumption of either methane or anraonia. In a newer process, which achieves
higher conversion efficiency,128 only methane and ammonia are used:
CH4 + NH3 -»• HCN + SHj.
This exothermic reaction is carried out in alumina reactor tubes coated inside
with platinum catalyst.
3-8
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Hydrogenation
The platinum metals and complex compounds of them have found extensive
application as hydrogenation catalysts in organic chemistry. Palladium is the
best-known agent for this purpose, but every metal in the group has been found
to have advantages for special purposes. Palladium may be introduced as powdered
monoxide and reduced in situ by the hydrogen, which produces the metal in finely
divided form. Alternatively, and more commonly, such inert substances as calcium
carbonate, magnesium oxide, activated carbon, and silica gel can be impregnated
with soluble palladium compounds. These are allowed to dry and then reduced to
yield a supported catalyst. The metal concentration on the support amounts to a
few percent at most, except in some special applications. Countless instances
of the use of palladium as a catalyst for hydrogenation appear in the literature;
a good summary is in a monograph by Wise,375
Major summaries of this subject have been presented by Rylander,37^
Augustine,27 and Bond,50 and shorter summaries by Bond52 and Wells, 55 The
use of osmium and ruthenium as hydrogenation catalysts has been described by
Bond and Webb53 and Webb;450 the characteristics of ruthenium-platinum oxide
catalysts for the same purpose have been outlined by Bond and Webster.54'55
t
Homogeneous hydrogenation by complexes has been summarized by Rylander,373 0?P-60~77)
A wide range of dehydrogenation or oxidation reactions in organic chemistry
have been performed with the aid of platinum-metal catalysts. Far the most part,
these reactions require higher temperatures than hydrogenation to shift the
equilibrium toward formation of the desired product. Under these conditions,
there is greater risk of side reactions. When a hydrogen acceptor (such as
3-9
-------
oxygen or nitrobenzene) is present, the desired reaction nay be achieved under
more moderate conditions, e,g, „ at lower temperatures, Palladium appears to be
the platinum-group metal most favored for these reactions and commonly is sup-
ported on carbon black at from 5 to 30 wt % metal.
The reactions for which this catalytic application is most successful are
aromatizations, i.e., conversion of fully or partially saturated molecules into
aromatic systems.372 feP» i"59* *375 Another kind of catalytic oxidation is the
selective oxidation of functional groups, e.g., oxidation of a primary alcohol
to an aldehyde; such selective oxidation reactions require the presence of oxygen
as a hydrogen-acceptor.
CoorfH nation Complexes of the Platinum-Group Metals as Catalysts
Over a period of not much more than 10 years, there has been extensive de-
velopment of platinum-metal catalysts of a new type, These are organometallic
coordination complexes for the most part, and they are homogeneous catalysts used
in a dissolved state. Brief general accounts of these new developments have been
presented by Bond-*! and Cleare.9^ Hydrogenation aided by homogeneous platinum-
metal catalysts was referred to previously.373 fa?' 60-77) A recent monograph
on platinum and palladium devoted considerable space to a discussion of this
newer aspect of their catalytic use.77 (pp' 386-395,443-448) Many e^p^ appear
throughout the monograph by Blander.373 (pp* 60"77)
One example of the sort of process made possible in this way is the conversion
of methanol to acetic acid with 99% selectivity through insertion of carbon monoxide
_ ofro
at pressures as low as 1 atm (101 kH/hr). The catalyst is a rhodium halide oon-
plex with an iodide promoter.
3-10
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There are some problems in connection with recovery of the expensive cata-
lytic materials when they are used in soluble forms, and some attempts have been
made to synthesize "homogeneous" catalysts with polymeric or macromolecular
ligands,258'264 the idea being to create the catalytic center characteristic of
a homogeneous agent in a polymerized and therefore insoluble matrix, which is
thus capable of ready separation,
Other Industrial or Commercial Catalytic Applications
This section groups a number of other applications of platinum-group metals
or their compounds as catalytic agents.
A comparatively new synthesis of hydrogen peroxide is based on the autoxida-
tion of 2-ethylanthraquinol; the guinol is converted to the corresponding quinone
and hydrogen peroxide. The latter is separated into water by countercurrent ex-
traction, and the quinone is reduced again to the quinol with a supported palladium
catalyst.24
The oxidation of ethylene to acetaldehyde with the aid of a palladium chloride,
PdC£2f catalyst has become known as the Wacker process.403,404 ^he palladium
chloride is in aqueous solution with added copper (II) chloride; palladium is re-
duced to the metal by ethylene, but its reoxidatijon to the starting compound by
a stream of air is aided by the copper chloride. Extension of this principle to
the production of acetone and methylethylketone by the oxidation of ptropylene and
butylene, respectively, has also been demonstrated.100
Osmium and ruthenium tetroxides have been used as oxidative catalysts in
organic chemistry in a variety of ways.160'161 *&• 66' 147) '374 one particular
reaction for which osmium tetroxide serves as a good catalyst is the hydroxylation
of double bonds whereby olef inic compounds are converted to vic-glycols with cis
configuration.161 <»• 66, 147), 374
3-11
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New ruthenium catalysts of exceptional activity have been developed for the
synthesis of nighntDlecularHweight polymethylenes f ran carbon monoxide and hy-
drogen under pressure. In both of two preparations described, the catalyst was
metallic ruthenium reduced frcm an oxide and suspended in nonane,^^
The production of heavy water involves hydrogen-deuterium exchange on a
catalytic surface. A recent account described a dnal temperature exchange between
water and hydrogen sulfide; the catalyst consisted of finely divided platinum sup-
ported on carbon. 376 jn other processes described, the exchange was between gaseous
hydrogen and water, 179 ^th finely divided platinum either in supported form or sus-
pended in the water.
Low-Temperature Catalytic
Hydrocarbons and light alcohols mixed with air and passed as a vapor over a
platinum surface will react, with emission of the heat of combustion. The prin-
ciple has been used occasionally for such devices as cigarette lighters. A
recent application has been in portable heaters that burn light hydrocarbons.
The fuel and air react at an impregnated catalyst bed, producing temperatures
of around 400° C. A wide range of recreational, agricultural, and industrial
applications of such heaters have been suggested, and a number of manufacturers
are producing units based on this principle.
ELECTRIC USES
Contacts for Relays and Switchgear
A large part of the palladium used by industry and utilities is for the pro-
duction of electric contacts. Telephone switchgear for dialing systems involves
millions of small contacts that are expected to provide dependable service for
long periods. Palladium, either gold-coated or alloyed with copper or silver,
3-12
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has been adopted in many countries for these switching contacts. The selection
of this metal or its alloys is based on good arcwjuenching characteristics and
minimal sticking, welding, or material transfer--owing to freedom from corrosion
or tarnish.
Voltage regulators for automobiles have been improved by the introduction
of platinum-iridium, platinum-gold<-silver, and palladium^ilver^nickel contacts,
all of which permit higher currents and extended life. Directional signals
incorporate silver and platinum contacts for a variety of reasons, including
hardness and good conductivity.
The use of platinum-group metals in electric contacts has been the subject
of numerous articles in Platinum Metals Review.37,79,80,158,209,326,349,377,423,446,448
Resistors and Capacitors
Windings in traditional wire-wound potentiometers and precision resistors are
made from an alloy of palladium and 40% silver, because of its low temperature
coefficient of resistance of this composition. Other alloys incorporating the
platinum-group metals are also used where other characteristics, such as mechanical
strength and higher resistivity, are important. ^^
Many resistors are made by firing solutions containing platinum-metal com-
4
plexes on glass, quartz, or mica in the form of rods, tubes, plates, or fibers.
A wide range of resistances and temperature coefficients of resistance can there-
by be obtained. Printed circuits as components of motor controllers, computers,
scientific instruments, etc., depend to a considerable extent on plating solutions
that contain platinum-group metals, as well as gold and silver,431'432
Similar plating and firing techniques have been applied to the fabrication
of ceramic capacitors in a multilayer construction. 232,421
3-13
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Electrochanical Electrodes
Apart from the familiar use of platinum electrodes in the analytic-chemistry
laboratory, platinum and platinums-clad electrodes find many applications in pre-
parative chemistry. In particular, anodes of this material resist oxidation them-
selves; because there is a high overpotential for the formation of oxygen
if they are shiny, many oxidative reactions can be carried out at such a working
electrode. For instance, the preparation of hydrogen peroxide by the anodic oxi-
dation of sulfuric acid was widely used, and, although superseded largely by oxi-
dation of 2-ethylanthraquinol,24 the original electrolytic process has been im-
proved324 and might be reactivated. A number of electrochemical oxidations form-
ing parts of organic syntheses have been described,12 and the removal of unwanted
chlorides from nitric acid has been achieved electrolytically at platinum anodes.463
Although the traditional material for fabrication of electrodes is a combina-
tion of platinum and 10% iridium, a number of applications have taken advantage of
platinum-clad anodes. For instance, a process has been described for production of
sodium hypochlorite from seawater;431,432 under other conditions, chlorates may
be produced at this electrode.320 Impressed current protection against corro-
sion win be referred to later, but the same sort of clad electrode finds applica-
tion for this purpose.
Spark Electrodes
The electrodes in spark plugs for aircraft engines are often made with a
platinum alloy. These must operate under extremely corrosive conditions and
suffer the risk of early disintegration through the deposition of lead from the
fuel at the grain boundaries. Alloys with 5 or 10% ruthenium, with palladium
and ruthenium together, or quite often with 4% tungsten are used for this
3-14
-------
purpose.34 Much-improved performance for the same purpose has been found
with pure iridium, although problems of fabricating articles with iridium
397
have delayed its use.
Grids for Power lubes and Radar Tubes
The grid structures in large thermionic tubes for use in radio trans-
mitters, modulators, and such industrial applications as induction heating
are fabricated from platinums-clad molybdenum (or sometimes tungsten) wire.
The characteristics of such material include mechanical strength and a high
electron work function.410,431,432
Fuel Cells
The requirement to generate electricity for satellites and manned space
vehicles has stimulated a great deal of research and development of fuel cells.
Because these, like conventional chemical cells, are based on converting the
energy of a chemical process into electricity, it is not surprising that cata-
lysis plays an important role. It is clear from the literature that platinum-
metal electrode systems predominate in the design of fuel cells because of their
permanence and their unique catalytic capabilities. '"
Production of Hydrogen
2+
Very recently, the complex tris (2,2 '-bipyridine) ruthenium (II) has been
shown to catalyze water-splitting reactions driven by sunlight. ^77 rjjjg process
works by one of the most efficient mechanisms for converting light energy into
chemical energy—photoinduced electron transfer. A key feature of this cata-
lyst is its insolubility in water, which is accomplished by preparing surfactant
analogues of bipyridine^ruthenium complex, which can be deposited as monolayer
3-15
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films on glass slides and which will reduce water when subject to ultra-
violet irradiation.
HIGH-TEMPERATURE USES
Platinum-Resistance Thermometers
Part of the range of the International Temperature Scale is defined in
terms of the platinum-resistance thermometer. This lies between the boiling
point of oxygen (-182.97° C) and the melting point of antimony (630,50 c).
For this purpose, extremely pure platinum is required with the ratio of its
resistances at 100° C and 0° C to be not less than 1%3910;1, A brief review
of the construction and applications of the platinum-resistance thermometer
has been presented elsewhere,335 and a more comprehensive monograph has been
issued by the National Bureau of Standards.352
Thermocouples
For good-quality thermocouples, especially for high-temperature measure-
ment, platinum and platdnura-rhodium elements are used. The positive element
is the alloy, whose composition must be carefully controlled if existing thermal-
electaxiiotive-force data are to be used, Generally, these contain 10% or 13%
rhodium. There has recently been a new international agreement on reference
tables for platinum-metal thermocouples.341 For work at higher temperatures,
thermocouple elements denoted "five*-bwenty" or "six-thirty" (referring to the
percentage of rhodium in each arm) have demonstrated superior performance. For
lower temperatures, thermocouple elements incorporating a palladium-gold allxy
(Pallador) have been proposed.41'298 Thermocouples of these types are used ex-
tensively in steel production, nonferrous metallurgy, glass manufacture, the
space program, and indeed almost anywhere when accurate informatim regarding
high temperatures is required.
3-16
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High-Temperatiire'-ffurnace Windings
The heating elements in many furnaces designed for use at high tempera-
tures and often under oxidizing or corrosive conditions commonly have resistor
wires made of platinunwnetal. Most are of an alloy of platinum with 10 or 20%
rhodium,165'336 but some trials have been made with an alloy with 40% rhodium,78
which compares favorably with rhodium itself for this purpose,
laboratory Ware for High Temperatures
Crucibles, combustion boats, tips of tongs, and other items of laboratory
equipment to be used for ignition or other high-temperature work when resistance
to chemical attack is important are generally fabricated from an alloy of
platinum and 10% rhodium. However, for some purposes, a "nonwetting" platinum
alloy may be prepared; this is not "wetted" by molten glasses or borate fluxes.
Another recent development has been the introduction of a dispersion-strengthened
platinum known as ZGS (zirconia-grain stabilized) platinum, 9 which is stronger
and offers more creep resistance than the customary platinura^rhodium alloys.
The preparation of various substances in the form of single crystals for
use in lasers, optical modulators, and other devices has increased in importance
in the last dozen years. Such crystals may be grown from their components with-
-*
in a flux by slow cooling or by the slow pulling of the crystal from a melt
(the Czochrolski technique) .431,432 The crucibles used for either method are
made of platinum or iridium, according to the temperature that they are required
to withstand. These crucibles can be used at temperatures up to 1,350° C for
platinum and 2,000° C for iridium98 without contamination of the molten contents.
3-17
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Strain Gauges
Platinum-nalloy resistance wires (.tungsten with, platinum or chroniura with
palladium) have been used in strain gauges for high-teroperature applications.45^
Flame Retardfmts
In apparent conflict with, catalytic oxidation activity of platinum, seme
platinum compounds have been shown to be capable of acting as flame retardants
when included in silicone rubber at very low concentrations, 178,291,396 How-
ever, because of the high cost of these materials, it is highly unlikely that
the compounds will became widely used for such purposes.
USES BASED ON OOPRDSION
There is a wide range of applications related to the corrosion resistance
of the platinum-group metals, some based on the intrinsic nobility of the plat-
inum metals themselves, some based on their ability to impart corrosion resist-
ance to or protect other metals, and others based on protection through applied
electromotive force, by which a metal to be protected is made cathodic or anodic.
A recent account of the fabrication of standard kilogram weights407 exempli-
fies the use of these metals because of *•>**?*• freedom from corrosion or tarnish.
Sintered platinum-alloy p^« for the ffiltration of highly corrosive fluids are
prepared by pressing tiny metallic spheres together and heating them at about
1,000° C.431'432 Platinum-lined furnaces for conducting such chemical reactions
as the fluorination of uranium compounds417 or plutonium oxide325 with hydro-
fluoric acid have been described.
The addition of comparatively small amounts of platinum metals has been
shown to increase considerably the corrosion resistance of various base metals
3-18
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or alloys. For instance, stainless steel,194'218 chromium193 and titanium23
195,405,406 JJ. . - , . .
have been so modified, the first two by the addition of platinum
and the last by the addition of palladium. The corrosion resistance of
titanium has also been increased by a thin layer of electrodeposited platinum107'322
or by a thin coating of palladium, ^°
The platinum metals, especially the alloy of platinum and 10% rhodium,
are applied in a great variety of ways in the manufacture of glass. Many
vessels and furnaces for handling molten glass are fabricated with platinum-
clad linings, molybdenum being a preferred support metal. The largest consump-
tion of platinum is for bushings that contain hundreds of small holes through
which molten glass is drawn or blown to make glass fiber. Here,, corrosion
resistance, great strength, and resistance to wear are important. In excellent
accounts of the use of the platinum metals in the glass industry,332'333'334
it was stressed that the important criterion for their selection is the protec-
tion of the purity of the molten glasses.
Finally, an important use for platinum or palladium is an electrolytic
method for protection against corrosion. As introduced, this was to protect
ships* hulls, propellers, and rudders against corrosion by seawater. These
parts of the ship were treated as one electrode of a cell, the other being a
number of platinum or platinum-clad electrodes normally on but insulated from
the hull; across these electrodes, an electromotive force was impressed such
that the hull, etc., were at a potential negative to the other electrodes.
This is known as cathodic protection,431'432 and its use has been extended to
steel piers, bulkheads, retaining walls, and pipelines, Base*fnetal anodes,
if used in this way, are corroded away and may be regarded as expendable, but
platinum (or titanium coated with platinum or palladium) remains essentially
3-19
-------
unattacked. The same principle has been used to protect against corrosion
in chemical plants, e.g. , in the case of paper-making machinery.-^ The
principle has been used with reversed polarity to protect steel by anodic
current (this is similar to passivation) ;189 the counterelectrode is a
platinum-clad cathode.
SPINNERETS AND BUSHINGS
As mentioned before, bushings for the production of fiberglass are made
from platinum-rhodium alloy. 332,333,334 Another use of this alloy in the glass
industry is in bubbler tubes to be inserted into molten glass. The practice of
forcing small gas bubbles through the melt results in agitation and better heat
utilization. The bubblers must be able to stand up under severe conditions
and are normally made of the alloy of platinum and 10% rhodium welded to a
nickel tube.331
Platinum alloys are also used to make spinning jets for the production of
viscose rayon. These are made with the alloy of platinum and 10% rhodium or,
perhaps preferably, with an alloy of gold and 30% or 40% platinum.182
i?'KHMOMAGNETIC
The alloy containing about 50 atomic percent each of cobalt and platinum
is said to make available a more powerful permanent magnet than any other ma-
terial known. It is also said to possess the advantages (over many other,
newer permanent magnetic materials) that it is malleable and ductile before
hardening and thereby easily fabricated. On the basis of this material, numerous
devices requiring small magnets have appeared, such as hearing aids, electric
1 ^7 2fi7
watches, phonograph pickup cartridges, and miniature relays,"''*0'
3-20
-------
HARDENING AGENTS
From what has been said in this chapter and elsewhere in this report, it
is clear that, for most applications of massive platinum or palladium (in con-
trast, for example, to finely divided catalytic material, which is often sup-
ported on something else), these metals must be hardened. Traditionally,
rhodium and iridium have been the hardening agents for this purpose, the
former being particularly suited to applications in which the alloy will be
used at high temperatures. The reason for restricting the use of iridium
as a hardening agent to alloys intended for cooler uses is related to the
measurable losses of iridium when it is heated in air or oxygen (See Chapter
4). In recent years, there has been an increase in the use of ruthenium for
the hardening of both platinum and palladium, dictated in part by price con-
siderations.
MEDICAL AND DENTAL USES
The primary medical use of the platinum-group metals in humans today is
in cancer chemotherapy, although palladium chloride and palladium hydroxide
have been used to treat tuberculosis and obesity, respectively.
In 1965, Rosenberg and coworkers described the bacterial effects of
various complexes of platinum. 36" further studies showed that neutral com-
plexes of platinum, such as cis-didtu\orodjlanitiineplatinum (II) and a number of
congeners (Figure 3-1), inhibited cell division, but not cell growth, in bac-
teria and that this led to filamentous growth. In 1969, they tested these
platinum complexes for antitumor activity against the solid sarcoma-180 tumor
in ICR mice and found that about 8 mg/kg body weight (which is a pharmacologi-
cally acceptable dosage) produced complete inhibition of the tumor. The
3-21
-------
Cl
NH.
NH.
III
pt(m
PI (in
Cl
NH:
Pt (IS)
ci
ROD
Cl
NH
VII
NH;
Pt(0)
Pt(E)
ci
'NH/
•NH.
Pt(E)
pt(n)
Cl
Cl
FIGURE 3-1. Structural formulas of some representative antitumor complexes of
platinum. I, cis-dicMorodiaircnineplatinum(II); II, cis-tetra-
cMorodiammineplatinum(IV); III, dichloroethylenediaTTmineplatin^a
(II); IV, oxalatodiammineplatinum(II); V, substituted (R) malonato-
dianmineplatinum(II); VI, cis-dichloro-bis (ethyleneimine) platinum
(II); VII, cis-dichloro-bis (cyclopentylamine) platinum (II);
VIII, cis-dichloro-bis (cyclohexylamine) platinum (II).
3-22
-------
National Cancer Institute later found that the same complexes were active
367
against the L1210 tuner in BDF mice, confirming their antitumor activity.
These complexes—particularly c^-dichlorcdianinineplatinum(II), which had been
chosen by the National Cancer Institute for clinical trials—have been ex-
tensively tested in many laboratories against a number of model tumor systems
(Table 3-4). Platinum drugs appear to be active against a broad spectrum of
transplantable tumors, virally induced tumors, and chemically induced tumors
and can cause advanced tumors in some systems to regress.
The active complexes have the following characteristics in common: they
are neutral coordination complexes of platinum (II) or platinum (IV); they ex-
change only some of their ligands quickly; two cis monodentate (or one bidentate)
leaving groups are required; the corresponding trans isomers of the monodentate
groups are inactive; the rate of exchange of the leaving groups should be neither
too low nor too high and should fall into a restricted "window of lability"
centered roughly on that of the chlorides; and the ligands trans to the leaving
groups are preferentially strongly bonded, relatively inert amine systems.
The antitumor effect is stereospecific, in that wherever cis complexes
have been found to be active, corresponding trans complexes have been inactive.
The excretion profiles and tissue distribution are approximately the same for
both isomers. Instead of differences in the availability of the isomers at
specific sites, it appears to be the stereoselectivity of the biochemical reac-
tion in the cell that leads to antitumor activity.
Injected platinum drugs are rapidly excreted, primarily in the urine; the
half-life for 80% of a dose is approximately 1.5 h in animals and less than
1 h in humans. The remaining 20% is excreted over a period of weeks. Of the
3-23
-------
TABLE 3-4
Beat Results of Antitumor Activity of cis-Diohlorodiainonimplatinum(II)
In Animal Systems^
Tumor
Sarcoma-180 solid
Sarcana-180 solid (advanced)
Sarcoma-180 ascites
Leukemia L1210
Primary Lewis lung carcinoma
Ehrlich ascites
Walker 256 carcinosarcjcma (advanced)
Dunning leukemia (advanced)
P388 lymphocytic leukemia
Retioulum cell sarcoma
B-16 melanocarcincma
ADJ/PC6
AK leukemia (lymphona)
Ependymoblastoma
RDUB sarcoma (advanced)
DMBA-induced mammary carcinoma
ICI 42, 464-induced myeloid and
lymphatic leukemia
Host
Swiss white mice
Swiss white mice
Swiss white mice
mice
BDF, mice
BALB/c mice
Fisher 344 rats
Fisher344 rats
BDF^ mice
Of mice
BDF. mice
BALB/c mice
AKR/LWmice
C57BL/6 mice
15-1 chickens
Sprague-Dawley rats
Alderly Park rats
Best Result*^
T/C - 2-10%
100% cures
100% cures
ILS - 379%; 4/10 cur
100% inhibition
ILS - 300%
100% cures; TI > 50
100% cures
ZLS - 533%; 6/10 cunj
I
ILS - 141%
ILS « 297%; 8/10 curd
100% cures; TI - 8
ILS - 225%; 3/10 curi
ILS - 141%; 1/6
65% cures
77% total regrei
3/9 free of all
ILS - 400%
a*todified from Itosenberg.363 The data are insufficient to permit useful averages to be
presented, with two exceptions: L1210 and B-16. These data represent optimal drug
dosages and optimal conditions.
b-/c m tumor mass in treated animals . QQ
' tumor mass in control animals * AUU*
ILS - % increase in life span of treated over control animals.
TI - therapeutic index (LD50/3ED90) x 100; ED9Q - effective dose to inhibit tumors by 90
3-24
-------
rapidly excreted portion, 95% it unchanged, in the case of both ci§-d±chlorodi-
anmineplatinum(II) and oii^alonatodianndneplatiniin(II) (the only two tested so
far) f the other 5% appears to be protein-bound.
The platinum conplexes are selectively taken up by the filtering and ex-
cretory organs of the body, primarily the kidneys, liver, spleen, and thynus.
No selective uptake of platinum complexes in tumor tissue has been shown. The
high uptake in the kidneys leads to damage to the proximal convoluted tubules
and causes nephrosis in mice and rats. Other forms of toxicity in these animals
are denudation of the intestinal epithelium (leading to the dose-limiting gastro-
intestinal damage), bone marrow depression, and hypotrophy of the spleen and
thymus. No histoohemically or physiologically detectable liver damage has been
reported in animals or man. Additional forms of toxicity in man are ototoxicity
(caused by destruction of the oochlear hair cells), nausea and vomiting (due
to a central nervous system reaction), and transient anemia. No gastrointestinal
toxicity has been reported in man, and the kidney toxicity is dose-limiting.
Tissue-culture and in vivo biochemical studies have shown that the platinum
drugs produce severe and persistent inhibition of DMA synthesis with little or
no inhibition of RNA and protein synthesis, at dosages equivalent to therapeutic
*
dosages. The degree of inhibition of DMA synthesis is dose-dependent. The
synthesis of DMA precursors and their transport through the plasma membrane are
not inhibited. CNA polymerase activity is not inhibited. These results are
>
consistent with the generally accepted working hypothesis that the anticancer
activity arises from a direct reaction of the platinum drugs with DMA.363
ois^DidiloTodianinineplatinum (II) reacts with CNA, in vitro in numerous
ways. It forms interstrand and intrastrand cross-links. It reacts monofunc-
tionally and bifunctionally with active sites on the bases. It does not appear
3-25
-------
to form stable products with the phosphates or sugars of the nucleic acids.
It is not an intercalating agent. The platinum is mainly localized in regions
of the DMA that are rich in guanosine and cytosine. Some of these reactions are
reversible or repairable. It is not yet known which type of reaction is signifi-
cant for anticancer activity.
Gottlieb and Drewinko^56 jjgye reviewed the results of phase I clinical
trials of cis-dic^orodianinineplatinum(II) in terminally ill cancer patients.
The results are compiled in Table 3-5. The overall rate of responses in those
trials was generally low, but some types of tumors responded more readily, as
listed in Table 3-6.
More recently, the drug has been tested in combination with other drugs.
Woodman, Venditti, and coworkers at the National Cancer Institute showed in
1973^69 that the platinum drug is additive, and in some cases synergistic, with
other anticancer agents in its activity against various animal tumors. These
results led to the chemotherapeutic use of cis-dicMorodiammineplatinum(II)
in combination with a wide variety of known anticancer agents in human patients.
A second modification in the drug use occurred in 1975, when Cvitkovic and co-
workers at the Sloan Kettering Institute developed a simple pharmacologic treat-
ment that largely ameliorated the kidney toxicity of the drug.-^ They hydrated
the patients with D-mannitol, an osmotic diuretic, before treating them; this
apparently protects the kidneys, but does not increase the percentage excretion
of the drug. Other side effects are now more prominent. Myelosuppression now
appears to be a dose-limiting factor. With this hydration treatment, the same
workers have been able to increase the dosage safely by a factor of 3. With
the high dosage in combination with other drugs, they have acehived a 95% re-
mission rate in patients with testicular cancer. The duration of the remission
3-26
-------
TABLE 3-5
Results of Phase I Clinical Studies with cis-Dichlorodianinineplatinum(II)
nvestigators
filtshaw and Carr
igby et al.
ippman et al.
i.11 et al.
3ssof et al.
»Conti et al.
illey et al.
>vach et al.
Total
in Terminally 111 Cancer
Institution
Royald Mrjrsden Hospital
Roswell Park Memorial
Institute
Memorial Hospital for
Cancer and Allied
Diseases
Wadley Institutes
Patients^
No.
Patients
19
50
21
63
Wilford Hall US Air Force 21
Medical Center
Yale University
Southwest Oncology Group
Mayo Clinic
10
57
51
323
Responses^
No. %
7
17
7
13
3
1
5
2
60
37
34
33
21
14
10
9
4
19
)ata from Gottlieb and Drewinko.
Ireater than 50% reduction in tumor mass.
3-27
-------
TABLE 3-6
Responses of Specific Cancers to cis-Dichlorodianinineplatinum(II)a
Diagnosis
Testicular carcinoma
Lymphona
Squamous cell carcinona
of the head and neck
Ovarian carcinona
No.
Patients
16
16
17
20
No. Responses
Complete^
7
2
0
0
Partial^
3
7
1
5
Improvements'2
3
1
6
3
Total
Response
Rate
81
63
41
40
to
00
studies shown in Table 3-5.
Other complete responses: bladder carcinoma (2); thyroid carcinona (1).
cNot complete, but >. 50% tumor reduction. Other partial responses: breast carcinoma (2); acute
myelogenous leukemia, endcmetrial carcinoma, renal carcinoma, thymona, neuroblastona, lung
adenocarcinoma, unknown undifferentiated primary (1 each).
_ tumor reduction, significant subjective improvement, or mixed response. Other improvements:
colon (3); multiple myeloma, breast carcinoma, acute myelogenous leukemia, lung carcinona,
prostatic carcinoma, unknown undifferentiated primary, undifferentiated sarcoma (1 each).
-------
is at least 14 months, and the relapse rate is very lew. The best previous
therapy produced a median life extension of 6 months, Thus, the addition of
the platinum drug to the prior combination therapy has produced significant
increases in the number, extent, and duration of remissions. The sensitivity
of different kinds of testicular cancers to the platinum drug has now been
verified by Einhorn et al.12713 at the Indiana University Medical Center and
by Merrin236a at the Roswell Park Memorial Institute.
Wiltshaw and Kroner466 have shown that ovarian carcinoma is also re-
sponsive to the platinum drug. Bruckner and coworkers6^ at the Mt. Sinai
School of Medicine have achieved a 70% response rate in ovarian carcinoma
patients with a combination of the platinum drug (at low dosages) and adria-
mycin. Some recent clinical results with combination and high-dose cis-
dic±loixxiiammineplatinum(II) therapy, collected in Table 3-7, show the poten-
tial value of this drug.
It is possible that a patient who was not sensitive to the platinum drug
initially ray become hypersensitive after a series of injections. This was
apparently observed in at least one case. However, little is known about the
sensitivity reaction, and cases where anaphylactoid reactions occurred can be
iK
attributed to impurities in the drug. Since the National Cancer Institute has
became more conscious of the need for high purity of the drug, these reactions
have not appeared. The neutral species (the active drugs) do not seem to
> •••••<«
generate allergic reactions.
The clinical results seen thus far indicate that the search for less toxic
but more effective analogues should be continued. In addition, the recent re-
sults of combination therapy suggest that this may be more effective than the
use of platinum compounds as single agents (see Table 3-7). A number of reviews
3-29
-------
TABLE 3-7
Preliminary Clinical Trials of cis-DicMorcxiLaniTUJieplatinvjmtll^)
Tumor Type
Testicular cancers
(netastatic)a
Ej>iderrooid carci-
noma of head and
neck (far ad-
vanced)*
Ovarian carcinoma
(advanced; prior
w therapy failed)0
T
0 Bladder cancer^
Drug Therapy
No.
Evaluable
Patients
Complete
Remissions
(Duration) , %
Partial
Remissions
Total
i Responses, %
cis-Dichlorodiammine-
platinim(II) 4- vin-
blastine + bleomycin
High-dose cis-dichloro-
diaitmineplatinxin(II)
cis-Dic&lorcdiatnmine-
platinum(II) + adria-
mycin
cis-DichloEodianinine-
platirajn(II)
39
26
18
24
85(3+ to 24+ mo.) 15(3+ to 24+ mo.)
8(2+, 6+ mo.)
33
23(1,2,3,4,5+,6+ mo.)
38 (measurable re-
sponses)
33
33
17 (measurable re-
sponses)
100
69
67
(89% survival
in remission)
50
Data from Einhorn and Rjrnas.127c
fcData from Wittes et al.467a
cData from Bruckner et al.65c
dData from Yagoda et al.472a
-------
and symposia104'362'363 have reported in detail the animal antitumor studies,
animal toxicology, analogue development, mechanisms of action, and clinical
trials of the platinum coordination complexes.
Various amounts of silver, copper, zinc, platinum, and palladium are added
to gold by manufacturers to change such properties as hardness, strength, color,
and cost. Gold is alloyed with other metals to improve its physical properties;
the products are the strongest and most versatile restorative materials used in
dentistry. Generally, gold alloys are based on a three-part composition—70%
gold, and the remainder copper and silver. To ensure hardness and to make heat
treatment possible, the proportion of copper is increased. To offset the lower-
ing of the melting-point range caused by the increase in copper content, platinum
or palladium is added; this addition may also improve the results of the heat
treatment. An addition of 10% platinum to a simple ternary alloy increases its
strength by 35%. However, owing to the high cost of platinum, its place in
dental alloys has been taken to some extent by palladium. Wrought-gold alloys
may contain up to 10% palladium. In the so-called white golds, 30-40% of the
gold is replaced by palladium; this lowers their resistance to tarnish, as well
as lowering the cost.21/370
4X
MISCELLANEOUS USES
Separation of Pure
Palladium has the remarkable property (described more fully in Chapter 4)
of being able to absorb or desorb hydrogen gas. Advantage is taken of this
property in the development of units to separate hydrogen from other gases
by passing gases that contain hydrogen through diffusion barriers of palladium
or a silver-palladium alloy. The hydrogen produced is ultrapure, and the
3-31
-------
purification units have been developed from laboratory scale to industrial
capacity. This technique and its practical applications have been described
in Platinum Metals Review.46,99,114,212,338,357,418,434
Jewelry
The use of platinum for jewelry appears, from statistics on sales to the
industry, to have declined over the last few years; such use of palladium has
remained about constant. Nevertheless, as Tables 3-2 and 3-3 show, the demand
A
is still substantial. Platinum is still used for small articles—such as rings
and settings for jewels—in the form of a hardened alloy, usually with 5-10%
iridium. Because of its lower cost, palladium has some appeal, and a limited
range of luxury goods—such as jewelry, cigarette cases, and the like—are
produced from this metal, generally alloyed with 4.5% ruthenium or 4% ruthenium
and 1% rhodium. White gold is an alloy that contains about 20% palladium or
nickel.
Reflecting or Ornamental Surfaces
There is a considerable demand for articles plated with rhodium. It has
a hard and highly reflecting surface. Although the reflectivity of rhodium is
not as great as that of silver, the absence of tarnish is a great advantage
over silver. Many items of silverware are rhodium-plated to protect them from
the otherwise inevitable dark tarnish. Other items requiring a good-looking
and durable surface—e.g., camera fittings and jewelry—are similarly coated,
either electrolytically or by vacuum deposition.
Mirrors and reflectors are often plated with rhodium, especially if the
conditions for their use may be corrosive, e.g., in scientific equipment or
lighthouse reflectors.350 Platinum has been applied on and off for nearly a
3-32
-------
century and a half to produce a silvery luster on ceramic glazes, and an inter-
esting contemporary account was given in a short article by Wynn.472 In addi-
tion to this decorative use, a platinum-gold layer fired onto the inner sur-
face of some mercury vapor lamps will reduce their infrared radiation through
multiple internal reflection.^44
Brazing Alloys
Many brazing alloys contain noble metals. One—containing 20% palladium,
5% manganese, and the remainder silver—has been used to join the thin-walled
tubing in the thrust chambers of the F-l rocket engine used, for instance, on
the Apollo spacecraft.431,432 Another, containing palladium with silver and
copper, has been used for the multistage jointing in the manufacture of heavy-
duty power tubes, such as magnetrons and klystrons, for high-temperature
operation.400'401 Another alloy for brazing tungsten consisted mainly of
platinum with a few percent of boron.
Protective Bursting Disks
For the protection of chemical process equipment, rupture diaphragms or
bursting disks (which burst if the pressure inside the equipment exceeds a safe
value) are often used. Platinum is the most useful material for the diaphragm
itself, because its great malleability permits sheets to be rolled to a uni-
form and specified thickness and because it undergoes minimal change during
319
operation, owing to the resistance of the metal to corrosive attack. A
number of examples of this application have been described.314
3-33
-------
Ganroa Radiography with Iridiuro-192
219/268
It has been shown that, for testing castings and Melded structures
by radiography, iridiun-192 offers several attractive characteristics and may
be used instead of cobalt-60.
Histologic Stain
Use as a histologic stain is peculiar to osmium tetroxide, which has been
so applied for many years. The conditions in the tissue are such as to bring
about reduction of osmium to the metal in a dark, insoluble form that is ideal
for highlighting the structure.160
Fountain-Pen Nibs/ Instrument Pivots, etc.
For a long tine, osmium alloys were produced that, because of their great
hardness, were used for special purposes, e.g., in nibs for fountain pens,
long-life phonograph needles, and instrument pivots. Their use in such applica-
tions has been all but completely superseded by changes in fashion or by intro-
duction of other materials. The use of a ruthenium alloy in the form of tiny
balls (0.02-0.04 in. in diameter) at the tips of ball-point pens has been
scibed4^1/432
3-34
-------
CHAPTER 4
PHYSICAL AND CHEMICAL PROPERTIES
PHYSICAL PROPERTIES
A selection of physical properties of the platinum-group metals is given
in Table 4-1.153/316,432 ^^ properties cited in the table are related, in-
sofar as possible, to the purest available specimens of metal. It is well
known, however, that minute traces of impurities cause appreciable changes in
such physical properties of these metals as hardness and electric resistivity.
Because the platinum metals have a marked tendency to absorb such gases as
hydrogen and oxygen, which have a significant influence on the physical L-»roper-
tir-s of the metals, it is easy to imagine the difficulty in establishing some
values. Mechanial properties of these metals, and others, depend on the amount
of cold working that has preceded their measurement. The mechanical and tensile
£jroperties of osmium and ruthenium are anisotropic, and this is attributed to
inequalities in spacing in their hexagonal close-packed structures.
For many years, osmium was described as the densest element in the
periodic table, but more recent measurements show that iridium is denser, by
about 0.2%. Osmium shows the minimal atomic volume among the transition ele-
•nents of the third long period. It was also believed for many years that
ijthenium exhibited allotropy; a transition temperature of 1035° C was thought
to be detectable betv/een two forms. It is now concluded that the allotropy
"does not exist. ^
An artificial radioisotope of iridium, iridium-192, may be mentioned
briefly because of its practical application as a source in industrial
I
4-1
-------
TKHLE 4-1
aone rnysicai fiujjeitieB ox
Property Platinum Iridium
Atonic number 78 77
Atonic weight 195.09 192.22
Stable isotopes 192(0.78) 191(38.5)
(% abundance) 194(32.8) 193(61.5)
195(33.7)
196(25.4)
198(7.23)
Density (at 20° C) , 21.45 22.65
.g/cn3
Crystal lattice
(closest packed) f^iKjr- ofrvjf!
Lattice constants
(at 20° C) , 8
a 3.9229 3.8392
c/a — —
Melting point, °C 1768 2443
Thermal conductivity,
W/on-°C 0.73 1.48
T.-inaar coefficient of
thermal expansion
x 106 (at 20-100° C) ,
per °C 9.1 6.8
•cue *• Aawnun-ii
Osmium
76
190.2
184(0.018)
186 a. 59)
187(1.64)
188(13.3)
189(16.1)
190(26.4)
192(41.0)
22.61
Hexagonal
2.7340
1.5799
3050
0.87
6.1
COUP MBULLS-
palla^iwn
46
106.4
102(0.8)
104(9.3)
105(22.6)
106(27.2)
108(26.8}
110(13.5)
12.02
Cubic
3.8906
1552
0.76
11.1
RhodJAin Ruthenium
45 44
102.9055 101.07
103(100) 96(5.7)
98(2.2)
99(12.8)
100(12.7)
101(17.0)
102(31.3)
104(18.3)
12.41 12.45
Cubic HPflCRffpni^E 1
3.8029 2.7056
1.5825
1960 2310
1.50 1.05
8.3 9.1
'^Data from Platinum Metals Review , Tugwell,432 and Goldberg and Hepler.
153
4-2
-------
TABLE 4-1 - continued
property fiatuiuu
Specific heat (at
cal/g-°C 0.03136
Heat .capacity (CL,
at 25* C) , e
cal/nole-oc 6.18
Entropy (S, at 25° C) ,
cal/nole-°C 9.95
latent heat of fusion
kcal/taole 4.7
irxoum
0.0307
6.00
8.48
6.3
(jomim j«i laniivi
0.0309 0.0584
5.90 6.21
7.8 9.06
7.6 4.2
i Knomiin
0.0589
5.98
7.53
5.15
0.0551
5.75
6.82
6.2
latent heat of
evaporation,
kcal/friole
Electric resistivity
(at 0°C),
uohm-cm
Tenperature co-
efficient of
resistance
(at 0-100° C) ,
per °C
Thermal neutron
cross section f
barns
Hardness' (annealed),
VHN
Tensile strength
(annealed),
tons/in.2
135.0
9.85
0.0039
9+1
40-42
(annealed),
tons/in.2 x 10-4
1.2
ic Buneptlbilit
at?/a x 10s
160.0
4.71
0.0043
425+15
200-240
80
2.75
0.133
162.0
8.12
0.0042
4.0
0.052
84.3
9.93
300-670 40-42
12.5
0.85
5.231
133.1
4.33
0.0038 0.0046
15.3+0.7 6.0+1.0 150+5
100-102
50
2.3
0.09903
155.0
6.80
0.0042
3.0+0.8
200-350
36
3.0
0.427
4-3
-------
TM3IE 4-1 - continued
Property Platinum Iridium Oanium Palladium Bhodiun Ruthenium.
Nbrk function ($) /
eV 5.27 5.40 4.8 4.99 4.90 >4.54
Ihennianic function
(A), anp/am?-K 64 170 120 60 100 —
4-4
-------
radiography of ferrous welds. It emits gamma rays with average energy 0.40 MeV
and has a half-life of 72 days.268
The numerical values of some properties given in Table 4-1 (e.g. melting
point, latent heat of evaporation, and hardness, or mechanical strength) re-
veal a progressive decrease in coherence or bond strength among atoms in each
triad (Ru, Rh, Pd; Os, Ir, Pt) as atomic number increases. Such trends can
be correlated with the progressive decrease in the number of electrons avail-
able for bonding in the solid state. These will occupy orbitals that have
been hybridized from s_, p_, and d states. The amount of d character that can
be contributed to these orbitals is presumed to decrease as electrons become
paired in atomic d orbitals or otherwise fail to engage in metallic bonding.
The tendency described continues beyond the group VTII metals to the correspond-
ing members of groups IB and IIB.3^
Radioactive isotopes of ruthenium are produced in the fission of
uranium-234, and a few of the characteristics of these species will be men-
tioned here. There are four known radioisotopes of ruthenium. Ruthenium-97
(half-life, 2.9 days; decay via electron capture and gamma-ray emission) and
ruthenium-105 (half-life, 4.4 h; decay via electron capture and moderate beta
emission) are formed only by neutron activation. Ruthenium-106 (half-life, 1
year; decay via low-energy beta emission) is formed only by decay of uranium-235,
but its daughter, rhodium-106 (half-life, 30 s), is a high-energy beta-emitter.
Ruthenium-106 and rhodium-106 are sometimes used as quality-control monitors to
measure the thickness of sheets of plastic and metal produced on a continuous
basis. The last isotope, ruthenium-103 (half-life, 40 days; decay via weak
beta emission), can be formed either by neutron activation or by uranium-235
4-5
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fission, where it represents 2.9% of the fission yield. The fission yield of
ruthenium-106 is only 0.38%.
Because such small quantities of ruthenium are produced during fission,
the only potential health problems that could be associated with this material
are due to its radioactivity. Current practice is to retain the ruthenium as
a sulf ide in a holding tank until its radioactivity has decayed to a safe
point.338a,465a
ALLOYING CHARACTERISTICS
The properties of alloys of the platinum metals with each other and with
other metals have been extensively studied (see, e.g., Vines and Wise,
numerous technical bulletins issued by the International Nickel Co. , and of
course Platinum Metals Review). The hardness of platinum is deliberately
raised for many of its uses by the addition of alloying elements. Of these,
nickel, osmium, ruthenium, copper, gold, silver, and iridium all produce con-
siderable increases in hardness, the effect per unit weight of added element
decreasing approximately in the order named. Rhodium and palladium produce
much less increase in hardness than do the preceding metals. Commercially
important alloys of platinum are prepared with copper, gold, iridium, rhodium,
and ruthenium. In recent years, alloys with cobalt have become important be-
cause of their strong ferromagnetic properties.
In much the same way, palladium forms alloys with advantages over the
pure metal for various practical applications. ^ In general, the alloying
elements tend to increase the resistivity, hardness, and tensile strength of
palladium. Copper, nickel, gold, iridium, rhodium, and ruthenium have been
used in this way, and an alloy with silver has been widely used in electric
contacts.
4-6
-------
Extensive or complete miscibility of one metal with another is generally
favored by similarity in crystal structure between the two metals, similarity
in atomic radii, and similarity in valence. On the basis of these three cri-
teria, extensive mutual solubility would be expected among the four platinum
metals with cubic close-packed structures (see Table 4-2). But limits of
solubility, imposed by differences in crystal structure would be expected
for alloys of these four metals with ruthenium or osmium. In fact, platinum
and palladium form solid solutions in all proportions with the elements
making up groups VTII and IB of the periodic table, except ruthenium and
osmium. The complete miscibility of iron with platinum or palladium is evi-
dently associated with stabilization of the gamma modification of iron by
small amounts of the second element. Iron alloys containing as little as
5% of palladium or platinum solidify as cubic close-packed crystals.
The formation of alloys by the platinum metals has been discussed in
terms of the numbers of electrons available for bond formation in the solid
2flR
state. ° As indicated earlier, these become fewer, with consequent lack
of cohesion, in passing from ruthenium to palladium or from osmium to platinum.
The role of the hardening elements added to palladium or platinum is to increase
•*•*•
the pool of d electrons and so augment the strength of the metallic bond.
Some of the differences in alloying between platinum and palladium suggest
that the latter has the smaller number of valence electrons and lower propor-
tion of d character in its bonding. Such conclusions are consistent with the
chemical evidence of preferred bivalence in palladium compounds and of preferred
quadrivalence in platinum compounds.
There is an unexpected, and as yet unexplained, feature of alloy forma-
tion in platinum and palladium; the occurrence of extensive miscibility gaps
4-7
-------
TABLE 4-2
Atonic Radii and Crystal Types of Group VIII and IB Elements
Property
Atonic radius, A
Crystal type0
Atonic radius, A*
Crystal type?
Atonic radius, A
Crystal type0
Group
VIUA
Iron
1.27
a BCC
Y CPP
Ruthenium
1.335
HCP
Osmium
1.35
H3>
Group
VIIIB
Cobalt
1.25
CCP
Rhodium
1.34
CCP
Iridium
1.354
CCP
Group
vine
Nickel
1.245
CCP
Palladium
1.375
CCP
Platinum
1.385
CCP
Group
IB
Copper
1.275
CCP
Silver
1.442
CCP
Gold
1.439
CCP
BCC = body-centered, cubic; CCP = cubic, closest-packed; HCP = hexagonal,
closest-packed.
4-8
-------
in sane alloy systems prepared at lower temperatures, whereas the same systems
prepared at higher temperatures exhibit complete irascibility. Thus/ palladium
forms a continuous series of solid solutions with rhodium and iridium at high
temperatures, but appreciable gaps in miscibility have been found in the same
system below 850° and 1500° C, respectively.343 The precise compositions of
the miscibility limits have not yet been established.
The alloying behavior of the other four platinum metals follows for the
most part the general principles outlined. Less is known about the character-
istics of osmium alloys, owing to difficulties in working with this hard and
high-melting-point element. Rhodium displays an unexpected reluctance to form
solutions with silver and gold, but otherwise is completely miscible with the
other cubic close-packed elements in Table 4-2, including y-iron. The alloys
containing about 50 atomic % each of iron and rhodium have an unusual magnetic
property: they are practically nonmagnetic at room temperature, but suddenly
become ferromagnetic when heated to 60° C.
OF SUBDIVISION ON CHEMICAL REACTIVITY
Platinum is relatively inert, with respect to chemical attack by oxygen
or many*acids, and a number of its uses are based on this property. However,
it should be stressed that the chemical reactivity of platinum and the other
elements in the group is markedly influenced by the state of subdivision of
the metal. Thus, the sponge ^obtained by igniting ammonium chloroplatinate is
more readily attacked than the compact metal. Similarly, platinum dissolved
in another metal, such as lead or silver, is much more readily attacked.
Platinum black or platinum finely dispersed on a porous bed, such as silica
4-9
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gel, is still more reactive and displays remarkable catalytic properties.
Similar conments apply to the other members of the group.
The "nobility" of platinum and its congeners arises from their bonding
in the solid state. Thus, the first-stage ionization potential of platinum
is 9.0 V, only a trifle higher than that of other transition elements. The
standard electrode potential QE?) — for 2e + Pt2+ tag) = Pt (s) — has been
estimated to be +1.2 V. An analysis of the energy quantities leading to such
a value of E°, in the manner of the Born-Haber cycle, discloses that the signifi
cant term is the very high sublimation energy of the crystal. The increase in
reactivity exhibited by samples with a high specific surface area, compared
with that of bulk metal, can be attributed to an increase in the number of
atoms with the higher energy associated with surface sites.
CHEMICAL REACTIONS WITH OXYGEN, HALOCTMS, AND ACIDS
The direct oxidation of the platinum-group metals is summarized in
Table 4-3. The formation of the volatile osmium tetroxide by finely divided
osmium occurs at room temperature and can be detected by its distinctive odor.
Although ruthenium also forms a volatile tetroxide, it differs from osmium, in
that its tetroxide does not form directly from the elements at moderate
temperatures.
Four of the metals other than osmium sustain a detectable weight loss
when heated in oxygen at high temperatures. Thus, platinum loses some weight
in oxygen at 1000° C. No such loss is observed when it is heated to this
temperature in a vacuum or in an inert gas. This observation is attributed
to the removal, through either volatilization or decomposition, of a film of
platinum dioxide, which probably coats the metal even at room temperature. ®
4-10
-------
TABLE 4-3
Reaction of Platinun-Group Metals with Pure or Atmospheric Oxygen
Extent of Oxide
Metal
Oxide Panned
Temperature, °C Taqperature, °C
Platinum Negligible
Palladium Superficial
Platinum (XVI oocideiPtD,
Platimm dioxide J *
Palladivm (II]
Palladivn
PdO
< 1000
> 350
> 870
Rhodiun Superficial
Iridiuc Siqperficii
Ruthenium Superficial
Rhodium (HI) oxide
Bhodium sesguioxide J
Iridium (IV) CBcide-iIrO,
Iridium Hjmi^a /
Osmium (VIII) oocLde|OB04
Oanium tetroodde J
Ruthenium (IV)
Rutheniun dioxide S
- 700
- 700
200
700
1100
1140
4-41
-------
Similar losses in weight occur with rhodium and iridium when they are heated
in air or oxygen to temperatures above 1100° C, the effect being greater in
the case of iridium. The weight loss is explained by the formation of volatile
rhodium dioxide, KhO2, or iridium trioxide, IrOg. Ruthenium, when heated in
air at 1000° C, sustains the greatest weight loss, believed to be due to forma-
tion of volatile ruthenium trioxide, RuQ3, with a vapor pressure of 10~^ atm
(101 N/m2) just above 1100° C.
Palladium has the capacity to absorb hydrogen—as much as 900 times its
own volume under standard conditions—over a range of temperatures.246 The
uptake of hydrogen corresponds roughly to the composition Pd^, but modern
studies appear to have largely ruled out the formation of such a discrete
substance. Instead, it is inferred that below 300° C there are two phases,
each consisting of a solid solution, whereas above this critical temperature
there is only a single solution phase. In each phase, hydrogen atoms are
held interstitially in such a way as to involve actual chemical bonding, as
deduced from changes in electric conductance and magnetic susceptibility.
t
To a smaller degree, platinum and rhodium exhibit a similar absorption of
gaseous hydrogen.
The principal products of direct reaction of the halogens with the heated
platinum-group metals are shown in Table 4-4. The temperatures shown are those
reconroended for good yield, but are not critical to the formation of some produd
Even at room temperature, palladium is corroded by moist chlorine or bromine, ari
a palladium anode is appreciably dissolved during electrolysis of chloride solu-
tions. Likewise, saturated chlorine or bromine water or an alcoholic solution
of iodine will corrode metallic ruthenium.
4-12
-------
TRBL£ 4-4
Reaction of PlatimnKSroup Metals with Fluorine and Chlorine
Reaction with Fluorine
Metal
Reaction with Chlorine
Products
Platinum Platinum tetrafluoride, PtF4
paii«<^<«n Palladium trifluoride, PtF*
j
Rhodium Rhodium trifluoride, RhF,
ifluoride, InFe
Ruthenium Ruthenium pentafluoride, RuFc
Oaniun Osmiun hexafluoride, OsFe
D
500
500
500
400
360
300
300
300
Platinum dichloride, Ptd
Platinum trichlCTri'V^
Platinum
PtCl2
Rhodium trichloride, RhCl3
Iridiun trichloride, Ird3
Ruthenium trichloride, RuCl3
Osmium
Osmium
500
300
600
600
600
400
<500
>650
4-13
-------
Table 4-5 summarizes the action of acids on the platinum-group metals. Re-
sistance to attack by common acids is shown in increasing order in this table.
The action of aqua regia on palladium and platinum yields chloropalladic acid,
H2PdClg/ and chloroplatinic acid, H2PtCl6; however, on evaporation of a solu-
tion of the former, the dichloride is the compound recovered. Rhodium can be
rendered susceptible to attack by aqua regia if it is dispersed in an alloy
that is dissolved by this acid.
In contrast with the acid reactions just given, the platinum-group metals
show a variety of responses to alkaline fusions, especially in the presence of
oxidizing agents. Thus, sodium peroxide or a mixture of sodium hydroxide and
sodium chlorate will bring oanium or ruthenium into soluble forms, usually sodiu
osmate, Na2OsO4, or sodium ruthenate, Na2RuQ4. These reactions are best carried
out with finely divided metal. These metals are also attacked appreciably even
by alkaline hypochlorite solutions. Rhodium is also attacked, but to a smaller
degree, by such alkaline fusions, as well as by fused alkali cyanides. Platinut
and palladium are appreciably corroded under conditions of alkaline fusion or
by fused alkali cyanide. The effect of such treatment on iridium is the least
among the group of metals, but it is not negligible.
The behavior of platinum toward a number of elements at high temperature is
or should be familiar to users of platinum laboratory ware. Carbon, phosphorus,
silicon, arsenic, etc., combine or alloy with hot platinum, so care must be takes
in heating compounds of these elements to avoid reducing conditions. Contact
with sooty burner flames or unburned gas may lead to embrittlement, owing to
formation of a carbide.
4-14
-------
TABLE 4-5
Attack of Platinum-Group Metals by Mineral Acids
Metal
Farm
Nature of Attack
Palladium
Platinum
Rhodium
Iridium
Compact
Sponge
Compact
or sponge
Compact
Dispersed
Compact
Sponge
Attacked by hot concentrated nitric acid and boiling
sulfuric acid; dissolved by aqua regia
Dissolved by all the above acids
Not attacked by single mineral acids; dissolved by
aqua regia
Attacked by boiling sulfuric acid or hydrohromic acid;
not dissolved by aqua regia
Dissolved at least partially by aqua regia
Practically unattacked by hot mineral acids or aqua
regia
Dissolved in Carius tube by hot hydrochloric acid plus
an oxidizing agent (nitric acid or sodium chlorate)459
Ruthenium Any
Osmium Any
Virtually unattacked by hot mineral acids or aqua regia
Virtually unattacked by hot mineral acids or aqua regia
4-15
-------
Rutheniun and iridium have been shown to resist chemical attack by a
number of molten metals when heated in argon atmospheres. Thus, crucibles
of these metals, especially iridium, can be used to contain a number of
normally very reactive elements at high temperatures.
SELECTED COfOUSDB
Binary Ccropounds
Platinum. The principal oxidation states of platinum are +2 and 44;
of these, the first is the more cannon. Most platinum compounds are coordina-
tion crfijpigxps? there is no <%if|v1 1 i "g evidence of the existence of simple
ypyamyg mptai long. The r*y"^"* rattlon chemistry of platinum is outlined tat-iar
in this chapter. Some of the simpler binary compounds will be described here,
with brief mention of scmp of the related complex ions, e.g. , those with the
halogens. In these onmplex ions, bivalent platinum assumes a
number of 4 (square planar) , and quadrivalent platinum, six (octahedral) .
Platinic chloride, Ptd^, is a red-brown crystalline solid that can be
formed by nigh-temperature chlorination of platinum, but is more conveniently
prepared by decomposing chloropl atinic acid with heat in a stream of hydrogen
chloride (165° Q or chlorine (369° C) . It is readily soluble in water,
alcohol, and acetone.
Chloroplatinic acid, HjPtClg, is formed when platinum is dissolved in
aqua regia. It is a dark-red crystalline solid whose aqueous solution is
yellow or orange, according to ccmasntration; in solution, it acts as a strong
f*?AAr ihe salts potassium hopc^m rmpp] $&_ ir»atg / B^PtClg, and fymc^viifT1 hexa—
chloroplatinate, (JSK^tCL, are sparingly soluble, and the former has been
4-16
-------
used in the gravimetric analysis of platinum. Many organic amines also form
insoluble chlorcplatinates that may be rea=ri for their t*e*rs**^&r\ ^H- ion .
Platinous chloride, PtCl^/ is a bncMiish^green solid usually
by heating platinic chloride in an atmosphere of chlorine (580° C) . It is
insoluble in water, but dissolves in hydrochloric acid to form a dark brown
solution of chloroplatinous acid, H^PtC^. The latter may also be prepared
by reduction of chloroplatinous add with
There are two fluorides of seme importance. Platinum nexafluoride,
PtF6, is a dark-red solid (m.p., 61.3P C) with only a narrow range of
existence in the liquid state (b.p. , 69.1° C); its vapor is brown. It can
be prepared by electrically heating platinum wire in fluorine close to a
surface cooled by liquid nitrogen. Being thermally unstable, it breaks down
by way of an unstable pp"*~afl wiride PtFc, to PtFj. The hpaeafinnricle is an
extremely powerful oxidizing agent; its place in chemical history is ensured
as the substance that first oxidized an inert gas (Xe+PtPg -> XePtFg) . The tetra-
fluoride, PtF^, may be utt^auBd by treating platinous chloride with fluorine at
200° C; it is a yellow-brown solid, slowly hydrolyzed by water.
Platinic oxide, PtO^, may be prepared by treating chloroplatinic acid
with sodium carbonate; the resulting residue is extracted with acetic acid,
and the insoluble remainder, consisting of yellow platinic acid, EUPt(OH)g
[or Pt(GH)4-2H^O], is heated below 100° C to yield the black dinyirtp. When
this is heated more strongly, the metal is obtained. The yellow "hydroxide"
is known as platinic acid; it is amphoteric, dissolving in either hydrochloric
or alkali r
4-17
-------
Platinous oxide, PtQ, may be obtained by carefully heating the black
hydroxide formed by the addition of an alkali to a chloroplatinite. This
gray oxide is subject to disproportion if heated too strongly, and platinum
and platinic oxide are formed. Treatanent of the hydroxide with hydrochloric
acid, again leads to disproportionation, with platinum and chloroplatinic acid
being formed.
Platinic sulfide, PtS2, is obtained as a black precipitate when hot
acidified chloroplatinate solutions are treated with hydrogen sulfide. This
compound is soluble in alkaline polysulfide solutions, and it is another form
in which platinum may be precipitated for gravimetric analysis.
Palladium. In its compounds, palladium most commonly exhibits an oxi-
dation state of +2, although it may less commonly be quadripositive (+4),
and in a few instances terpositive (+3). A few univalent complexes of palladia
have also been reported. 177,296 ijke t^e other platinum metals, it has a strong
disposition to form coordination complexes. Those of bivalent palladium show
a coordination number of 4 for this element and a square planar structure.
Palladous oxide, PdO, appears as a black powder when palladium sponge
is heated in oxygen. It may also be prepared (for instance, in making pal-
ladium catalyst) by fusing palladous chloride, PdCl2/ with potassium nitrate
at 600° C and then leaching out the water-soluble residue. The oxide is in-
soluble in water and boiling acids (including aqua regia). It can easily
be reduced by heating in hydrogen, and the metal so produced is an active
hydrogenation catalyst. When alkali is added to aqueous palladous salts,
a yellow hydrous oxide, Pd(OH)2/ is produced. This loses water and turns
4-18
-------
black when heated to 500 or 600° C. The dissociation pressure of palladous
oxide reaches 1 atra (101 kN/fa2) at 875° C.
Palladous chloride may be prepared by direct union of the elements at
500° C; above 600° C, it dissociates to the elements. It is a red deli-
quescent solid; from its aqueous solution or from solutions of palladium
dissolved in aqua regia, crystals of palladous chloride dihydrate,
PdCl^^HnO may be obtained. Evidence of the existence of the corresponding
acid --chlor opal lad cms acid, ^PdCl^-- Is open to question, but salts derived
from it, such as K^PdCl^, are obtained by adding the stoichiometric amount
of the appropriate metal chloride to aqueous palladous chlorice and evaporat-
ing to dryness. The corresponding bromide and iodide are dark solids,
insoluble In water, but dissolved by an excess of the halide ion as complex
«o
ions,
Fluorine reacts with metallic palladium or with palladous chloride to
form palladium trifluoride, PdF3, a black solid. This is an active oxidizing
agent. On reduction, it yields palladous fluoride, PdF., generally contaminated
with palladium. Pure PdF2 may be produced by treating palladium trifluoride
with selenium tetrafluoride; it is a violet crystalline solid, completely
hydrolyzed by water.
Palladium forms a number of compounds with sulfur (and also with selenium
and tellurium) . For example, the following sulfides have been deduced from
the phase diagram and, in part, characterized by x-ray measurements:
Pd4S, Pd-^Sg, Pd-uS5, PdS (palladous sulf ide) , and PdS2 (palladium disulfide) .
These are dark and comparatively inert chemically, and they show some semi-
metallic characteristics.
4-19
-------
Palladous nitrate, Pd(N03)2, may be formed by dissolving finely divided
palladium in warm nitric acid. The salt may be obtained as crystals from
this solution, but it may be contaminated with basic salts and is very
hygroscopic. The solution readily hydrolyzes, especially if heated.
Rhodium. Rhodium shows a decided preference for the oxidation state
+3 in its compounds. However, the oxidation state +4 is found in rhodium
tetrafluoride, RhF^, in a poorly characterized hydrous dioxide, Rh(OH)^,
and in a few fluoro- and chloro- complexes, such as cesium hexachlororhodate
(IV), C^RhClg. A sole instance of the oxidation state +6 is the hexafluoride,
RhFg. Oxidation states lower than +3 occur among the carbonyls and carbonyl
halides and in a number of recently synthesized complexes. Rhodium is said
to be the only element in the second or third transition series that possesses
a definite, well-characterized aquo ion; this is the yellow rhodium hexaquo
ion, Rh(HLO)g+3 found in aqueous sulfate or perchlorate solutions.
The oxide, Rfc^O-j, results from heating the finely divided metal to
red heat in air; it can also be prepared by igniting rhodium (III) nitrate,
Rh(NC>3)3. It is a gray crystalline solid with the same crystalline structure
as corundum, and it does not dissolve in acids. When alkali is carefully added
to rhodium (III) solutions, a yellow precipitate of hydrous rhodium (III)
oxide, said to be Rh^^-^O, is formed. This will dissolve in acids or in
excess alkali, and on ignition it forms the anhydrous oxide. If too much
i is used, a black precipitate is produced that does not dissolve in
acids; this is believed to be
4-20
-------
Rhodium trifluoride RhF3, is produced by the action of fluorine on
metallic rhodium or rhodium trichloride, KhCLj, at 500-600° C. It is a red
solid that is unreactive toward water, aqueous acids, and aqueous alkalis.
Hiis synthesis of rhodium trifluoride also results in the simultaneous produc-
tion of a small amount of rhodium tetrafluoride, RhF4, a blue solid.
Rhodium (III) chloride can be prepared in various ways, and its proper-
ties depend on the method of preparation. With direct union of the elements
at 250° C, the product RhCl^ is a red powder insoluble in water and acids.
If the yellow hydrous oxide is treated with hydrochloric acid and the solution
carefully evaporated, dark-red crystals of rhodium chloride tetrahydrate,
RhClg'-tt^O, are produced; these are water-soluble. If these are dehydrated
in a stream of gaseous hydrogen chloride, a water-soluble anhydrous salt is
produced.
Several rhodium (III) salts containing the aquo ion, RhG^OJg"^, have
been prepared by dissolving the yellow hydrous oxide in the appropriate acid.
The salts that have been characterized are formed by oxyacids. Rhodium
perchlorate, RhfClOj^'&^O, is an example of such a compound, and its struc-
ture is known from x-ray diffraction. Rhodium alums, such as KRh(S04)2'12H2Of
have been known for a long time. Rhodium sulfate occurs in two forms: one
is a yellow crystalline solid, RhL^SO^^-l^O; the other is red,
Kh^SO^-'G^O. The ye^-CM sulfate is a normal ionic salt from whose
solutions barium sulfate can be precipitated. The red salt is obtained
by evaporating the yellow solution to dryness at 100° C; from its solution,
no barium sulfate can be precipitated. Evidently, in the latter compound,
three sulfate ions are coordinated to the metal in a nonlabile complex.
4-ZL
-------
Iridium. Iridium exhibits a greater variety of oxidation states in its
compounds than rhodium, its close congener in the periodic table. Among its
simpler compounds, the preferred oxidation states are +3 and +4, with the former
being more cannon. Some examples of compounds in which iridium displays other
oxidation states are mentioned below, and a wide range of formal oxidation
states have been encountered in its coordination complexes. In contrast with
rhodium, iridium shows no evidence of an aqueous cation, so most of its solu-
tion chemistry involves complex ions.
Iridium dioxide, IrC^, is the most cannon oxide and has the rutile struc-
ture. It is formed by direct union of the elements at about 1000° C, but it
decomposes at about 1120° C. When quadripositive iridium salts (e.g., IrClg"2)
are treated with alkali, an intensely blue hydrous oxide is precipitated.
When dried under nitrogen at 350° C, this yields the dioxide in a reasonably
pure state. Addition of alkali to a terpositive iridium salt (e.g., IrClg-3)
in an oxygen-free atmosphere yields a green or blue-black hydrous sesquioxide.
This is a gelatinous material, soluble in excess alkali and apt to absorb
atmospheric oxygen with oxidation to the dioxide. If this preparation is de-
hydrated, even with oxygen excluded, it fails to yield a pure Ir203. One tri-
oxide, IrO , has been prepared by fusion of the metal with alkaline oxidants,
such as sodium peroxide; however, this compound remains poorly characterized.
Iridium hexafluoride, IrF6, is a yellow solid (m.p., 44° C) that fumes
strongly in air and reacts vigorously with water. The pentafluoride is also
a very reactive yellow solid (m.p., 106° C). A third fluoride, IrF^, is a
black solid that is difficult to prepare.
4-22
-------
Iridium trichloride, IrCl3, is formed by direct union of the elements at
450-600° C, the reaction apparently being accelerated by sunlight. It is
olive-green, brown, or black, according to particle size. It is not soluble
in water. A hydrated iridium chloride, dark-green and water-soluble, is formed
by reaction of hydrochloric acid on the dioxide. A tetrachloride of somewhat
doubtful quality has been formed by the reaction of chlorine or aqua regia with
(NH4)2IrClg and by some other syntheses.
Iridium reacts with sulfur, selenium, and tellurium; the compounds formed
have been identified as intermediate phases in the two-component systems with
iridium. These include Ir2S3, IrS2, Ir3S8, IrS3(?), Ir2Se3, lrSe2, IrSe3,
IrTe2, and IrTe^. These are all dark solids and are quite resistant to
acids.
Ruthenium. Ruthenium is known to occur in compounds in at least eight
oxidation states, but the most common are +2, +3, and +4. In general, the
chemistry of ruthenium resembles that of osmium much more than that of iron.
The tetroxide, KuD4, has already been mentioned; it is a yellow molecu-
lar solid (m.p., 25° C; b.p., 100° C) and is highly toxic. It is produced
when acidic solutions of ruthenium compounds are heated with strong oxidizing
agents. In contrast with osmium tetroxide, it is not formed by direct union
of the elements, nor is it produced by the action of nitric acid on ruthenium
compounds. When ruthenium tetroxide is dissolved in alkali, it is immediately
reduced first to a green perruthenate, BuO/", and then to an orange ruthenate,
RuQ4-2.
The dioxide, RuO_, is formed by heating ruthenium in air at 500-700° C.
It is a black crystalline solid with the rutile structure. It is not dissolved
4-23
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by acids, but is reduced to the metal when heated in hydrogen. Addition of
alkali to ruthenium (III) solutions results in precipitation of dark hydrous
oxides; similar precipitates occur when alkaline solutions of rutheniun
tetroxide are treated with ethanol and boiled. In neither case are the sub-
stances formed well characterized.
Ruthenium disulfide, RuS2, is a gray-blue crystalline solid known in
mineral form as laurite; it is structurally analogous to pyrite, Fe^* ^
can be prepared by direct union of the elements at high temperature. The
compound is chemically unreactive.
Treatment of the element with fluorine produces ruthenium pentafluoride,
RuF5, a dark-green solid (m.p., 85.6° C; b.p. , 227° C) with a colorless vapor.
It is very reactive, is hydrolyzed by water, and is reduced when heated with
iodine. Treatment with an excess of iodine yields ruthenium trifluoride,
, a brown solid; but treatment with iodine and IF,- results in formation
of yellow crystals of the tetrafluoride,
Chlorination of the element yields ruthenium trichloride, RuCl^, a
black solid that is insoluble in water and of which there are two crystalline
modifications. A hydrated form, prepared by evaporating a hydrochloric acid
solution of ruthenium tetroxide in an atmosphere of hydrogen, is formulated
as RuClyEjjO. This is soluble in water, but the fresh solution contains no
chloride ion and should be regarded as a complex. This aqueous solution
undergoes slow hydrolysis with precipitation of a hydrous oxide.
Ruthenium tetrachloride, RuCl^ and a hydroxychloride, RuOBCl,, are
formed when hydrochloric acid solutions of ruthenium tetroxide are evaporated.
There is reason to believe that these compounds are structurally more complex
than the formulas suggest. "'
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Perruthenates and ruthenates have been mentioned already in connection
with the alkaline solutions of ruthenium tetroxide. Ruthenates and per-
ruthenates may also be produced by direct fusion of the metal with a mixture
of an alkali-metal nitrate and hydroxide. In the solid state, these substances
are black; their aqueous solutions—green and orange, respectively—are not
particularly stable.
Osmium. In compounds and complexes of osmium that have been described,
it exhibits each of the nine oxidation states from 0 to +8. The more common
values among its siirpler compounds are +3, +4, +6, and +8. Like most of the
platinum metals, osmium does not form a simple aqueous cationic species.
Osmium tetroxide, OsO4, is a colorless molecular solid (m.p., 40° C;
b.p., 101° C) with a characteristic pungent odor suggestive of ozone. It
is a highly toxic substance, and exposure of the eyes and the respiratory
tract to it must be avoided. It is formed directly by combination of the
elements, e.g., when the metal is heated in air above 200° C. Also, and in
contrast with ruthenium, the tetroxide is formed when any of many osmium com-
pounds are heated with nitric acid. When osmium tetroxide is dissolved in
water, it remains in the molecular form, but it is converted by alkali to the
oatate ion, HOsOj" or OsO^OH^"^/ which is yellow. It was undoubtedly this
ion that early workers mistook for chromate before the identification of
osmium.
A dioxide, OsO2, can be prepared either by heating the metal in a stream
of nitric oxide at 650° C or by heating osmium in a stream of nitrogen and
osmium tetroxide vapor at 600° C. It is a dark solid, possibly dimorphic,
with one form having the crystal structure of rutile. This oxide dissolves
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in hydrochloric acid to form chloroosraic acid (IV), KLOsClg. Addition of
alkali to the latter solution regenerates osmium dioxide in a hydrous form.
Osmium hexafluoride, OsFg, is a yellow-green solid (m-p. r 32.1° C;
b.p., 46° C) prepared by direct union of the elements at 250° C. In the
chemical literature before 1958, this ccmpound was erroneously described as
OsFg. It is readily reduced by iodine and can be hydrolyzed by water. A
pentafluoride is formed from the hexafluoride by ultraviolet irradiation
or by reduction by iodine dissolved in IF5. It is a green solid On.p. / 70° C),
melting to a blue liquid (b.p., 226° C) and yielding a colorless vapor. There
is also a yellow tetrafluoride (m.p., 230° C) that can be prepared from the
hexafluoride by reduction.
There is a good deal of contradictory information about the chlorides
of osmium. ^1 & red crystalline tetrachloride can be prepared by direct
chlorination of the metal under a pressure of 7 atm (709 kN/to2) • TSiis dis-
sociates when heated at 470° C in a stream of chlorine to form a dark-gray
solid trichloride. The tetrachloride is soluble in water or alcohol/ al-
though the solutions are not particularly stable, whereas the trichloride is
insoluble and not very reactive.
There are a disulfide, a diselenide, and a ditelluride; each can be
formed by direct combination of the elements above 600° C. All are dark
solids with the crystal structure of pyrite and show very little chemical
reactivity.
Osmium and ruthenium form various oxy-congtounds that have no counter-
parts among the other platinum metals, but these may be compared with the
ferrates. The osmate (VIII) ion formed when osmium tetroxide dissolves in
alkali has already been mentioned. Noteworthy is the observation that, when
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the tetroxide is reacted with alkali, osmium, In contrast with ruthenium,
suffers no iirmediate change in oxidation state. However, if a mild reducing
agent (e.g., alcohol) is added to the alkaline solution, the element is con-
verted to the osmate (VI) ion, OsO2 (.OH) 4~2, evident from the color (pink).
Both osmate structures are octahedral about the metal atom. If aqueous osmate
(VIII) is treated with concentrated aqueous ammonia, an unusual compound re-
sults, called an osmiamate, OaQ^T, in which a nitrogen atom is bound to
the metal by what appears to be a multiple bond. Structurally, this ion
appears to be a distorted tetrahedron; similar species are found for rhenium
and molybdenum.
Thermodynamic Data on Binary Alloys. Considerable information regard-
ing the platinum-group metals and their simpler compounds is available in
compilations ifif thermodynamic data, 147,153,207 which should be consulted for
further details. Information regarding binary intermetallic compounds formed
as components in alloy systems can be found in such sources as the Metals
Handbook.180
Coordination Compounds of Platinum and Palladium
The coordination chemistry of platinum and palladium has attracted con-
siderable attention in recent years, largely because the metals have been the
source of a large number of compounds of great intrinsic interest. Use of
the metals and their compounds as homogeneous and heterogeneous catalysts has
been the primary reason for the rapid development of the organometallic
chemistry of these metals. Research in this subject is extensive and has
resulted in a vast quantity of information on the reaction of the metals and
their compounds with organic molecules. More recently, the discovery that
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cds--didilorodiamomineplatinuni CII) exhibits anticancer activity (see Chapter 3)-
has stimulated tremendous interest in the effects of platinum compounds on
biologic systems.^65
Throughout the history and development of modern coordination dhanistry,
these elements have been of particular interest. The square-planar geometry
of the bivalent oxidation states made possible the study of cis and trans
isomers in such complexes. One of the most studied properties of these com-
plexes is the labilizing effect that some ligands have at the cis_345 and
trans32 positions. Research has resulted in a better understanding of the
mechanisms of substitution reactions involving metal complexes. The trans
effect has also enabled the systematic synthesis of the geometric isomers of
a given complex.148
Oxidation States and Stereochemistry. For the different oxidation
states of platinum and palladium, the partially filled shells are d shells—
5d and 4d, respectively. These d orbitals project well out to the surface
of the atoms and ions, so the electrons occupying them are strongly in-
fluenced by the surroundings of the ion and, in turn, are able to influence
their environment significantly. Thus, many of the properties of a particu-
lar oxidation state are quite sensitive to the number and arrangement of the
d electrons present. For this reason, the coordination number and stereo-
chemistry for the individual oxidation states are different.
The most common oxidation state of both metals is +2.^-" Almost all
the complexes of this oxidation state have a coordination number of 4 and
a square-planar geometry. There are also many compounds with the elements
in the zero and +4 oxidation states. However, these oxidation states are
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much more ccnroon for platinum than for palladium. In the +4 oxidation state,
both metal ions are in an octahedral ligand environment. Zerovalent platinum
and palladium complexes have coordination numbers between 4 and 2, with 4
being the most common. The 4-coordinated, zerovalent complexes are tetra-
hedral, inasmuch as in these complexes the electronic configuration of the
metals is d10, rather than d8s2. Compounds in which platinum and palladium
have oxidation states other than zero, +2, and +4 are rare. There are a few
compounds of platinum with oxidation states of +1, +5, +6, and possibly +3;
the only other oxidation state of palladium is apparently +1.
Complexes of the Zerovalent Metals. Zerovalent platinum and palladium
form complexes with phosphine,256 arsine,256 phosphite,2^ isocyanide,257
cyanide, acetylide,2^ ammonia,449 and nitric oxide^2 ligands. Carbon
monoxide forms only a polymeric complex with a platinum (0) in the absence
of other ligands. However, it forms a variety of mononeric and polymeric
complexes with phosphine complexes of platinum (0). The strong a-donor
ability of phosphine increases the electron density on the metal, making it
more susceptible to metal-to-carbon monoxide ir-back-donation.257,449
Most of the preparative routes involve reduction of complexes of the
bivalent metals in the presence of the ligand to be complexed.66'256
However, in a few instances, complexes have been prepared by heating the
metal in the presence of the ligand. For example, [Pd(PPh3)2] can be
formed by heating metallic palladium with triphenylphosphine in the presence
of excess triethylsilane.172
The zerovalent complexes of platinum and palladium that have been most
widely investigated are those containing tertiary phosphine ligands. These
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complexes are air-stable in most instances and are soluble in a number of
common organic solvents. For this reason, they are used as the starting
material for the synthesis of many of the other zerovalent complexes. Some
of the properties of these complexes are given in Table 4-6.
Complexes of the Bivalent Metals. Bivalent platinum and palladium form
complexes with ligands containing donor atoms from almost every group in the
periodic table. In the following discussion, a brief description of some of
the more important complexes of several groups will be given.
• Hydride Complexes of the Bivalent Metals: The first hydride com-
plexes of platinum and palladium were reported in 1957.87 There has since
been a rapid expansion of the field, resulting in a number of reviews on the
topic.84'111 A wide range of methods have been devised for their preparation.
A typical synthesis^1 is the reaction of cis-[ (PRg) ^PtCl^ 3 with hydrazine in
dilute aqueous or alcoholic solution to produce trans-[ (PR3)2PtHCl]. All
the known hydride complexes are square-planar, and most have a trans con-
figuration. The platinum complexes are considerably more stable than the
palladium compounds. For example, trans-[ (PEt3)2PtHCl] can be distilled
under high vacuum at 130° C and 0.01 torr (1.33 N/rtn2) ,91 whereas the pal-
ladium compound is isolated as a solid and is always contaminated with de-
composition products.88 The properties of both the metal and of the ligand
that produce a high ligand-field stabilization energy in the complex also
produce a hydride complex that is more stable with respect to air and water.
One of the most interesting and important reactions of the hydrides involves
86
insertion of an unsaturated organic compound into the metal-hydride bond.
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TABLE 4-6
Properties of Zerovaient Platinum and Palladium Complexes
Ocnplex
[Pt(EPh3)4J
[Pt
[(PPh3)2Pt(C2H4)]
Color Malting Point, °C
YeUow 118 (decorap.)
Yellow 125-130 (deoonp.)
Yellow 130-132
White 122-125
Yellow 100-105 (deconp.)
Remarks
Stable in air for
several hours
Stable in air for
several hours
Stable in air
Stable in air
Stable in air for
only short period
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Reactions of this type are involved in the homogeneous hydrogenation and hydro-
silation of olefins, as well as the catalytic isomerization of olefins by both
platinum (II) and palladium (II) complexes.
• Complexes with Group IVA Elements; There are many complexes in which
ligands containing silicon, germanium, tin, and lead bond directly to either
platinum (II) or palladium (II). Carbon is also in this group, but complexes
involving the metal-to-carbon bonds usually have different properties. There
are several reviews on complexes containing bonds between metals and group
IVA elements. ^/89 A number of methods have been used to prepare these com-
plexes.42'89 Treatment of cis-[ (PEt?) qPtCLg] with ^X)^ (X = silicon,
germanium, and palladium; R = organic group) has been used to prepare
trans-[ (PEt3)2PtCl(XR3) ] in benzene. The complexes of platinum and palladium
that have been prepared in the pure state are all crystalline. It appears that
platinum complexes are more stable than the corresponding palladium complexes;
the order of stability of the group IVA ligands is CL^X > PlvjX > Me3X. The
stability order of the platinum complexes is approximately SN " Ge > Si " Pb;
for the palladium complexes, it is Ge > Pb » Sn " Si. The presence of tertiary
phosphine or arsine ligands appears to be virtually essential for the formation
of stable complexes in which the group IVA elements, including carbon, are
bound to platinum or palladium.
• Complexes with Group VA Elements; Some of the most interesting com-
plexes of platinum (II) and palladium (II) are those involving ligands that
contain the group VA donor atoms nitrogen, phosphorus, arsenic, and antimony.
Nitrogen in almost any environment binds strongly to both metals, and this
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has led to the preparation of complexes with a wide range of nitrogen ligands.
Tertiary phosphine ligands have been very important in the development of the
organometallic chemistry of platinum and palladium through their ability to
form very stable complexes with both metals. All four group VA elements
form strong a-bonds to the metals. Phosphorus, arsenic, and antimony are
also capable of accepting back-donations of electron density from the metal
through Tr-bonds. This accounts for the considerable difference in properties
between the nitrogen-containing ligands and with ligands containing the heavier
group VA element.
Probably the most important nitrogen-containing complexes of platinum
(II) and palladium (II) are the bis-amnine complexes, [Ptl^X^] (L = ammonia or
amine; X = halogen). These complexes exist as cis and trans isomers and have
been very instrumental in the development of modern coordination chemistry.
Studies on these complexes also led to the discovery of the trans effect in
platinum (II) complexes in the 1920's.^ Mare pertinent to this discussion are
the biologic effects of cis-dicdilorodiarimineplatinum (II) recently observed.
This complex has been found to be a potent anticancer agent and exhibits a
rather high toxicity.
The tertiary phosphine, arsine, and stibine complexes of platinum (II)
and palladium (II) are stable in air, are soluble in organic solvents, are
readily recrystallized, and have well-defined melting points. The stability
of the complexes decreases in the order PRj > AsR3 > SbR . The monomeric
platinum (II) complexes generally exist as both cis and trans isomers. The
phosphine and arsine ligands are always trans. The corresponding stibine
complexes contain the cis isomers at up to 40% in equilibrium in solution.
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• Bivalent Complexes with Group VIA Elements; Platinum CH) and
palladium (II) form stranger complexes with ligands containing sulfur than
they do with oxygen-containing ligands. The stabilities of complexes of
sulfur, selenium, and tellurium are very similar. The actual sequence de-
pends on the nature of the other ligands bound to the metal. The relative
stability of complexes involving oxygen-containing ligands is H20 > ROH >
the opposite is true for sulfur-containing ligands: H2S < RSH < R2S- Complexes
of bivalent platinum and palladium containing H2X (X = group VTA element) are
known only when X is oxygen. The compound [Pd^O^j^fClO^^ has been pre-
pared, but the aquo complexes are rarely isolated, because water is a very
poor ligand for these metal ions. Hydroxy complexes are known, but are also
rarely isolated. The corresponding HX~ complexes involving the other elements
of group IVA cannot be prepared. However, thioethers, selenoethers, and
telluroethers all form complexes with both metals. The telluroether complexes
of platinum (II) are much less stable than their sulfur and selenium analogues.
Other types of ligands that contain group VTA elements and form complexes
with platinum (II) and palladium (II) are sulfate ion,126 carbonate ion,294
nitrate ion,248 urea,307 thiourea,44 and many organic polydentate ligands.177
Complexes of the Quadrivalent Metals. Platinum readily forms platinum
(IV) compounds and complexes, whereas palladium is reluctant to form palladium
(IV) compounds. The palladium complexes are more stable than simple palladium
(IV) compounds, but only a few are known; apart from the complexes formed on
dissolution of palladium in concentrated nitric acid, they are mainly the octa-
hedral halide anion complexes.
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None of the group IVA elements form complexes with palladium (IV) .
Platinum CIV) forms cyanide, cyclopentadienyl, and many organonetallic
complexes. Stable platinum (IV) complexes with other group IVA ligands have
been isolated by the addition of hydrogen halides to platinum (II) complexes
that have ligands containing these elements.
Platinum (IV) forms a large number and a wide variety of complexes with
nitrogen-, phosphorus-, and arsenic-containing ligands. There are very few
palladium (IV) complexes with ligands involving these donor atoms. The most
extensive and typical series of platinum (IV) complexes are those which span
the entire range from the hexamnines, [Pt(amine)g]X4, to the hexahalide com-
plex, [PtXg]2".429 Other complexes of the quadrivalent metals with group VA
elements are tertiary phosphine,^ tertiary arsine, aUqrlcyanide and aryl-
cyanide,155 g^ azide^S ligands.
There are very few complexes of palladium (IV) that contain ligands with
group VIA donor atoms. Platinum (IV) , however, forms a much wider range of
complexes, with such ligands as dialkylsulfides, dialkylselenides, nitrate,
carbonate, and sulfite.
Organometallic Complexes. Several reviews have been written on the
organometallic chemistry of platinum and palladium. 293,304 Both platinum
and palladium form complexes containing metal-carbon o-bonds. Complexes of
this type with both platinum (II) and platinum (IV) are known, whereas palladium
forms such complexes only in the +2 oxidation state. Complexes of these metals
involving metal-carbon o-bonds contain other ligands, such as phosphines,
arsines,71 bidentate thioethers,71 selenoethers,391 bipyridyl,71 pyridine,235
and triethylstibine. Treatment of a halometal complex with an anionic
4-35
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alkylating agent, such, as a Grignard or organolithium reagent, is the most
widely used method for preparing both the platinum (II) and palladium (II)
complexes. 7^
The platinum (II) alkyl and aryl complexes are colorless crystalline
solids that are not oxidized by moist air. The palladium (II) complexes are
less stable. Both metals form a wider variety of organometallic complexes
than any other transition metal. When the complexes are heated, they appear
to undergo homolytic fission, with the formation of organic radicals. For
example, when [ (PEt-KUCEU] is heated at 100° C in a sealed tube, a
mixture of ethane and ethylene is obtained, with a trace of methane.
Olef in complexes of platinum are the oldest class of organometallic
complexes known and have therefore been extensively investigated. A number
of their reactions are of commercial interest. Platinum and palladium
form olefin complexes in both the bivalent and zerovalent oxidation states.
One method of preparing the platinum (II) and palladium (II) complexes is
to react the metal (II) salts, such as ^ [PtCl^J, with the olefin in
aqueous or ncnaqueous solution. 1° Platinum (II) olefin complexes are much
more stable than palladium (II) olefin complexes, with respect both to dis-
placement of the olefin with a halogen and to reactions of the complex with
a variety of reagents, including water and most nucleophiles. The olefin
and acetylene complexes of the zerovalent metals are stable, generally white,
crystalline materials with fairly high decomposition points. The decomposi-
tion temperature for one of the several complexes is given in Table 4-6.
The importance of electaDn--withdrawing substituents attached to the multiple
bond is indicated by the decreasing thermal stability of [
in the order R = CN > F > H.
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Another important class of ozgancnetallic complexes of platinum and
palladium is the ir-allyl complexes. The difference between these complexes
and normal olef in conplexes is that a three-carbon ir^allyl group is involved
in the bonding of the former. Palladium (II) forms a wide range of ir-allyl
complexes, whereas platinum forms very few. Several methods have been re-
ported for the synthesis of these complexes.250'380'392'462 The platinum and
palladium ir-allyl conplexes are yellow or occasionally red crystalline com-
plexes, most of which are easily handled in air at room temperature. The
palladium complexes are hydrolyzed readily at room temperature in the
presence of water.
Complexes of Ruthenium
The chemistry of ruthenium has been extensively reviewed by Griffith.161
A wide range of complexes are known for ruthenium in the 0, +2, +3, and less
commonly +4 oxidation states. The largest number and variety of complexes
are found for ruthenium (II). Extensively studied subjects involve the
chemistry of trialkylphosphines and triarylphosphines, the corresponding
phosphites, and, to a lesser extent, the arsines. Other ligands often
associated with the PR, group in ruthenium complexes are halogens, hydrogen,
alkyl and aryl groups, carbon monoxide, nitric oxide, and alkenes. Some
typical complexes containing PR^ ligands are RuCl2(PPH3)3, RuCl2(00) (PPh^/
RuHj (N2) (PPhJ 3, Rod (NO) (PPl^) 2, and RuCl2 (RCN) 2 (PPkj) 2 •
The chemistry of nitrogen donor ligands is of special interest, owing
to formation of (N2) conplexes of ruthenium (II). The first N2 complex to
be prepared was [Ra(NH3)5N2]Cl2,11 which is a frequent precursor for the
synthesis of many other ruthenium (II) anmine complexes. The hexanmine can
4-37
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be prepared by reduction of zinc dust with strong arrmoniacal solutions of
ruthenium halides. The Ru(HH3)5 group has strong w-bonding proper-
ties and forms complexes with N2, carbon monoxide, and similar ir-bonding
ligands. Ihe aquopentamnine reacts with both N22^ and nitrous oxide26 by
displacing the water molecule. Ethylenediamine and other organic anmines
also form a wide variety of complexes with ruthenium (H) .
Ruthenium (III) forms several types of ammine complexes, The reduc-
tion of ruthenium ammines of the type [Ru(NH3)ejL]+3 with Cr+2 and other
reducing agents has been studied in detail .274 ihe hexammine, [Ru(NH3)
reacts with water very slowly at room temperature, but reacts rapidly with
nitric oxide to form [Ru(NH3)5NO]+3. Another type of ammine complex formed
by ruthenium (III) is "ruthenium red." The structure of "ruthenium red"
appears to be that of a linear trinuclear ion with oxygen bridges between
the metal atoms, [ (NH3)5Ru-O-Ru(NH3)4-O-Ru(NH3)5]+6.125
The formation of nitric oxide complexes is a characteristic feature
of ruthenium chemistry. The vast majority of ruthenium-nitric oxide complexes
are of the general type [Ru(II) (NO)L^] (L = almost any ligand) .
In addition to the ccnoplexes already discussed, there are several
well-characterized oxygen-ligand complexes of ruthenium, such as oxalates,
3, and acetylacetonates, Ru(acac)3. Many sulfur-donor complexes
Qn
are also known. u
Complexes of Hhodium and Iridium
The chemistry of rhodium and iridium has been thoroughly treated by
Griff ith.-^ Most of the chemistry of rhodium involves the oxidation states
-1, 0, +1, and +3. The most common oxidation states of iridium are +1, +3, and
+4.
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The coordination chemistry of rhodium and iridium in the +1 state pri-
marily involves ir-acid ligands, such as carbon monoxide, PRg, and alkenes.
Both square-planar and 5-coordinate species are known for both elements.
The rhodium (I) and iridium (I) complexes are usually prepared by seme form
of reduction of ShCl^SHgO or KjIrClg in the presence of the complexing
ligand. Some typical rhodium (I) complexes are [Kh(CO)2Cl]2, trans-
KhCl(OO) (PPh3)2r HhH(CO) (EPh3)3, and BhCl(00) (PPh^. The complex KhH(CO) (PPh3)3
undergoes a wide range of reactions, but its main importance is as a hydro-
formylation catalyst for alkenes.61 QjLoro-tris- (triphenylphosphine) rhodium,
RhCl(PPh3)3, is widely used as a homogeneous hydrogenation catalyst. KhCl(PPh3)3
also undergoes a wide range of oxidative-addition and other reactions. 29,49, 430
The most important iridium (I) complexes are trans-IrCl (00) (PPh3)2 and its
analogues containing other phosphines. Many studies involving their oxidative-
A A O
addition reactions have been reported.
Both rhodium (III) and iridium (III) form a large number of cationic,
neutral, and anionic octahedral complexes. The cationic and neutral com-
plexes of both elements are generally kinetically inert, but the anionic
complexes of rhodium (III) are usually labile. Both rhodium and iridium
form cobalt-like ammines of the types [MLg]+3, [MLgX]*2, and [ML4X2]+.
The salts are made in various ways, but usually by the interactions of aqueous
solutions of FhCl3(aq) with the ligand. Some typical cationic complexes are
[Rh(NH3)5Cl]+2, trans- [Khpy4Cl2]+, trans- [Irpy4Cl?]+, and [FhH(NH3)4H2O]+2.
Neutral complexes with carbon monoxide, PR, pyridines, etc., as ligands are
prepared directly from KhCLa'SHjO or IrGlg . Typical examples of some
neutral complexes are FhC^L-, IrCl_py3, and Rh(acac)3
4-39
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Complexes of rhodium (II) are rare and are primarily confined to binuclear
carboxylates of the type [BhfOOCR^^* T^6 complexes have a tetra-bridged
structure that involves a Eh-Kh bond. These compounds are effective anti-
tumor agents when given to tumor-bearing mice^^ and are potent enzyme-inhibito]
Moltimetallic Cluster Compounds
Strictly speaking, "cluster compounds" are defined as discrete units
containing three or more transition elements of the same or different types
in which strong metal-metal bonds are present. However, the term is also
used often to include complexes in which at least two transition-metal atoms
are held together by bridging species, including carbonyls, triphenlyphosphines,
and sulfur species. Several species include platinum-group metals. MDSt of
these complexes are covalent structures and are relatively insoluble (and may
be unstable) in water.
Considerable interest has been manifested in these complexes recently,
because they offer the possibility of varying the intermetallic distances in
zerovalent systems. This feature makes them promising candidates for highly
selective catalytic reactions.
The physiologic activity of these bridged cluster compounds is not
known. Although they may not be very soluble in water, they may be soluble
in lipids, and this could allow them to be transported easily into living
cells. As interest in these species expands, it is recommended that they be
tested for toxicity.
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CHAPTER 5
ANALYSIS AND DETERMINATION
The analytic chemistry of the platinum-group metals (and gold) has
surveyed in two volumes compiled by Beamish and van Loon,33'35 which pro
an authoritative source for much of the material in this chapter. There
also an excellent review by Walsh and Hausman447 that surveys the state
the art up to about 1962. Reports from a recent symposium on the analyt
chemistry of the platinum-group metals have been published.19
This chapter will cover methods of sampling the platinum-group meta
and bringing them into solution, appropriate methods of separating them
each other and from base metals, and various "wet-chemical" and instrume
methods for their determination.
SAMPLE PREPARATION
The sampling of various materials containing platinum-group metals
difficult, because such items are often not homogeneous. This is partic
larly true of catalysts, which may be in the form of powder, pellets, or
rods containing a wide variety of contaminants after use. Many catalyst
are concentrated on the surface of supports, so fines are richer than th
bulk in metal concentration. Care mast then be taken to sample both fra
tions and to blend each individually; in the case of catalyst powders, t
requires utmost care in thorough homogenization. Catalysts containing
organic matter and those on carbon supports can be burned and the residu
prepared for sampling by grinding, sieving, and blending. However, beca
of the possible risks in burning residues containing flammable and catal
5-1
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material, a snail portion should be treated first as a guide in selecting
the proper and safe procedure.
Decomposition and dissolution of the platinum metals present special
problems, because of the general inertness of these "noble" metals.
Behavior of Mineral Acids
Although the platinum metals are referred to as "noble" — i.e. , not
readily attacked by single mineral acids or by oxygen — some are corroded
to an extent that depends on the presence of impurities or on the degree
of subdivision or dispersion in another metal (see Chapter 4) . For example,
the use of nitric acid to remove copper from a platinum precipitate can re-
sult in losses of the latter into the acid copper solution. Similarly, the
presence of mercury will permit nitric acid to dissolve some platinum. In
fire-assay methods (described below) , in which the platinum metals are iso-
lated in a "button" of lead alloy, platinum and rhodium are attacked appreci-
ably if the button is dissolved in nitric acid.
Aqua regia, a mixture of three or four parts of hydrochloric acid and
one part of nitric acid, is well known as a dissolving agent for gold and
platinum. However, not all the platinum metals are quantitatively dissolved
by this reagent (see Chapter 4) . 1!he solvent role of aqua regls depends on
the presence of nitric acid — a powerful oxidizing agent capable of electron
removal, e.g. ,
Pt° = Pt4* + 4e—
and hydrochloric acid, which provides a source of chloride ions to form a
_2
stable complex with the oxidized metal ions, namely, PtClg . Other com-
binations of oxidants and complexing agents will also have great dissolving
5-2
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power; for instance, air and potassium cyanide solution vail dissolve even
gold.
Finely divided precipitates of platinum or palladium may be dissolved
in aqua regia; but, if such solutions are evaporated, care must be taken not
to overheat the residues, inasmuch as palladium may consequently become ex-
tremely resistant to redissolving, even by aqua regia. Massive platinum,
as in heavy-gauge wire, is sometimes incompletely dissolved by aqua regia,
and other methods must be used. Finely divided rhodium is soluble in hot,
concentrated sulfuric acid; but, because of the stability of the rhodium
sulfate complex formed, the metal is precipitated from this solution as
sulfide, which is then dissolved in hydrochloric acid. No combination of
mineral acids can dissolve ruthenium, iridium, or osmium.
Behavior of Alkalis
Fusion of the more intractable platinum metals with potassium hydroxide
and either potassium nitrate or peroxide is usually effective in bringing them
into soluble forms. By such means, rhodium, ruthenium, iridium, and osmium
can be completely oxidized and later dissolved in mineral acids. Such fusion
mixtures react with palladium, but less readily than the metals listed above.
Caustic fusion is not applicable to the dissolving of platinum for analytic
purposes, but may corrode platinumware.
Action of Chlorine
All the platinum metals can be solubilized by direct chlorination at high
temperatures t650-700° C) in the presence of an alkali chloride, or by heat-ing
in a sealed reaction tube containing hydrochloric acid and an appropriate oxidant.
5-3
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Fusion with Base Metals
Fusion of the more resistant platinum metals with zinc, tin, or bismuth
produces alloys or intermetallic compounds that are more readily soluble in
mineral acids or oxidizing fusions. Such treatment is particularly effective
for larger pieces of the metals. Indeed, fusion of platinum with zinc may
result in finely divided metal that reacts violently on addition of acids.
Fusion with lead can be used as a practical means of separation of the
platinum metals into two groups: platinum, palladium, rhodium, gold, and
silver dissolve in molten lead; iridium, ruthenium, and osmium form a sepa-
rate, extremely insoluble crystalline phase. However, some analytic chemists
have found that part of the iridium, ruthenium, and osmium remains associated
with the lead phase; thus, this separation technique is questionable.
Pyrosulfate Fusion
Fusion with potassium pyrosulfate at 700° C in a quartz crucible is
particularly useful for materials containing rhodium, such as palladium-
rhodium alloys, which are not ordinarily soluble in aqua regia.
SEPARATION OF THE PLENUM-GROUP METALS
In recent years, a number of new and more efficient methods for separating
the platinum metals have been developed—such as ion exchange, solvent ex-
traction, and diromatography—in addition to the more traditional techniques
of fire assay and precipitation. In many respects, fire-assay methods, which
involve a general technique for removing the platinum metals from ores or
alloys to a medium favorable for analysis, are superior to some of the more
modern techniques. Separation by selective precipitation (gravimetric de-
termination) will be discussed later. It should be noted that ruthenium and
5-4
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osmium, which form low-boiling tetroxides (see Chapter 41, can be readily
separated from the other metals by volatilization.
Fire Assay
Methods included under the classification of "fire assay" involve high-
ternperature treatment of samples intimately mixed with graphite, an oxide of
lead or copper, and a flux of borax and sodium carbonate, to produce an alloy
containing the noble metals (including gold and silver) and a glassy slag
containing all the extraneous materials. The slag is easily separated from
the alloy button on cooling. If lead is used, the button can be fired again
in air on a bone-ash cupel. Lead oxide (litharge) is formed and, being molten
at the temperatures used, soaks into the porous cupel, carrying most of any
remaining base metals with it. The precious metals remain as a small bead.
Sane loss of platinum metals—such as ruthenium, osmium, and iridium—may
occur in cupellation, so direct dissolving of the lead or copper button is
usually preferable.
Solvent Extraction
Removal of one member of the group with an organic solvent provides a
useful, convenient, and selective separation technique. Of many schemes
that have been proposed, two general categories may be distinguished:
chelate extraction systems in which the platinum metal is incorporated into
a neutral organic complex that is preferentially dissolved in an organic
phase, and ion-association systems in which the platinum metal is in an
anionic complex species (e.g., halide or pseudohalide, such as thiocyanate
and complexes) that couples with a suitable cation (such as quaternary
anroonium, phosphonium, or arsonium ion or a cationic dye) to form a neutral
5-5
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extractable species. Many of these complexes are characteristically and
intensely colored, so the metal may be spectrophotometrically determined
at low concentrations in the extract.
For example, platinum and palladium may be selectively separated from
most of the other metals of the group by extraction with dipyrrolidinodithio-
carbamate in chloroform. Iridium is separated from platinum, palladium, and
rhodium by extraction with diantipyrylpropylraethane. A mixture of ruthenium,
rhodium, and palladium in hydrochloric acid solution can be treated with a
chloroform solution of triphenylmethylarsonium chloride to remove palladium;
further treatment of the aqueous solution with 8-quinolinol in butyl Cellosolve
removes ruthenium, leaving rhodium in the aqueous layer.
Chromatographic methods involve multistage countercurrent contact between
an immobilized phase (liquid or solid) and a mobile phase (liquid or gas).
This general category encompasses processes involving ion-exchange mechanisms,
as well as solvent extraction (called partition dmxatography). The im-
mobilized phase is dispersed in a relatively thin layer supported by (or form-
ing part of) finely divided granules that can be packed in a columnar bed or
spread thinly on a glass or metal plate or even on papers of various sorts.
Selective absorption or binding of ions of interest from the sample
solution when passed through a column packed with a suitable ion-exchanger
constitutes a very convenient separation scheme. As mentioned previously,
the platinum metals form many anionic halide or pseudohalide complexes;
anion-exchange resins are used in their separation. The anionic complexes
can be eluted from the column with either concentrated hydrochloric acid or
5-6
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2 M perchloric acid. Cation-exchange resins have been used to remove base
metals, as cations, from solutions that contain platinum-metal complexes as
anions.
Paper chrcmatographic procedures have been developed for separating and
qualitatively testing for the platinum metals with systems based on the ex-
traction of hal ide anionic complexes of the metals into butanol.
METHODS OF utgriaKMINATION
Spectrochemical Methods
Emission Spectroscopy. Emission spectroscopy consists of excitation
of samples with an arc or a spark under carefully controlled conditions so
as to generate characteristic spectral lines whose intensity, recorded on
photographic plates or measured by a photomultiplier tube, is related to
the concentration of elements in the samples. This is a well-established,
sensitive, and convenient way to analyze minor or trace constituents.
Such methods have been developed for the platinum-group metals, and
some thoroughly proven procedures have been selected for publication by the
American Society for Testing and Materials.1? For this application, emission
spectroscopy has been generally preferred by European analysts to atomic-
absorption techniques, whereas the reverse is true in North America and Japan.
Prior separation or concentration steps to bring the element sought to a suit-
able degree of detectability may be required with both techniques. And there
is a requirement for the preparation of standards for calibration; not only
must these contain the sought element(s) at known concentration (and usually
a reference element at fixed concentration to serve as an internal standard),
5-7
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but, because of so-called matrix effects, they often must contain about the
same proportions of other elements as do the samples for analysis.
The limits of defect-ability of the metals in a direct-current arc are:
palladium, 3 ppm; platinum, 10 ppm; rhodium, 3 ppm; iridium, 100 ppm;
ruthenium, 80 ppm; and osmium, 500 ppm. Similar limits are obtained with
a high-voltage spark.
Atomic-Absorption Spectroscopy. In atcmLc-absorpjtion spectroscopy (AAS),
the metals of interest are converted to atoms in the vapor phase. Light of a
wavelength characteristic of the sought element is then passed through the gas,
and the amount that is absorbed by these gaseous atoms is measured and related
to the amount present. Formation of the gaseous element can be achieved either
by aspiration of a solution containing metal ions into the reducing part of a
flame or (more recently) by flameless reduction of the sample heated in a
carbon furnace. Another fairly well-developed method of improving sensitivity,
as well as the selectivity, when flames are used involves aspiration of ex-
tracts of the sample into organic solvents, rather than aqueous solutions into
the burner.
A somewhat related technique, atomic-fluorescence spectroscopy CAPS),
differs from AAS in that the atoms of the element(s) to be determined are
subjected to excitation through illumination, but then lose part of the ex-
citation energy through fluorescence emission. The latter is measured by a
sensitive detector as with AAS, except that the beam of incident radiation
is arranged at right angles to the direction along which the fluorescence is
measured.
5-8
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These spectroscopic techniques are susceptible to interferences result-
ing from substances in the sample that may interfere with the observed final
measurement. Such interferences may be physical, such as alteration of the
aspiration or nebulization of the analytic solution; or they may be chemical,
such as something that influences the population of free atoms of the analyte(s).
These effects call for careful control of the preparation of samples for vapor-
ization; they may be alleviated to some degree by addition of releasing or pro-
tective agents (sometimes called "buffers") to the analyte. Such interferences
are quite pronounced in the analysis of some of the platinum metals. For example,
the addition of lanthanum can increase apparent spectral sensitivity in the de-
termination of rhodium.
Examples of applications to the platinum metals, taken from a recent
monograph,2^ illustrate the sensitivity of the method. It may be added
that somewhat lower values obtained for these elements by the use of flameless
excitation have been reported recently.^ Most recent publications indicate a
*5/"(T >| O*7
preference for this method of atomization. ' See Table 5-1 for sensitivities.
Inductively Coupled Plasma Spectroscopy. A development of considerable
importance in spectroscopic analysis has been the introduction of high-frequency
plasmas"f 133,134,241 gg a means of atomic excitation. These attain much higher
temperatures than flames and furnaces. The special advantage claimed for this
technique is the capability of performing simultaneous multielement determina-
tions in the parts-per-billion range with small volumes of sample. This repre-
sents a significant extension of the range and versatility of conventional
emission spectroscopy.
5-9
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TABLE 5-1
Atonic Spectrosoqpy of Platinum-Group Metals'2
Atonic Absorption Spec
Element Sensitivity / ppn
Iridium 7.7
Osmium 1-3
Palladium 0.2
Platinum 2.2
Rhodium 0.35
Ruthenium 0.25
Atonic Fluorescence Spec
.% Detection Lumit, ppm Detection Limit, ppm
0.02
0.1
0.03
4
0.06
0.15
10
80
firm Kirkbright and Sargent.
5-10
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Recently published detection limits for palladium, platinum, and rhodium
were 0.007 ppm, 0.08 ppm, and 0.003 ppm, respectively, with this technique.
381
Spectrcphotometry.JOU- The platinum metals form highly colored complexes
with many inorganic and organic reagents, and the literature35 contains hundreds
of spectrophotometric procedures. Only a few of the most widely used will be
mentioned here.
Platinum (IV) in hydrochloric acid reacts with tin (II) chloride to form
rapidly a stable yellow-orange (403 nm)* complex that is useful for accurate
determination of platinum at 3-25 ppn. The sensitivity can be enhanced by ex-
traction of the complex into oxygen-donor solvents. A further increase in
sensitivity might be achieved by using the near-ultraviolet absorption peak
at 310 nm.447 Gold and palladium interfere strongly, and the other platinum
metals must be absent for highest accuracy.
5-(pj-Diinethylaininobenzylidene)-rhcdanine, which can also serve as a
reagent for palladium, can be used for platinum at 0.5-6 ppm. A dark-red
complex (545 nm) is developed in acid solution. Silver, gold, copper, rhodium,
and palladium interfere. The arHition of dimethylglyoxime permits a 12-fold
excess of palladium to be tolerated.
2,3-Quinoxalinedithiol forms colored complexes with both platinum (624 nm)
and palladium (517 nm), with the possibility of simultaneous determination of
both metals.
* Wavelengths for maximal absorption shown in parentheses.
5-11
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Palladium forms a complex (594 nm) with pyridir^2-aldehyde-2-quinolyl-
hydrazone that can be extracted into chloroform and used to determine the
metal at 1.5-7 ppm in the presence of relatively large proportions of associ-
ated noble and base metals. Palladium can also be determined at 0.1-10 ppm
by means of the intensely colored red-brown complex PdI4"2 (410 nm) formed
with excess iodide.
Rhodium forms a cherry-red complex (510 nm) with pj-nitrosodimethylaniline,
useful at 0.15-1.1 ppm; however, associated base metals and platinum metals
interfere. p_-Nitrosodiphenylamine can also be used; its decreased sensitivity
is compensated for by requiring less rigid control of conditions used for
color development. Rhodium can be determined in the presence of platinum,
copper, and iridium by the colored complex formed with tin (II) chloride after
separation of platinum as an iodide complex tributylphosphate in hexane.
Ruthenium forms a red complex (510 nm) with 2,4,4-tri-(2-pyridyl)-2-triazi
at a pH of 2-4; this complex can be used for the determination of ruthenium at
1-4 ppm. Distillation of ruthenium beforehand is usually necessary, because of
interferences.
Iridium may be determined in the presence of platinum, palladium, and
rhodium by extraction with thenoyltrifluoroacetone in the presence of ethylene-
diaminetetraacetate (EEXEA), with measurement of absorbance of the extract at
440 nm. The preferred range of concentration of metal in the extract for
measurement in a 1-cm cell is 7-35 ppm.
Osmium forms a blue complex (600 nm) with thiocyanate, which, when ex-
tracted into ether or octanone, can be used for determinations of this metal
up to 15 ppm without interference, except from platinum and antimony.
5-12
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X-ray Fluorescence Spectroscopy. X-ray excitation of the elements of
higher atonic number (> 20) results in the production of characteristic
fluorescence that provides the basis for highly selective, sensitive, rapid/
and nondestructive analytic methods. This method has been applied to the de-
termination on active alumina catalysts of platinum and palladium at 0.25-0.75%,
with a standard deviation (at the 0.6% level) reported as 0.006% and 0.0025%
27*5
by different workers. In another instance/ the same metals at approximately
3 and 4 ppm in a nickel matte were determined, with results comparing favorably
with those of other methods. For this second application, the precious metals
were concentrated by absorption on filter paper impregnated with ion-exchange
resin/ and determinations were made by comparison with standards. The other
platinum metals have also been determined by x-ray fluorescence.
Spark-Source Mass Spectrometry. Ma«B-spectrometric techniques have been
extended by the use of an electric spark8 to substances not readily volatilized.
This multielement technique has been used to determine nickel/ copper/ lead,
silver, and palladium as impurities in platinum at from 0.5-14 ppm with reasonable
accuracy. The procedure requires preliminary separation by ion exchange, with
electrodeposition of these metals onto a gold electrode, from which they can be
volatilized. This particular application is based on isotopic dilution.
Surface Analysis. Three new techniques, each applicable to the analysis
of elements at sample surfaces, are in various stages of development: ion-
scattering spectrometry, scattered-ion mass spectrometry, and electron
228 229
spectrosoopy for chemical analysis. They have recently been described, '
and their analytic capabilities outlined for the nonspecialized reader. The
limited number of suitable instruments yet in service and the early state of
5-13
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development of specific analytic procedures suggest that not too many appli-
cations to platinum-metal analysis have been described. However, it is known
that ion-scattering spectrometry has been successfully used to determine plat-
inum metals in supported catalysts where the metal is on the surface of a
carrier of greatly different atomic mass.
Electron-Probe Microanalysis.20'231 Electron-probe microanalysis depends
on excitation of the characteristic x-ray spectra of elements by a narrowly
focused beam of electrons directed to strike a small part of a sample to be
analyzed. The emitted x-ray spectrum is measured by an x-ray spectrometer;
qualitative information concerning the elements present in the sample is
given by the wavelengths emitted. Quantitative analysis is achieved by
measuring the intensity of the emission and making various corrections.
There have been some remarkably successful applications of this method
to the analysis and characterization of new minerals of the platinum metals
(among others), which were referred to in Chapter 2.
Neutron-Activation Analysis
Activation techniques are probably unequaled for the purpose of de-
termining submicrogram traces of most metals. With a flux of 5 x lO""1*-
neutrons/cn^-s, neutron activation is at least one order of magnitude more
sensitive for most platinum metals than the best of the spectrophotometric
methods.
The sensitivities listed in Table 5-2 are predicated on irradiation of
a sample for 1 month at a neutron flux of lcr2/cn?-8, followed by a 2-h
decay during which radiochemical purification with a quantitative yield is
carried out.
5-14
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TABLE 5-2
Sensitivity of Neutrpn-Astivation Analysis for Platinum-Group Metals and Gold
Radioisotope Estimated
Produced (half-life) Sensitivity, g
Gold-198 (2.7 d) 5 x 1(T12
Rttthenium-105 (4.5 h) 1 x 10~9
Osniuii-193 (31 h) 1 x 10"9
Hhodiun-104 (4.3 min 44 s) 5 x l(f3
Iridium-194 (19 h) 1 x 10"11
Platinun-197, -199 (18 h) 1 x l(f9
Pal.ladium-109 (13.5 h) 1 x l(f10
5-15
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Activation procedures have been applied to analysis of platinum catalysts,
A 2-h irradiation at a relatively low neutron flux of !QQ/ca?~B was sufficient
to detect platinum at 17 ppm and ruthenium at 38 ppm, without the necessity of
any chemical separation steps. The other platinum metals should be as readily
analyzed.
A complete list of isotopes, natural and artificial, of the platinum-
group metals is given in Walsh and Hausman.447
Electrochemical Methods
Polarography. Polarographic behavior of the platinum metals has been
extensively studied. Platinum may be determined in the presence of palladium
at 0.02-0.2 yg/ml by the catalytic wave in 2 M hydrochloric acid. Palladium
at 20-60 yg/ml may be determined in an EDOA solution at a pH of 5-7 without
interference from the other platinum metals. It is possible to determine
0.5-25 ug of rhodium in 20 ml of a sodium chloride-hydrochloric acid solution
by oscillographic polarography without interference from platinum, palladium,
or gold. The polarographic reduction wave of iridium (IV) to iridium (III)
in hydrochloric acid can be used for 1-450 ng/ml in the presence of rhodium
and palladium. Osmium and ruthenium can be simultaneously determined by
oscillographic polarography in sodium chloride-hydrochloric acid solutions.
Coulometry. Controlled potential coulometry has been used for accurate
determination of microgram, as well as milligram, quantities of platinum,.
palladium, rhodium, iridium, and ruthenium. Coulometric generation of titrants,
as well as direct reduction of the metal ion, has been effectively used.
5-16
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r. Amperometric titzations are based on the use of polarographic
waves as indicators of the consumption of the electroactive species by its re-
action with a suitable titrant. Thus, 0.5-2.1 mg of palladium can be titrated
atnperometrically as it is oxidized fron oxidation state +2 to +4 by standardized
hypochlorite solution. Iridiun and ruthenium can be titrated in the same way
on reduction by ascorbic acid or hydroquinone as titrant.
Gravimetric Methods
Platinum may be precipitated quantitatively from hydrochloric acid
solutions in the form of ammonium hexachloroplatinates, (NH4) J?tCl^j very
few base metals interfere, but iridium, rhodium, or palladium may interfere
somewhat, and reprecipitation may be necessary (see Chapter 2).
Dime thyIglyoxime, C4H6(NOH)2/ is the most important precipitant for
palladium, which, in acid solution, is precipitated as Pd ^H^N^C^) 2 essen-
tially without interference from any of the other platinum metals or base
metals.
No fully acceptable gravimetric methods are available for the other
platinum metals, but procedures are available for use in restricted appli-
cations.35 A classic case is the precipitation of ruthenium, after distilla-
tion and absorption of its tetroxide, by thionalide (a^mercapto-N,2-naphthyl-
acetamide), followed by controlled ignition to the metal.
Volumetric Methods
Palladium may be titrated by potassium iodide in the presence of up to
a 2-fold excess of platinum and a 200-fold excess of copper (II); the end
point is found potentiometrically. Vanadium (II) may also be used as a
titrant for palladium. An indirect method for the same metal, with which
5-17
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platinum does not interfere, involves the addition of an excess of EDTA
and back-titration with zinc (II).
Platinum (IV) may be titrated with iron (II) sulfate containing tri-
ethanolamine in an alkaline medium; under these conditions, iridium (IV)
is reduced to iridium (III), and palladium (II) is reduced to metal. Copper
(I) chloride may be used to titrate platinum in the presence of palladium.
Ruthenium (IV) may be potentiometrically titrated to ruthenium (VIII) by
lead tetraacetate; alternatively, reduction of ruthenium CIV) to ruthenium
(III) by iron (II) has also been used.
5-18
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CHAPTER 6
TOXICOLOGY AND PHARMACOLOGY
The toacioology and pharmacology of the platinum-group metals include
the study not only of the six metals themselves, but also of their compounds.
One recent introduction of catalytic converters using platinum, palladium,
and rhodium to control motor-vehicle exhaust emission makes it possible for
quantities of the metals and their compounds to enter the environment (R. F.
Hill, personal ccranuncation; and Brubaker et al.65) . Radioruthenium can also
enter the environment through the waste effluent from nuclear fuel reprocessing
plants, where it is one of the most abundant fission nuclides released. The
exact amounts and chemical forms that may be lost are not known, but it is
imperative to consider carefully the toxic and pharmacologic properties of
these materials, if intelligent recotroendations about their use are to be
made.
Other potentially significant uses of platinum-metal compounds are their
application as antitumor chemotherapeutic agents, several of which are under-
going limited testing.
The purpose of this chapter is to assess the physiologic activity of
these materials, inasmuch as the human population may ultimately be exposed
to them through the skin, by inhalation, and orally.
ACUTE TOXICITY OF PLATINUM AND PALLADIUM COMPOUNDS
was one of the first to emphasize that the toxicity of
platinum and its salts and complexes varies. The compounds tested were
anmonium salts containing divalent and tetravalent platinum with various
6-1
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numbers of anrnine ligands. He concluded that an increase in the valence of
the noble-metal component of a complex salt was asBociated with a stronger
"curare-like" action of the salt. The acute toxicity of a number of platinum
coordination complexes has been investigated.363 There is general agreement
about the lethality of specific compounds, but the acute toxicities of the
various compounds vary over a wide range, as indicated by the 1^50 «8* of botil
soluble and insoluble complexes. Studies363 of the toxicology of the anti-
tumor agent cis-[Pt(NH3)2Cl2] have been riondiictfvl on mice and rats and have
been summarized by Rosenberg. The important histologic changes observed
were denudation of intestinal epithelium, bone marrow depression, thymic
and splenic atrophy, and acute nephrosis. Studies in dogs and monkeys re-
vealed similar toxic effects, with damage to the renal tubules prominent,
especially at high doses. Other important effects were damage to the bone
marrow and gastrointestinal epithelium.
The acute LD4Q of platinous chloride, PtCl2, administered as a single
dose to outbred albino rats and followed for 2 weeks is approximately
26 mg/kg of body weight.
The LD5Q of palladium dichloride, PdCl2/ has been determined after intra-
venous, intraperitoneal, oral, and intratracheal administration for several
species. Some of the data for rats and rabbits are presented in Table 6-1.
The greatest toxicity was seen after intravenous or intratracheal administra-
tion, and the least after oral administration. To examine the effect of the
*!£> is the dose at which 50% of the treated animals die.
50
6-2
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TABLE 6-1
LD.Q.J of Palladium Chloride0
Approximate LD50,
mg/kg of body Eiight Route of Administration*?
5 Intravenous
70 Intraperitoneal
200 Oral
6 Intratracheal
5 Intravenous
aoata fron Moore ct al.
272
^Clonic and tonic convulsions were noted after intravenous administration.
The manner of death after administration by the other routes was not
mentioned.
6-3
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chemical nature of the salt on toxicity, values for the LD50 of palladium
chloride, K2PdCl4, and (NH^jPdC^ were determined after intravenous ad-
ministration. The LD5o«s/ expressed in micromoles of palladium per kilogram
of body weight, were very similar for these compounds. Results of preliminary
experiments indicate that palladium chloride or palladium sulfate, PdS04,
when injected intravenously, acts as a nonspecific ^rrh'iy irritant, as well
as a peripheral vasoconstrictor. Inasmuch as the chloride salt strongly
dissociates in solution, the palladium itself may be the irritant.
In a study!97 of the toxicity of various lead, manganese, platinum,
rutheniun, and palladium salts after oral administration, the toxicities of
the salts were in decreasing order: PtCl4, Pt(SO4)2'4H20 > PdCl2*2H2O,
RuCl3 > MnCl2-4H2O, PdS04, PbCl2, PtCl2 > Pt02 > MnO2, PdO. Thus, on oral
administration, the two soluble tetravalent platinum salts were the most
toxic, and the highly insoluble salts, including the oxides of platinum and
palladium, were the least toxic.
Palladiun chloride has a slight diuretic effect in rats, and lethal
intravenous doses in rabbits cause hemolysis, albuminuria, and diuresis.
The heart, kidney, liver, bone marrow, and blood cells are the sites of
tissue damage. 262 ^he dermal irritancies of 11 platinun and palladium
compounds were evaluated with albino male rabbits. The procedures and
evaluation criteria were adapted from those in use by the National Institute
for Occupational Safety and Health. Four materials were evaluated as unsafe
*
for contact with intact or abraded skin, as judged by severity of response:
(C3H5PdCl)2/ (NH4)2PdCl4, (NH4)2PdCl6, and PtCl4. One was evaluated as safe
for intact, but not abraded, skin: ldCl. Two were found safe for intact
6-4
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skin, but for abraded skin only if protected: KgPdC^ and PdCl2. The re-
mainder were considered safe for intact or abraded skin (irritancy grade,
less than 1 on a scale of 4): Pd(NH3)2d2, PdO, PtO2, and PtCl2.72
Two platinum compounds (PtO, PtCl2) and two palladium compounds (PdO,
PdCl2) were examined for ocular irritancy in rabbits.^16 in each rabbit,
10 mg of the test material was deposited on the surface of the right eye,
and the left eye was used as a control; six rabbits were used per compound.
The rabbits were examined for ocular inflammation 24, 48, and 72 h after
application. At these doses, neither of the platinum compounds was irritating,
and no reaction was noted with PdO. All six animals that received PdCl2 showed
corrosive conjunctiva! lesions and severe inflammation of the cornea and anterior
chamber of the eye. These effects were seen at 24 h and persisted at 48 and
72 h.
Dermal and ocular irritancies of compounds of metals other than platinum
and palladium can be obtained from other sources.^43
REACTIONS WITH ENZYMES, NUCLEIC ACIDS, BACTERIA, AND VIRUSES
The interactions of platinum-group metals and their complexes with bio-
polymers is still poorly understood, despite at least two strong motivations
for studying them—the use of heavy-metal derivatives for isomorphous replace-
ment in proteins for x-ray crystallographic determinations of structures, and
the antitumor activity of many of these complexes.
Platdnum-Protein Interactions in X-ray Crystallography
Crystalline proteins steeped in various mother liquors produce clear
x-ray diffraction patterns. To reduce these patterns to atomic structures,
6-5
-------
the phases must be determined. To accomplish this/ a heavy atom could be
attached to specific sites of the protein if it does not distort the diffrac-
tion patterns (iscmorphous derivatives) . A solution of the heavy-atom com-
pound is added to the mother liquor, and the crystals are allowed to inter-
act for long periods to saturate possible binding sites. Among the most
successfully used heavy atoms are those of the platinum group, particularly
the [PtCl4J= ion. As expected, the evidence (reviewed recently by Petsko
et al. 3) suggests that the [PtClF ions selectively attack specific pro-
tein sites, such as disulfide bonds, methionines, histidines, NH2 terminal
groups, and sulfhydryls, with a rare few proteins showing binding at asparagine
(hexokinase 2) and arginine (lysozyme chloride) . The major binding sites are
listed in Table 6-2. These are consistent with expectations that the
[PtCl ]= ions, as well as other platinum complexes, preferentially bind
to nitrogen and sulfur. The bonds are mainly covalent, with S^2 substitution
at the chloride positions. Stereochemistry is a major determinant of the
specific sites of interaction, because of the rigid square-planar structure
of the Pt (IV) complexes. The chemistry of the complexes in the mother liquor
is not well understood. Dickerson et al.^2^- have suggested the formation of
an intermediate Pt (IV) complex with the protein ligands attached at the fifth
and sixth coordination sites of the resulting octahedral complex. Petsko
et al. 313 argued that this is less likely to occur in the nonoxidizing environ-
ment of the protein mother liquor and favored a scheme whereby some of the
chlorides are replaced by NHj or (PO4) = of the mother liquor to produce Pt-
diammine complexes, which then react with the protein sites.
6-6
-------
Survey of
TABLE 6-2
" Binding to Protein Crystals*2
Protein
Carboxypeptidase A
a-Oiymotrypsin
Y-Chymotrypsin
PMS-6-Chymotrypsin
Concanavalin A
Cytxxihrome
Horse-oxidized
Tuna—reduced
Erythrocrurorin
Hexokinase 2
Lysozyme chloride
Prealbumin
RLbonuclease S
Subtilisin BPN
Thermolysin
Sites of [PtCl4]2" Binding2'
S-S 138-161;
Met 103;
His 303: N-Term.
S-S 1-127;
Met 192; N-Term.
S-S 1-127;
Met 192;
Met 192;
S-S 1-127;
His 57
Met 130
Met 65; His 33
Met 65
Met 131; His 94;
His 111
As 1
Arg 14
-SH
Met 29
Met 50; His 64
His 250; His 216
aData from Petsko et al.313
bS-S, disulfide link; Met, methioniiie; His, histidine; -SH, sulfhydryl;
Arg, arginine; As, asparagine.
6-7
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Rabertus et al.359 recently reported a structural determination of
yeast phenylalanine tRNA to a 3-8 resolution using trans-PtQBfcQgClg and an
osmium-adenosine triphosphate complex as two of the isomorphous derivatives.
Ihe trjns-Pt(NH3)2C!2 has a major binding site on the anticodon oligonucleo-
tide sequence GnrA-A-Y-A-i|» , one of the most significant regions of the tRNA
molecule.348 Interestingly, the cis-Pt(NH3)2Cl2 did not show a similar inter-
action at this site, nor did it yield a satisfactory isomorphous derivative.
This emphasizes the major importance of stereochemistry in determining the
reactions of these complexes with biopolymers.
Platinum-Protein Interactions in Enzyme Inhibition
PdCl2 is known to bind to various proteins—such as carboxypeptidase,
casein, papain, and silk fibroin—but the binding sites are not known.
Enzyme inactivations by PdCl2 ^aere reported by Spikes and Hodgson. 2 Only
trypsin and chymotrypsin were rapidly inactivated at a pH of 4.2, and even
these were much less affected at a pH of 8.9. The authors speculated that
the inactivation may occur through free sulfhydryl or cystine groups in
trypsin and through cystine groups in chymotrypsin. Catalase, lysozyme,
peroxidase, and ribonuclease were not inhibited at all.
Inhibition of the two enzymes, leucine aminopeptidase and malate
dehydrogenase, by a variety of platinum complexes has been studied by a
group at Auburn University for some years. Leucine aminopeptidase is
inactivated by pTBT4, Pt(En)Br2, and Pt(Dien)Br+, with the rates and final
percentage of inhibition decreasing in sequence. These studies were done at
a pH of 8.0. Hie authors^4 suggested that at least two halide-leaving groups
are required for high inactivation rates. Additional studies showed that the
6-8
-------
monoaquo complex PtBr^ttloO)" inhibits leucine aminipeptidase and malate
dehydrogenase more rapidly than PtBr4+2, which is consistent with the sug-
gestion by the authors and others that the slow aquation is the rate-limiting
step in reactions with biopolymers:
Pt(X)halide + U slow PtWlO + halide
and
f ast
S as y Pt(X)S + H20,
where S is some biopolymer group bound to the platinum.263
Although lytCl and Bb2PtBr4 produce the same equilibrium inhibition
(after 24 h of incubation) of malate dehydrogenase, the bromide complex
approaches the equilibrium approximately 5 times faster than the chloride
complex. This is consistent with the ratio of the known rates of aquation
of these two ligands and supports the hypothesis that aquation is the rate-
limiting step in substitution reactions with protein ligands. ^0
As a general rule, neutral platinum diloroammine complexes that are
active antitumor agents are in the cis configuration; the corresponding
trans isomers are inactive. Thus, stereospecificity is of great biologic
significance. This provides a powerful tool for elucidating the mechanism
of action at a molecular level. It is assumed (but not proved) that the
cis isomers attack significant sites by a bidentate binding (chelation) at
neighboring sites, whereas the trans isomers attack by monodentate substitu-
tion or, if bidentate, at obviously different spatial sites. Inasmuch as
both the cis- and trans-Pt (NH^) 2GU isomers inhibit some enzymes (malate
dehydrogenase and liver alcohol dehydrogenase) with the same association
inhibition constant, it is proposed that the inhibition of these is by a
6-9
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monodentate substitution. For other enzymes (yeast alcohol dehydrogenase
and lactate dehydrogenase), the association inhibition constant (or equi-
librium association constant), K^ is approximately 20 times higher for the
trans iscmer than for the cis isomer. It is suggested that a bidentate
chelation over a wider gap is the essential interaction of binding and
inhibition.141 The nature of the binding ligands of these proteins is
still undetermined. Table 6-3 shows some equilibrium association constants
for a number of different platinum complexes with malate dehydrogenase.
Platinum-Complex Interactions with Amino Acids
Amino acids have at least four groups that react readily wititi platinum
and other platinum-group metals: -N^' -CC>2/ -SCH^, and the imidazole ring
nitrogens of histidine. Extensive work by the Russian school, and particu-
larly the Volsteyn laboratory, has elaborated the kinetics and thermodynamics
of reactions of [PtCl^] and of cis- and trans-Pt (NH.J ^Clo with these amino
acid groups. These have been reviewed in some detail by Thomson et al., 2*
MsAuliffe and Murray,261 and D. R. Williams.464
A variety of reaction products are formed, depending on the ratio of the
concentrations of the reactants, the pH, and the isomeric structure of the
platinum complex. For example, [PtC^]"2 reacts with glycine to give the
bis-(cis)-glycinato Pt(II) complex and the trans isomer, in a 3:1 ratio. In
these cases, both -NH2 and ~C°2~ of eacn 9lvcine are bound to the platinum.
At a higher pH and with excess glycine, the Pt(glycine)4 complex results.
Thomson et al.425 pointed out that, as a general rule, at neutral pH the
preferred form is the closed-ring structure, but at higher and lower pH the
6-10
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TABLE 6-3
Equilibrium Association Constants (Ke) for Platinum
Complexes and Malate Dehydrogenase7*
Complex
[M] "1 x 102
PtCl
~2
cis-Pt(NH3)2Cl2
trans-Pt(NH3)2C^2
Pt(NH3)3Cl+
Pt(NH3)4+2
PtClg-2
PtBTg-2
PtBr4-2
8,700
290
3.0
3.2
No observed enzyme inhibition
No observed enzyme inhibition
5,200
4,300
9,200
, vtere [E] = enzyme concentration,
g
[I] = free platinum complex concentration, and JE-I] = enzvmr -platinum
-complex concentration. Data from Friedman and Teggins.14!
6-11
-------
rings are open. In the platinum chloroamnines, only the chloride is sub-
stituted, and the trans labilizing influences of the incoming amino acid
groups are very slight. However, in the reaction of the -SCH3 group of
methionine (in excess) with cis-Ptdtt^^C^, the trans labilizing effect
of the Pt-S bond is strong, and the ammonia ligand that is trans to the
-SCH3 is replaced by the -NH2 of the amino acid, to form a bidentate closed
ring on one side of the molecule and an open monodentate ligand opposite
it.
Mcftuliffe and Murray^^ studied the reactions of palladium (II) with the
siilfur-containing amino acids—in particular, methionine. X-ray crystallography
has confirmed the structure of the resulting complex as the dimer connected
by hydrogen bonds fraa -che
,COOH
Cl
NH2 CH
/\
Cl
CH,
CH.
CH.
carboxylic acid. With S-methylcysteine, the results were
Pd2 (Cyst) 3C1- 2H20 and Pd (Cyst) Cl.
6-12
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Platinum-Nucleic Acid Interactions
The fact that sane platinum coordination complexes are effective anti-
cancer agents in animals and man has provoked a series of studies of these
complexes in many laboratories on their mechanisms of action at the molecu-
lar level. The results of such studies, which strongly ijnplicates DMA as
the target cell receptor, have been reviewed by Rosenberg,363 Roberts,355
Thomson, 424 Q^ Robins.360
Platinum-Nucleic Acid Interactions in Vivo. cjte-DicMorodiamineplatinum
(II) and various analogues cause tumors in animals to regress. Several of
these drugs are now in clinical trial, and they have been studied widely for
their actions in cells. Apparently, the effective drugs' ligand structures
remain intact after injection into animals, owing to the high extracellular
chloride concentration (0.112 M) , which represses chloride dissociation.
Thus, the drugs are passively transported through cellular membranes. How-
ever, the low intracellular chloride concentration (0.004 M) allows dissocia-
tion of the chloride ligands and their sequential replacement with aquo
ligands; the resulting monoaquo or diaquo species react with specific cell
receptors to produce biologic actions.
The effects of the drugs on the synthesis of cellular biopolymers
(ENA, RNA, and proteins) have been studied by Harder and Rosenberg175 and
by Howie and Gale^4 in both tissue culture and mice. The generally con-
sistent results are:
• at low doses (equivalent to the amount found in tumor tissue,
5 ug/g of tissue after a therapeutic dose) , synthesis of new
ENA is selectively and persistently inhibited
6-13
-------
• HNA and protein synthesis is only moderately affected
• the degree of inhibition of synthesis is dose-dependent, and
the inhibition reaches a nadair about 4-6 h after removal of
the drug pulse. This dose is not frankly cytotoxic; after
3-4 days, the resulting giant cells revert to apparently
normal cells. At much higher doses, the drug is frankly
cytotoxic, and all biosynthesis is inhibited. At the lower
dose, synthesis of DNA percursors is not inhibited, nor is
their transport through membranes affected. The inhibition
of DNA synthesis cannot be reversed by extensive dialysis or
washing of the cells; this indicates tight binding of the
platinum to the cell receptor.
All these results suggest that the primary lesion in the cell caused
by the drug is an attack on the DNA of the cell. Table 6-4, derived from
Ranter,22a shows the distribution of the platinum molecules among the various
cellular biopolymers when cis- and trans-Pt (II) (NH3)2C12 is given to the
cells at doses that would reduce the surviving cells to 0.37 of the initial
number. In conclusion, the reactions of the platinum complexes with DNA
lead to biologic activity.
PlatininHDNA Reactions in Vitro. Hbracek and Drobnik201 followed the
interactions of cis-Pt(II) (NH3)2C12 and DNA with ultraviolet difference
spectrophotometry. They reported a shift in the absorption maximum of DNA
from 259 to 264 nm. At a platinum:phosphorus ratio of 1:1, there was also
a marked hyperchranicity- At lower platinum:phosphorus ratios, they found
a significant amount of renaturation after a heating and cooling cycle, and
6-14
-------
TABLE 6-4
Yield of cja- and
Bound to
HeLa Cell Ma
-------
they suggested that this was due to cross-linking of the doublestranded DNA.
Chemicals that are known to stabilize the ENA structures (e.g., putrescine,
spermidine, histones, mg+2, SO4~2, and N03~ions) did not change the reaction
rates or the final equilibrium values. High chloride concentrations did
retard the reaction rates, probably by slowing the hydrolysis reaction of
the platinum drug, which is assumed to be the rate-limiting step. The
measured activation energy (EH) for the reaction was 18,000 + 630 cal/mol.
The tvo cis chloride-leaving groups of cis-Pt(II) (NH3)2Cl2 are 3.3
A apart. Therefore, it appears unlikely that the DMA is cross-linked at
the N^ position of guanosine in the two opposing strands, as has been sug-
gested for the attack of bifunctional alkylating agents on DMA. Roberts and
Pascoe356 studied the cross-linking ability of both cis- and trans-Pt (II)
(NH3)2C12 in DNA, both in vivo and in vitro, with the BUdR labeling technique.
They concluded that the cis and trans isomers are both capable of cross-linking
DNA, but a much higher dose of the trans isomer is required to produce the
same amount of cross-linking as the cis isomer. The evidence of cross-linking
of DNA has been verified with other techniques in other laboratories.
Thomson424 jj^ suggested that, in addition to forming interstrand cross-
links, the platinum drug may cause intrastrand cross-links involving the
6-NH2 groups of adenine in an ApT sequence, the 2-NH groups of guanine in
the narrow groove, and the 6-NH2 groups of cytosine in a CpG sequence in a
deep groove.
Because only a small number of the platinum molecules bound to DNA pro-
duce cross-linking, there must be other modes of binding. Stone et al..^ have
shown that the platinum binds predominantly in QC-rich regions of the DNA,
and, indeed, they can assay the GC content of any DNA after reaction with
6-16
-------
the platinum drug by using density-gradient oentrifugation. The platinum
drug does not react well with histones, and chromatin appears to bind some-
what less of the drug than comparable amounts of pure ENA. Mansy259 studied
the reaction of cis-Pt(II) OJH3l2Cl2 and ENA with laser Raman light-scattering.
The results show no evidence of interaction of the platinum complex with either
the phosphate or sugar moieties of the DNA. Robins,361 however, in his studies
of the rate of reaction of dichloroethylenediamineplatinum (II) with ENA and
other precursors, found that the presence of the phosphate group increased the
reaction rate. The lack of evidence of phosphate binding from the laser
Raman studies could be explained by assuming that the phosphate-platinum
interaction is a short-lived intermediate in the final reaction of the drug
with the ENA bases.
Bacterial and Viral Effects of Platinum-Group-Metal Complexes
Metal complexes have a long history of clinical use in bacterial infec-
tions. However, the involvement of platinum-group metals in chemotherapy
started in 1953 with Shulman and Dwyer3" and co-workers. They showed that
the relatively inert chelate complexes of tris-phenanthroline ruthenium are
active bacteriostatic and bactericidal agents against gram-positive micro-
organisms, but are less active against gram-negative microorganisms. Sub-
stituted derivatives are also somewhat active against the Landshutz ascites
tumor in mice. Unfortunately, these types of charged conplexes exhibit
intense neuromuscular toxicity ("curare-like" behavior) and can be used
only topically; however, clinical tests did indicate their usefulness in
the treatment of dermatosis, dermatomycosis, and monilial nail-fold infec-
ticns.399
6-17
-------
Rosenberg and his co-workers366 first called attention to the effects of
simple platimm-group-nietal complexes on miojoorganisms in 1965. In recent
years, many investigators have studied these systems, and a multiplicity of
different effects have emerged.
Bactericidal Effects. For the simpler complexes— such as
and the corresponding salts of iridium, osmium, and palladium — the dominant
effect on microorganisms is bactericidal. As a general rule, charged
species in solution are bactericidal, and not bacteriostatic, as shown by
viable-cell counts. The effective concentrations of the drugs vary from
1 yg/fal for (NH^^PtClg to 40 ug/ml for l^RuClg. The distribution of a
bactericidal charged metal complex among the various chemical components of
the bacterial cells is shown in Table 6-5, the data were obtained by use of
radioactive platinum-191 labels. Although the great preponderance (95%) of
this complex is bound to cytoplasmic protein, as would be expected from the
evidence presented above, no conclusion can be reached on the mechanism of
the bactericidal action, other than that it is unlikely to be a direct attack
on cellular ENA.
The neutral complexes generally force filamentation, but they are
bactericidal at higher concentrations (> 38 yM) . Shimazu and Rosenberg395
showed that mutant Escherichia coli with deficient DMA repair mechanism
(B__ -, , B 0) are more sensitive to the drug than normal E. coli B or an
S— J. S— £. — __—
ultraviolet-resistant strain (B/r) ; this implicates an interaction with
bacterial DMA as the lethal lesion for the neutral complex. There is some
evidence that microbial resistance to the bactericidal activity of these
6-18
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TABLE 6-5
Average Distribution of
Percentage of Radioactivity
Component
Metabolic intermediates
Lipids
Nucleic acids
Cytoplasmic protein
Double-Negative
Species
1
3
1
95
Neutral
Species
19
6
30
45
aDouble-negative and neutral species of ammonium hexachloroplatinate,
^Derived from Renshaw and Thomson.
6-19
-------
drugs does not develop. Sensitivity to these drugs has not been examined
in a wide array of bacteria. These subjects require much further work.
The bacteriostatic activity of rhodium complexes has been studied by
Bromfield and co-workers.59 The most active complexes are trans-[Eh L4X2]Y,
in which L is a substituted pyrimidine, X is a halide, and Y is Cl, Br, N03,
or ClO^. These are active against gram-positive bacteria at concentrations
of 0.1-5 yg/ml, but require concentrations 100 times larger for activity
against gram-negative bacteria. Cobalt and iridiun analogues were tested,
but are not active. The authors suggested that cobalt complexes are too
labile, whereas iridium complexes are too stable. Rhodium complexes are of
intermediate lability and are therefore able to pass intact to the site of
action in the cell (unknown) and react with the target site.
Filamentation Effects. Some platinum-group-metal complexes, when present
in dilute concentrations in the growth media of microorganisms, selectively
inhibit cell division without inhibiting growth, thus forcing the bacterial
rods to form long filaments. Table 6-6 shows seme typical results-^ of
different metal complexes with E. coli as the target organism. Table 6-7
catalogs the effect of one complex, the cis-dichlorodianinineplatinum(II),
on a variety of gram-positive and gram-negative organisms. The latter group
are by far the more sensitive. In addition, Bromfield et al.59 reported that
[JMpyridineJ^C^Cl and [Kh^-ethylpyridine^B^jBr induce long filaments
(20-500 times normal length) in 100% of the E. coli at concentrations of
5 yg/ml and 0.4 yg/ml, respectively.
Most of the biochemical studies of this effect involve cis-dichloro-
diamnineplatinum(II) and E. coli. Here, filamentation is a reversible effect;
6-20
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TABLE 6-6
Effects of Group VIIIB Transition-Metal Connppunds in Producing
Elongation in E&dTievCahia colJQ- ~~
Metal Compound
(NH4)2PtCl6
RhCl3
(NH4)2BhCl6
K2RuCl6
RuI2
RUNOC13
RuBr2
RuNO(N03)3
K3Rh(N02)6
Effective Concentration,
yg/ml
1-20
30-100
20-30
40-50
40-100
20-30
20-40
10-20
10-100
20-60
15-30
25-75
Elongation^
(estimated)
100%, 25-100X
75%, 5-25X
75%, 5-20X
50%, 5-20X
50%, 5-10X
25%, 10-20X
25%, 4-6X
10%, 10-20X
10%, 4-6X
10%, 3-5X
10%, 3-5X
5%, 3-5X
aDerived from Rosenberg et al.
Growth is the estimate of the relative percentage of filamentous cells in
a drop of culture fluid; the relative length of the filaments is expressed
in comparison with a control of normal-size cells after 6 h of incubation
in a synthetic medium.
6-21
-------
TABLE 6-7
Elongation Effect of cis -Dichlorodl£miiQiieplatin\im (II) on Bacteria
in Nutrient Media**
Organism
Elongation^
Escherichia ooli B
E. coli C
E. coll K-12
Aerobacter aerogenes
Alcaligenes faecalis
Proteus mirabilis
Pseudomonas aeruginosa
KLebsiella pneumoniae
Serratia marcescens
Bacillus cereus
B. licheniformis
B. inegaterium
B. subtilis
Lactobacillus sp.
Clostridium butylicum
Gorynebacterium sp.
Streptococcus lactis
£3. faecalis
Staphylococcus aureus
Sarcina lutea
Neisseria catarrhalis
5 yg/mlc
25
50%, 10-25X
40%, 10-15X
5-10X
2-15X
2-10X
2-5X
5-10X
2-10X
25%,
50%,
25%,
10%,
25%,
50%,
20%,
Normal
Normal
Normal
Normal
Normal
Normal
Normal
Normal
Normal
Normal
Normal
Normal
2-5X
95%, 10-50X
70%, 10-25X
20%, 10-20X
Toxic
10%, 10-15X
25%, 5-10X
Toxic
Toxic
Toxic
Normal
Normal
Normal
Normal
Toxic
Normal
Normal
Normal
Normal
Normal
Normal
Normal
75 yg/iaL0
Toxic
Toxic
Toxic
Toxic
Toxic
Toxic
Toxic
Toxic
Toxic
25%, 2-10X
90%, 2-5X
20%, 2-5X
20%, 2-4X
Toxic
Toxic
Toxic
Normal
Normal
Normal
Normal
Normal
aDerived from Rosenberg et al.364
Expressed in the same terms as those of Table 6-6; incubation time, 12-24 h.
^Concentration of platinum complex.
6-22
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removing the bacterial filaments to a fresh median free of the drug causes
reversion to normal colonies of the bacteria. This appears to differ from
filamentation caused by such agents as the nitrogen mustards, in which case
filamentation is terminal. The synthesis of new DNA is not markedly impeded
at the low concentrations of the drug that cause filamentation, and the bac-
terial DNA appears throughout the filament, either in clumps or as continuous,
long, axial fibers. Filamentation is not reversed by pantoyl lactone, divalent
cations, or temperature shock, which do cause reversal for most causative agents.
As shown in Table 6-5, the neutral complex is distributed among the various
classes of cellular components differently from the charged complex. About 30%
is associated with nucleic acids, compared with 1% of the charged species of
the drug. An indirect effect of the drug on bacterial respiration and reversible
changes in the inducibility of B-galactosidase have also been reported.
Gillard, Harrison and Mather (cited in Cleare^) tested a large number of
rhodium complexes for their ability to inhibit cell division in E. coli, in an
attempt to correlate it with some physicochemical property of the complexes.
MDst of the active complexes had polarographic half-wave reduction potentials
between -190 and +90 mV, with respect to the standard hydrogen electrode.
Inactive complexes were generally more negative. This may be related to a
required reduction of Rh (III) to Rh (I) for activity. Steric factors and
lipophilic character also seem to be correlated with activity.
Lytic Induction of Lysogenjc Bacteria.. A most interesting new bacteriologic
effect of cis-dichlorcdianirdLneplatinum (II), discovered by Reslova,347 is
that it induces bacteriophage virus from lysogenic strains of E. coli. This
is consistent with earlier results with some organic antitumor agents; they
6-23
-------
are also potent depressors of latent viral information in lysogenized bacteria.
The platinum drug produces detectable increases in phage-particle production,
even when present in the bacterial growth medium at concentrations as low as
0.1 yM. This is well below the threshold for forcing filaments (= 1 yM) or
for toxicity (= 30 uM) . The number of induced (lysed) cells is increased
over background at even lower concentrations (- 0.03 uM) of the drug. The
platinum drug induces lysis in a wide variety of lysogenic strains and dif-
ferent bacteria. A positive correlation was found between the ability of
a platinum complex to induce lysis and its antitumor activity.
In the direct induction of lysogenic bacteria, the inducer in the cell
may directly or indirectly inactivate the phage represser in the lysogenic
cell. Another means of induction is even more indirect. In this case, the
platinum drug is added to a nonlysogenic strain with a sex-specific F
factor. 174(PP- 105-106) Tnis p factor (a DMA-containing replicon) is trans-
ferred by sexual conjugation to a lysogenic receptor cell, which then under-
goes lysis. Because only DMA is transferred from the treated cell (F ) to
the recipient cell (F~), it is very likely that the platinum-drug lesion on
the DMA is responsible for induction.
Viral Inactivation in Vitro and in Vivo, viral particles incubated with
various platinum complexes for different periods can be inactivated.240,398
This is tested by transferring the treated virions to a suitable host organism,
where their presence is manifested by a detectable pathologic effect. Bacterio-
phages Teven, T-,, T7, R-,7, and $X and SV 40, RDUS sarcoma virus, influenza virus
Newcastle disease virus, and fowl pox virus have been shown to be inactivated
by in vitro treatment with various platinum drugs. The most potent inactivator
6-24
-------
was the cis--diaquodianinineplatinum (II). Inactivation was rapid (minutes) and
complete with this drug.
In vivo viral inactivation is measured by inoculating a suitable host with
the live virus, treating the host later with the platinum drug, and following
the course of pathologic events in the host.174 It is of some significance
that a delay between viral inoculation and drug treatment can occur without
significant loss of activity. During this delay, the viral particles disappear
into the cells (eclipse phase) and start replication. The results of one such
study are shown in Table 6-8 for fowl pox virus in an embryonated egg host.
The dose of the drug used was not frankly cytopathic to the host cells.
Inactivation appears to require the aquation of the chloride-leaving
groups of cis-dichlorodianroineplatanum (II), in agreement with other studies
showing that the interaction of the drug with nucleic acid occurs via the
intermediate diaquo form. It is therefore most likely that the viral inactiva-
tion is due to a reaction between the platinum drug and nucleic acid, but possi-
ble reactions with other biopolymers cannot be ignored. The kinetics of in
vitro inactivation are first-order and reflect the kinetics of binding of the
platinum drug to the virion. In the case of the SV 40 virus, the inactivation
of infectivity and ability to induce V antigen occurs faster than the inactiva-
tion of the T antigen induction. The inactivated virions retain their immuno-
genic properties.
HJAEMftOOLOGIC DISPOSITION OF THE PLftTINIM-GROUP METALS
The fate of platinum in rats has been studied by administering platinum-191
or platinum-191,193 as a chloride or as a sulfate intratracheally, orally, intra-
muscularly, and intravenously,272f387
6-25
-------
TAKE 6-8
Antiviral Activity of gja-DiaqucdiagTTnineplatinum (II)
against Fowl Pox Virus in
Delay between Reduction of
Virus and Drug, h Pox Lesion, %
0 97.2
2 93.3
4 98.2
6 78.2
8 91.3
10 66.7
24 23.7
aDerived from Harder.174 Fifteen FPV particles suspended in 0.1 ml
were inoculated at time zero into an artificial air cell previously pre-
pared on the chorioallantoic membrane (CAM) of each embryonating white leg-
horn egg. At various intervals, 0 or 3.9 yg of cis-Pt (II) (NH3)2^0)2 in
0.2 ml or water was inoculated onto the CAM at tEsTsame site that had pre-
viously received the virus. On the 5th-6th day of incubation, the CAM was
excised and washed and the visible pox lesions in the membrane from treated
and untreated eggs were counted.
6-26
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The route of administration is important in determining the retention of
platJJium-191, as shown in Figure 6-1. Retention is greatest after intravenous
administration, next greatest after Intratracheal administration and least after
oral administration. In addition, retention after intratracheal administration
is greater than that after inhalation (Figure 6-2) ,272 In agreement with these
data is the fact that most of the platinum-191 given orally is excreted in the
feces, whereas platinum-191 given intravenously is excreted in both the urine
and the feces, with urine containing a great quantity of it. Tissue distri-
bution studies387 7 days after platinum-191,193 administration showed that the
kidneys contain the greatest amount of radioactivity per gram, followed by the
spleen and liver. Data derived from pregnant rats272 given platinum-191 on the
eighteenth day of gestation and sacrificed 24 h later revealed a small amount of
transplacental passage of the isotope (0.01%/g of fetus), but there appears to
be placental binding or accumulation.
The fate of palladium has also been studied in rats with 103PdCl_ given by
the intratracheal, inhalation, oral, and intravenous routes.272 As with plat-
inum, the amount of palladium-103 retained after a single dose depends on the
route of administration. The retention declines rapidly after oral administra-
tion to about 0.4% of the initial dose after 72 h; less rapidly after intra-
tracheal administration; and least rapidly after intravenous administration,
with approximately 10% of the initial intravenous dose being retained within
the body 76 days later. The amount of palladium-103 retained after intra-
tracheal administration is higher than that after inhalation exposure. In
addition, the retention time for palladium-103 declines rapidly in the suckling
rat after oral administration, in a fashion similar to that in the adult; how-
ever, the amount absorbed and retained with time is significantly higher
6-27
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Percent of Initial inPt
48 12 18 20 24 28.32
D«y» After Dosing
FIGURE 6-1.
Whole body-retention of platinum-191 in adult rats after
intravenous, intratracheal, and oral administration.
(Reprinted from Moore et al. )
6-28
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60
50
40
30
20
10
Percent of Initial 1f1Pt
Retained
••Adult Inhalation
a Adult Intratraeheal
4 8 12 16
Days After Dosing
20
FIGURE 6-2.
Whole-body retention of platinum-191 in adult rats after inhala-
tion and Jjitratracheal administration. (Reprinted from Moore
et al.27 .)
6-29
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(Figure 6-3). It is interesting that, for a given route of administration,
platinum and palladium appear to be excreted at about the same rate.
The fate of rhodium in rats was studied with carrier-free rhodium-105
given intravenously or orally. 124,387 Rhodiu^ios ^3 not found to a great extej
in any of the tissue 96 h after oral administration, the kidneys having the
highest amount—0.04% of the administered dose. However, after intravenous
administration, 45% of the dose was still retained by the rat, with the
greatest amount being in the kidneys—1.5%/g of kidney. Prolonged applica-
tion rhodium chloride to rabbit skin results in degenerative changes in the
kidneys and liver (this is also true for platinum, but not for palladium and
ruthenium). *^°
The fate of carrier-free iridium-192 was measured in rats 2 h, 7 days,
and 33 days after intravenous injection.124,387 TWO hours after injection,
18% of the iridium had been excreted in the urine, and the greatest tissue
concentrations on a percent-per-gram basis were in the liver, kidney, and
lung. Seven days after injection, 38% had been excreted in the urine and
14% in the feces. The kidneys, liver, and spleen contained the highest
amounts on a per-gram basis. Thirty-three days after intravenous injection,
44% had been excreted in the urine and 36% in the feces. Iridium-192 was
still concentrated in the kidneys, liver, and spleen, both in percent per
organ and in percent per gram. The fate of iridium-192 is similar to that
of platinum. When iridium-192 is administered orally, only small amounts
are found in the tissues and organs 7 days after injection, with 94% being
excreted in the feces and 3.6% in the urine. The fraction absorbed from
the digestive tract is estimated at 10%. Data on retention of ruthenium
after inhalation have been reported by Casarett et al.'^
6-30
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80
70
60
GO
40
30
20
10
• Suckling Rat
Oral
Percent of Initial ™Pd
Retained
Adult IV
0 4 8 12 18 20 24 28 32
Dayi After Do«ng
FIGURE 6-3. Whole-body retention of palladium-103 (given asPdc^) in
adult rats after intravenous, intratracheal, and oral ad-
ministration, and whole-body retention of palladium-103
in suckling rats after oral administration. (Reprinted
from Moore et al.272)
6-31
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The fate of carriers-free ruthenium has been evaluated by several investi-
gators.124/ 169,170,414 studies have suggested that less than 0.5% is absorbed
after an oral dose of 103^+4 Or 106Ru+4 to rats, whereas chickens may absorb
3% of an intragastric dose of ruthenium-106.171 Studies with five different
ruthenium-106 compounds showed that the chemical form or compound in which
radioruthenium is administered influences the total amount absorbed. Thus,
for up to 2 weeks after intragastric administration to cats, 2.4% of RuD4 is
absorbed, but 12.3% of RuNO(N03)3.414 The distribution of ruthenium in the
organs of cats is similar to that observed in other species. Thus, 5 days
after administration, kidney, bone, and testes retain the highest proportion
of the ruthenium-106. Nelson et al.288 have shown detailed autoradiographs
of adult male and pregnant female mice at intervals of up to 32 days after
intravenous injection. The metabolism of rutheniun in man has been examined
by Yamagata et al.473 and by Webber et al.451
The retention of osmium after intravenous and intramuscular administration
in rats has been reported by Durbin et al.124 A few other studies of the toxi-
cology of osmium have also been reported.63'152'213'221'408'457
In a study of the effects of various salts of platinum or palladium on
the characteristics of the microsomal drug metabolizing enzyme system, with
doses far exceeding anticipated environmental exposure, it was shown that the
dietary administration of various salts of platinum or palladium for a week
generally decreased or had no effect on the characteristics of drug metabolism
by isolated microsomes. Four or more weeks after dietary administration of
these salts, there was either no effect or, if anything, an increase in the
measures used.197 Considering the high doses used, one can only conclude that
6-32
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probable doses to which the human may be exposed would have little or no
effect on the microsoral nrixed-function oxidase system. Additional studies
of the effects of platinum and palladium on different organ systems have also
been reported.135b,272a,448a,461a
A recent study has attempted to establish baseline data for platinum con-
centration in human tissue before the widespread use of catalytic converters.-^3
(A more detailed discussion appears in Chapter 8.) In addition to the tissue
analyses, the following data are available for 92 of the 97 autopsy sets
studied: age, race, sex, occupation, and cause of death. Clinical summaries,
where available, were studied; these gave no indication that platinum tumor-
suppressant therapy might have been a source of platinum exposure. Similarly,
occupational information available did not indicate occupational exposure to
platinum. Of a total of 1,313 samples obtained from the 97 subjects and
analyzed for platinum, 62 contained detectable platinum; 45 subjects had de-
tectable platinum in one or more tissue samples. Thus, although platinum
was detected in only 5% of the samples analyzed, 46% of the subjects had de-
tectable platinum in at least one type of tissue. Among the cases of detected
platinum, the range of platinum concentrations was 0.003-1.46 pg/g (wet tissue),
the mean was 0.16 ug/g, and the median was 0.067 yg/gm.
The authors ranked the sites of platinum deposition in the subjects
studied according to the frequency of detection of platinum in various tissue
sample types. In order of decreasing frequency, they were: subcutaneous fat,
kidney, pancreas, and liver. The presence of platinum in subcutaneous fat
raises the question of transport, in that most platinum ccmpounds are lipid-
insoluble. Conversion to lipid-soluble compounds through methylation of
platinum compounds is a possible explanation suggested by the authors.123
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However, the relatively small number of samples that actually contained
platinum casts serious doubt on the statistical reliability of such analysis.
INHALATION TOXIdTY OF PLftTINUM-GRDUP METALS AND COMPOUNDS
In view of the possible emission of small particles of platinum and
palladium through attrition from automotive catalytic converters (see Chapter
8)^65,214 inhalation would be the most likely mode of human exposure. Indeed,
inhalation of some platinum-containing salts or acids can provoke rather severe
physiologic responses in people who have been "sensitized,"139/210,317 as will
be explained in more detail in Chapter 7.
The most common current occupational exposure to noble metals is through
inhalation of dusts by refinery workers involved in producing these materials.21"
It is not at all uncommon for an employee of such a plant to develop an asthmatic
or dermatologic allergy that disappears when the person is removed from areas
where the air is contaminated with these materials. It is thought that the com-
plex platinum salts cause release of histamine that accounts for the asthma or
hay-fever symptoms.301'302'303'379 Apparently, the platinum metals themselves,
even in a state of very fine subdivision, do not invoke significant physiologic
responses when inhaled; the active species are some complex compounds of the
metals. With respect to the two most common metals, compounds of platinum are
by far more active allergens than those containing palladium. Recognition of
this susceptibility has led the Occupational Safety and Health Administration
to set a maximum of 2.0 yg/m3 (based on 24-h exposure) for exposure to airborne
soluble platinum salts.^3? The standard is similar for osmium tetroxide,^37
33|
and the maximal allowable exposure to soluble rhodium salts is set at 1.0 yg/fo -I
6-34
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CHRONIC EXPOSURE TO PLftTINUM, PALLADIUM, AND REIATED COMPOUNDS
Animals
In experiments reported by Schroeder and co-workers384'385 toxic effects
of small doses of scandium, hexavalent cshrcmium, gallium, yttrium, indium,
rhodium, and palladium on growth and survival in mice were evaluated. They
raised 958 mice in an environment limited in metallic contamination and gave
than metal at 5 ppm in drinking water from weaning until natural death. Body
weight was measured every month for 6 months, at 12 months, and at 18 months.
Compared with controls, the feeding of gallium was accompanied by significant
but not marked suppression of weight at 14 of 16 measurements (eight in each
six); of scandium, at 10 measurements; of indium, at eight; of palladium,
at seven; of rhodium, at six; of yttrium, at 12; and of hexavalent chromium,
at eight. Survival of gallium-fed females was less than that of controls,
whereas survival of palladium-fed males and yttrium-fed males and females
was greater. Tumors were found at necropsy in 16.3% of one group of controls
and in 27.4% of the scandium-fed, 26.0% of the gallium-fed, 13.0% of the indium-
fed, 28.8% of the rhodium-fed, and 29.2% of the palladium-fed groups. Malig-
nant tumors were increased in the rhodium and palladium groups, at a minim-ally
significant level of confidence (p_ < 0.05), all but one tumor being malignant.
In a second series, tumors were present in 26.8% of controls, 27.6% of the
chromium-fed mice, and 33% of the yttrium-fed mice. All tumors in the latter
two groups were malignant. Therefore, rhodium and palladium appear to exhibit
slight carcinogenic activity in mice.384
A silver-palladium-gold dental alloy imbedded subcutaneously for 504
days caused tumor formation in seven of 14 rats. 3 Bnplants of a silver-
palladium-gold-copper dental alloy (70.02, 24.70, 5.23, and 0.03%, respectively)
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in the oral submucous membranes of rabbits had only mild effects, but liver im-
plants in rats temporarily constricted capillaries of the liver parenchyma, and
testicular implants in rats caused seminiferous-tubule degeneration. 166
Ridgway and Karnofsy353 evaluated PdCl for teratogenic effects in chicken
eggs and found it to be nonteratogenic . The LD^Q was greater than 20 mg/egg on
the fourth day of incubation.
Studies evaluating platinum, palladium, and other members of this group
for mutagenic, teratogenic, and long-term effects (other than the study reported
above) are lacking, and investigation is urgently needed.
Osmium metal apparently is nontoxic, but the toxicity of the tetroxide
in both animals and man is well known. The volatile osmium tetroxide affects
the eyes, causing a halo phenomenon, and also affects the lungs in both experi-
mental animals and man.63 More information on somium appears in these references
Humans
The toxic and potentially toxic effects of platinum in humans are thought
to involve the water-soluble platinum salts (potassium hexachloroplatinate,
potassium tetrachloroplatinate , sodium chloroplatinate , and ammonium chloro-
platinate) , and not platinum itself.1'139'210'354 However, Schwartz et al.386
and Ledo-Dunipe reported that dermatitis had resulted from contact with
platinum oxides and chlorides, and the sensitization of skin to platinum during
the process of soldering has been reported.
The subject of allergy to platinum and its compounds is covered more
thoroughly in Chapter 7 and is introduced here because of its relevance.
"Platinosis" has been defined as "the effects of soluble platinum salts on
354
people exposed to them occupationally." The condition is characterized by
6-36
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pronounced irritation of the nose and upper respiratory tract, with sneezing
and coughing. In some cases, radiographic examination of the lungs has indi-
cated a low-grade pulmonary fibrosis. The first symptoms, arising during
exposure and persisting for about an hour after leaving work, are pronounced
irritation of the nose and upper respiratory passages, with sneezing, running
of the eyes, and coughing. Later, the "asthmatic syndrome"--with cough, tight-
ness of the chest, wheezing, and shortness of breath—develops, becoming
progressively worse with the length of employment.63 Hunter et al.210 re-
ported that concentrations near the threshold limit value for platinum salts
produce symptoms in some people. However, exposure to platinum-metal dust at
much higher concentrations does not produce these symptoms among the industrial
population.354 The hypersensitivity response attributed to platinum is an im-
portant consideration in this matter. Evidently, a considerable proportion of
the population responds to platinum salts as though they were allergens; once
a person is sensitized, he never seems to become asymptomatic in a platinum
atmosphere. There are predisposing factors that should be considered re-
garding exposure of the general population to any amount of these materials.
People with light complexion, light hair, blue eyes, and delicately textured
skin appear to be considerably more susceptible than darker-skinned, brown-
eyed people.354 Long exposure of men to platinum and platinum compounds in
refining has been associated with subsequent formation of allergic reactions
and dermatitis. However, even after 18-20 years of repeated exposure to
threshold-limit concentrations of platinum compounds, no increase in incidence
of cancer has been observed.2x4'354 Information on the mutagenesis of platinum
271
and ruthenium complexes was given in a paper by Monti-Bragadin et al. x
6-37
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A recent preliminary industrial-hygiene survey of workers who handle the
catalytic material was conducted by the American Cyanamid Company (R. M. Cline
and W. V. Andresen, personal corraunication). Workroom air concentration of
platinum was near the threshold limit value (2 yg/m ) for the water-soluble
salts of platinum.15 However, this material was considered to be the dust of
the final catalyst product, not the water-soluble salts. All the workers had
received annual physical examinations that demonstrated no signs of allergic
reactions or other disease process, even after more than 10 years of exposure.
A urinary-excretion study of these workers showed a significant increase in
excretion of platinum at the end of a work cycle, compared with the beginning.
In addition, the concentration of platinum was greater in workers at the
beginning of a work cycle than in controls who presumably were not exposed to
the dust.
Regarding other possible adverse health effects, including postulated
354
aggravation of preexisting cardiorespiratory problems, Roberts stated that
"platinosis" does not appear to alter general health in any way other than to
irritate the upper respiratory tract and to cause contact dermatitis.
Among the platinum-group elements, the allergic reaction is evidently
peculiar to platinum. The only reported case276 of contact dermatitis from
palladium occurred in a research chemist who had been studying various precious
metals for several months. His face, hands, and arms showed patches of eczema.
The condition cleared up completely on avoidance of palladium exposure and
treatment with betamethasone valerate ointment.
Palladium chloride has been used (ineffectively) to treat tuberculosis
at a dosage of about 18 mg/day. Oral dosages up to 65 mg/day apparently
6-38
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produce no adverse effects. Topical application of palladium chloride as
a germicide does not cause any skin irritation.
Palladium hydroxide has been used to treat obesity- Colloid palladium
hydroxide injections (5-7 rag/day) reportedly caused a 19-kg weight loss in a
3-month period, with necrosis at the injection site.262
Palladium has been found in teeth that contained palladium-alloy fillings.
Presumably, small amounts of palladium are solubilized by body fluids. Traces
of palladium have also been found in human liver.^2^
Additional information on acute and chronic toxicity and pharmaoologic
properties of the platinum-group metal ccmpounds can be found elsewhere.28a,28b,
65a,129a,145a,238a,412a,413a,419a,425a,428a,451,473
6-39
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CHAPTER 7
ALLERGY TO PIATINUM COMPOUNDS
The first report of undue reactions to platinum compounds, by Karasek
and Karasek230 in 1911, concerned photographic workers who suffered from
severe upper and lower respiratory tract and skin disorders. Similar clini-
cal manifestations—rhinitis, conjunctivitis, asthma, urticaria, and contact
dermatitis—have since been reported mainly in chemists and workers engaged
in the refining of platinum.183'210'244/301/311'354 until recently, such
subjects provided the only clinical material for the study of allergy to
platinum compounds, and they are still the major source. However, reports
by Khan et al.233 in 1975 and others of anaphylactic reactions in patients
under treatment with antitumor platinum coordination compounds3*'7 now provide
an additional' source. The findings in these subjects are guides to the in-
vestigation of the role of platinum compounds in allergic sensitivity and
to the planning of prospective serial studies involving the ordinary popula-
tion in case environmental exposure becomes significant. It appears that
platinum compounds are unique in their ability to cause allergic reactions;
ccropounds of the other platinum-group metals (with the possible exception
of rhodium) do not show such activity..
The assumption that the clinical manifestations are due to allergic re-
sponses, rather than toxic or irritant effects, is based on three traditional
criteria: the appearance of sensitivity is preceded by previous exposure
without apparent effect; only a fraction of exposed subjects react to exposure
and show evidence in the form of results of skin and provocation tests; and
7-1
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the affected subjects shew increasingly high degrees of sensitivity to
amounts that are almost always far below those encountered at work or under
ordinary circumstances. It is necessary to distinguish between environmental
exposure, which is regarded as unlikely to cause sensitization, and the in-
finitely smaller doses that are capable of eliciting reactions in already-
sensitized subjects. Furthermore, prolonged clinical exposure to small doses
of allergen may sensitize people, as has been shown in some forms of extrinsic
allergic alveolitis, and it is not improbable that this applies as well to
asthma.
With respect to many chemical agents, only a low proportion of exposed
subjects show evidence of sensitivity. By contrast, the incidence of sensi-
tivity to platinum compounds among refinery workers can be very high, showing
the allergenic potency of complex platinum salts, and it can develop rapidly.
In one factory,2^ 71% of 91 employees were affected, 57% with asthma and 14%
with skin manifestations; in another factory,3°1 65% of the subjects were
affected. A 5-year survey354 of 21 subjects showed all to be affected with
about 40% regarded as asymptomatic, but showing conjunctivitis and nasal
mucosal changes, and about 60% having respiratory tract or skin manifestations
or both. In most cases, complete cessation of exposure is followed by dis-
appearance of the clinical disorders, presumably because the causal agents are
unlikely to be encountered in ordinary life.
There is only scanty immunopathologic, but very suggestive clinical, in-
formation on the mechanisms likely to be responsible for the different forms
of clinical reaction to the platinum compounds. In the absence of satisfactory
in vitro iitmunologic tests, direct tests performed on the sensitized subjects
themselves serve in this respect as models. The analysis of the allergic
7-2
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reactions has to be based/ by analogy, on the four main types of allergic
reaction. 5 These may serve as guides to relevant test procedures already
available or urgently requiring development.
T¥PE I ALLERGy.; IMMEDIATE, ANAPHYIACTIC
The Type I allergic reaction is immediate and is mediated by antibodies
that can sensitize mast cells and basophils. This is the most striking and
likely to be one of the most informative types of allergic reaction for in-
vestigation, not only of occupational, but also of nonoccupational environ-
mental exposure. Clarification of various immunologic terms and assessment
of how it should be sought are therefore essential. Their relevance to
platinum sensitivity is illustrated below.
Type I, IgE-Mediated Allergy
The production of IgE antibody is characteristic of, although not con-
fined to, a group in the population, termed "atopic," who readily become
sensitized by ordinary exposure to common environmental allergens and who are
none likely to do so on exposure to occupational agents, also. This group of
subjects can serve as biologic monitors of environmental allergens, hence their
importance in this and similar contexts.
The term "atopy" is, however, used in a variety of ways: to describe
one or more of the clinical manifestations, to indicate the presence of Type I,
I$E allergy, or some combination of both. Not only are there differences of
opinion on the definition of the clinical entities—for example, asthma329—
but they may or may not be associated with the presence of IgE antibody.
7-3
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Use of the term "atopy" to describe the inmunologic reactivity of the
subject, irrespective of the presence or absence of clinical manifestations,
would emphasize the outstanding feature of these subjects, however defined,
and would provide a generally acceptable criterion for classification of
30R
subjects as atopic or nonatopic. The main proviso here is that the test
procedures — skin and serologic — should be such as to give the most specific
information.
Skin-Test Procedures
Their high allergenic potency makes it necessary to take great care with
platinum salts/ as shown by severe reactions to scratch and intracutaneous
139 ,244, 441
The main methods for skin-testing for Type I allergy are the prick test,
the scratch test, and the intracutaneous test. The other methods of testing
may suffice for some clinical purposes; but, because it affords the greatest
scientific accuracy and for other reasons, the prick test is the most acceptabl
being the simplest, the safest, and the most precise. It is performed by prick
ing with a gentle lifting motion through a drop of test solution into the super
ficial epidermal layers, with a separate fine hypodermic needle for each prepar
tion. When it is carefully done, and preferably on the flexural surface of the
forearm, no blood is drawn and no reactions are elicited at control or negative
sites, except in subjects with dermographism. This makes it possible to read
even very small wheals as unequivocally positive. These reactions should have,
in addition to the wheal, an erythematous flare and are often accompanied by
itching. The painless simplicity of the method makes it easy to do a series
of tests, starting with the weakest concentrations and repeating doubtful tests
7-4
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rapidly in duplicate or triplicate, as desired, to ensure a confident interpreta-
tion. Repetition of tests for this reason is seldom necessary in practice.
Reactions to the prick test show the best correlation with tests for
specific IgE antibody in the serum and, when there are symptoms, with the history
and provocation tests.415 Prick tests also reduce considerably the production
of nonspecific reactions due to the trauma of testing or to the effects of
histamine-liberating agents. A comparison of the threshold concentrations re-
quired to elicit wheal reactions to histamine and to histamine-liberators shows
/ 1 O
that the prick test, which introduces 3 x 10"6 ml into the skin, requires a
concentration 103-104 greater than that for the intracutaneous test, which intro-
duces an estimated 0.02 ml into the skin. Concentrations of extracts of cannon
allergens are usually 10-100 times higher for prick tests than for intracutaneous
tests. This still leaves considerable leeway for avoiding nonspecific wheal
effects of test preparations of cannon allergens or other agents.
The exquisite sensitivity of the test is shown by the positive reactions
to prick tests that introduce into the epidermis 3 x 10 ml of platinum salt at
~9 96 311
a concentration of 10 g/ml. ' This gives an absolute skin-test dose
of 3 x 10"^ g of the test compound! This degree of sensitivity emphasizes
the need for test procedures that avoid nonspecific reactions, whatever their
source, and the care required for excluding even very slight contamination of
inactive platinum compounds with the highly allergenic complex platinum salts.
3he production of urticaria in some sensitive subjects is probably due to the
local effects of the platinum salts on the skin and to systemic effects of in-
haled or ingested platinum salts.
Scratch tests are not as suitable as prick tests, because they produce
more traumatic nonspecific wheals and are more difficult to standardize.
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With the prick test,354 it was found that most unexposed persons react to
10~2 to 10~1-gAa concentrations of sodium chloroplatinate, and many to a
379
10~3-g/ml concentration. This salt has been reported to be the most potent
histamine—liberator of the complex platinum salts.
Prick Tests and Atopic Status
Evidence of specific IgE antibody to common relevant environmental allergen!
is strongly suggested by wheal reactions to carefully performed prick tests;
this suggestion is confirmed by such tests as the radioallergosorbent test
(RAST) for specific IgE antibody in the serum. In practice, prick tests with
a small battery of extracts of conroon allergens suffice to classify subjects
for epidemiologic purposes into atopic and nonatopic immunologic groups, as
defined308 earlier.
A double-blind, statistically controlled study of repeated prick tests
with a battery of conmon allergens at monthly intervals for up to a year in
"it
a group of about 50 asthmatic subjects showed no evidence of the induction
of sensitivity to extracts that initially gave negative tests. There was a
remarkable consistency of positive reactions to the relevant allergens, with,
if anything, a tendency for reduction in the sizes of the wheals. These find-
ings are pertinent to the use of such tests for immunologic screening of the
population (atopic versus nonatopic) and to the epidemiologic use of repeated
serial tests.
The main extracts used so far have been of house dust; of the house-dust
allergy mite Dermatophagoides, either D.pteronyssinus or D.farinae; of pollen;
and of moulds and animal danders. As might be expected, the subjects who have
positive reactions to one or more of these tend to become sensitized to other
C. W. Clarke and J. Pepys, personal communication.
7-6
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allergens more rapidly and more frequently. This has been shown in workers en-
gaged in the manufacture of biologic detergents, in whom the chances of Type I
sensitization were twice as high in atopic as in nonatopic subjects.159'290'312
With respect to platinum-refinery workers, a similar association was re-
ported354 in subjects with a history or family history of hay fever, asthma,
urticaria, and contact dermatitis. In workers of different ethnic populations
in two refineries, one in the United Kingdom and the other in South Africa
(I. Webster, personal communication), the relationship of atopic status (with
regard to prick-test reactions to common allergens) to the appearance of posi-
tive reactions to 10~9- to l(T3-g/ml solutions, mainly of (NH4)2[PtClg], is
shown clearly.
In a prospective study of 212 UK workers, 64 were regarded as atopic,
having had positive prick-test reactions to one or more of the common allergens
(the remaining 148 were regarded as nonatopic). None reacted to the platinum
salts at 10~3 g/ml before starting their employment. Later, a subgroup of
50 workers who had positive prick-test reactions was subjected to further study.
Of these 50, 40 also had clinical symptoms of allergy to the platinum salts.
Twenty of the 50 were atopic. Thus, 20 of the original 64 atopic subjects had
become sensitized, compared with 30 of the 148 nonatopic subjects; this differ-
ence is significant at the 1% level. Another nine subjects had negative reac-
tions to prick tests, but had symptoms suggestive of allergy to the salts. The
high degree of sensitivity in such subjects, 18 of whom were tested further with
other salts, is shown in Table 7-1 ;96 many of these and others reacted to con-
centrations of 1(T7 - 10~9 g/ml.
In the other refinery, of 169 subjects, including some who had previously
been engaged in the industry, 39 had positive prick-test reactions to
7-7
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TABLE 7-la
Prick Test Results
oo
• t . J . (
Teet Concentration Given Verloui Subjecte with Meeo Dteei e» Weal
Teet
Flellmm Coaplex Ser. A I
(NH4)2|PtCl6) (e) 10-*(3) 10-5(3)
(b) 10"5 10"6
(0
(d)
(HH4)2PtCl4 10-6 10-6
(mi4)2|Ptlr6|
Ce2|Pt(N02)Cl} (•) 10-*(+) 10-5(4-)
(c)
Ce2Pt|N02)2Cl2 10-1(±) 10-*(2)
Ce2|Ft(H02)3Cl| (e) 10-1(1) 10~1(2)
(b)
K2Pt(ND2)4
(Ft(MI])4MFt(Ml3)Cl3l
£le|Ft(NH])2Cl2)
rreng|rt(NU))2Cl2]
el»-|Ft(Cll3MI2)2Cl2|
tiene-|Pl
-------
(NH4)2[PtClg] or Na^PtClg]; 17 of these were atppic, and 22 nonatopic. The
incidence of atopy in the total group is not known. Of the 17 atopic subjects,
six were skin-test sensitive after 1 month, five after 2-3 months, two after
5-8 months, and four later than this. Thus, in 11 of the 17, sensitization
was evident by the fifth month. Of the 22 nonatopic subjects, sensitization
was evident in six by the end of 5 months. Only one nonatopic subject showed
sensitization within the first 2 months. The remainder required periods of
exposure longer than 5 months for sensitization. The difference between the
incidence of sensitization among the atopic and nonatopic subjects at 2 months
was highly significant; at 5 months, it was significant at the 5% level. Of
people with positive prick-test reactions to (NH^IJPtClg], one reacted to
10~7 g/ml, 10 to 1CT5 g/ml, seven to 10"4 g/rol, and three to 10~3 g/ml. Of
1 —5
those who reacted to Na2[PtClg], one reacted to 10""' g/ml, 13 to 10 g/ml,
five to 10"4 g/fal, and five to 10~3 g/onl.
Analysis of Prick-Test Reactions to Platinum Compounds
The high degree of prick-test sensitivity to the complex platinum salts
96
in sensitized workers made possible an analysis by Cleare et al. of the
allergenic relationships of a number of different compounds.
The test materials consisted of a wide range of halide, nitro, and amine
complexes (Table 7-1). These were prepared by established methods and had
satisfactory elemental analyses. Because refinery workers tested were all very
sensitive to [PtClg] ~ or [PtCl4]2~, presence of these species as impurities
at very low concentrations could give a positive test for an otherwise innocuous
material. For example, contamination, not detectable by elemental analysis, of
7-9
-------
1(T3 could elicit reactions at concentrations of 1(T3 of the test material in
subjects capable of reacting to the contaminant itself at 10 g/ml.
The results show that an allergic response is elicited by charged com-
plexes containiiig at least one chloro ligand and that, allowing for the
inaccuracies inherent in the measurement of wheal reactions, allergenicity is
related to the number of chloro groups in the molecules, as shown by the follow
ing allergenicity ranking;
(NH4)2[PtCl6] ~ (NH4)2[PtCl4J
> Cs2[Pt(N02)3Cl2] > Cs
the last compound was inactive.
Amine complexes showed the same general pattern. An important difference
is that both cis and trans dichloro complexes—[Pt(A)2Cl2J, where A =
are neutral and give no reactions. The only exception is cis-[Pt(OHEtNE^)
which occasionally elicits a reaction at high concentration. This compound is
very soluble, in comparison with the other neutral species, and may suffer fron
slight contamination; 0.001% of [Ptd^]^"" would be sufficient to cause a positiv
response. A charged species containing t*ro cis chloro ligands [Pt(gly)Cl2] is
slightly active. Amine species with one chloro group are either slightly active
or inactive, irrespective of charge, although contamination may be a problem.
The fully substituted amine [Ptftffl^^d^ is totally inactive, as is
[Pt(tu)4]Cl2, another complex in which all the coordinated chlorides have been
replaced.
7-10
-------
Changing f ran chloro to bromo ligands yields complexes that are less
physiologically active, so that the order (NH4)2[PtCl6] > (NH4)2fPtBr6]
is observed in all cases. [Pt(dien)Br]Br and [Pt(dien)l]l are inactive, as
is the corresponding chloro complex.
The reactivity of the complexes depends largely on the leaving ability
of the ligand that is being replaced. Table 7-2 shows the general conclusions
that can be drawn from these results. Reactions 1 and 2 (Table 7-2) are prob-
ably most relevant, in that coordination positions other than the leaving group
under study are occupied by an inert trichelating ligand. Thus, such leaving
groups as the halogens are consistently fairly labile and reactive, whereas
nitro, -NO, , and thiocyanato, -SCN~, groups are more inert and much less re-
active. The platinum-amine linkage is very stable and inert to nucleophilic
attack; i.e., amines are very poor leaving groups.
The results of the allergy tests can be explained on this kinetic basis.
Ocmplexes containing strongly bound ligands with poor leaving abilities are
not allergenic, presumably because there is little or no reaction with pro-
teins—e.g., [Pt(NH3)4]Cl2, K2[Pt(NO2)4], and [Pt(tu)4]Cl2. Allergenic ac-
tivity increases with the number of halide ligands, the chloro complexes being
more effective than bromide, as forecast by Reaction 2.
Three postulates are examined for the platinum-protein interaction. Pro-
tein reaction^ 13,460 with more than one platinum coordination position, such as
protein bridging, is unlikely, because compounds with only one reactive group
are allergenic. The antibodies elicited by exposure to [PtCl^] seem capable
of reacting with different structures within the approximately planar coordination
around the platinum (II) center attached to a protein or other macromolecular
carrier.
7-11
-------
TABLE 7-2
Relative Lability, k2 (X)/k^ (N3) , of Ligands
Displaced fron Pt(II)
[Pt(dien)X]+ + py -»- [Pt(dien) (py)]2+ + X~ (D
(water, 25° C)
Cl>Br>I>N > SCH > N09 > CN
3 c
40 27 12 1.0 0.36 0.056 0.02
[Pt(dien)X]+ + Y~ •*• [Pt(dien)Y]+ + X~ (2)
(water, 30° C)
I s Br > Cl > N3 (> N02)
73 70 33 1.0 ?
[Pt(bipy) (N02)X] + Y~ -> [Pt(bipy) (N02)Y] + X~ (3)
(nethanol, 25° C)
I > Br > Cl > N02 > N3
900 240 140 7 1.0
fron Cleare et al..96 Basolo et al. ,31 Belluco et al. ,40 and
77
dien = diethylenetriamine,
py = pyridine
bipy = 2,2'-bipyridyl
'w A_A
«. \j4j
7-12
-------
•The fact that Pt(IV) and Pt(ID chloro species are equally aLtergenic
in the same individual may be due to reduction of Pt (3V) to Pt (II), as
postulated for the anticancer agents. Pt(W) species are usually kinetically
inert, unless there is a reaction through a Pt(II) intermediate. The possi-
bility that Pt(IV) will react in its own right in such a highly sensitive
biologic system is slight, because PtCH) is not much more allergenic than
pt(IV). However, one cannot rule out the possibility that the sensitivity
to Pt(IV) compounds observed in the refinery workers was due to their working
under refinery conditions where they were initially sensitized to Pt(IV).
Prick tests were made on one highly sensitive subject who showed positive
reactions to (NH4)2[PtCl4] and (NH4)2[PtCl6] at 10~9 g/tol. Additional tests were
made on the same subject with equimolar conjugates of these salts containing
human serum albumin, transferrin, and ovalbumin and with a mixture containing
the salt and a conjugate of human gammaglobulin in a 3:1 molar ratio of pro-
tein to platinum salt. The platinum content of these conjugates ranged from
6 to 27 ppm (by weight). All produced negative reactions; this raised ques-
tions about the nature of the carrier protein when the salts themselves are
used for testing and about whether the spatial arrangements of the platinum
ions are such as to enable bridging of IgE antibody molecules on the surface
of the skin mast cells. Ishizaka and Ishizaka2^^ found that 7 S molecules
with a molecular weight of about 30,000 are necessary for this bridging, which
triggers the release of histamine from the cells. It is conceivable that the
platinum conjugates do not fulfill this requirement.
In summary, the allergenicity of the platinum compounds in the refinery
subjects is confined to a small group of charged compounds containing reactive
ligand systems (such as chloride and, to a lesser extent, bromide) and is
7-13
-------
directly related to both their charge and their overall reactivity. The re-
finery workers sensitized to specific platinum complexes apparently do not
react to the majority of platinum compounds.
Allergic Reactions to a PlatinumHjoordinationr the latter compound and DDP did not
produce reactions in refinery workers who were highly sensitive to
(NH4)2[PtCl6] and (NH4)2[PtCl4]. The patient reported by Khan et al.233
did not, however, show sensitivity to some square-planar complexes, such as
platinum blue, platinum(II) 1,2-diaminocyclohexane malonate, and platinum (II)
ethylenediamine malonate. Significant histamine release was obtained on incu-
bation of the patient's leukocytes with DDP. A raised serum content of IgE
was found, as has also been the case in three of 22 sensitized refinery workers
The differences between the refinery workers and the affected patient may
be attributable to differences in their modes of sensitization and the chemical
agent responsible. The possibility of contamination with minute amounts of a
complex platinum salt of the therapeutic platinum coordination compounds has
to be considered, but this would not explain the absence of reaction to DDP,
7-14
-------
etc. , in the refinery workers, nor would it account for the positive reactions
to DDP, diaquo-DDP preparations, and other coordination ccqpounds, as well as
to the complex platinum salts, in the affected patient.
RadJoallergosorbent Tests for Specific IgE Antibodies to Platinum Compounds
and Conjugates in Sensitized Refinery Workers
Radioallergosorbent tests31'40'77'220'222'460'461 (RAST) have been used
with a wide range of conjugates for the presence of an IgE antibody specific
to platinum conjugates. In spite of the high degree of Type I skin-test
reactivity to the platinum salts, it has not been possible to demonstrate
IgE antibodies.
The agents used so far for linking to paper disks for the RAST include
conjugates of (NHg^CPtClg] and (NEfy) 3 [PtClg ] with human, bovine, and rabbit
serum albumin, ovalbumin, gelatin, and collagen; bovine gamma globulin; whole
dialyzed human and rabbit serum; albumin-depleted human serum; and human skin
sections. Molar ratios of coupling of platinum to protein have ranged from
10:1 to 1:100.
Platinum compounds were also linked to Sepharose (Pharmacia) particles
via an amino acid derivative. These included ^[PtB^QK^^]/
(NH4)2[PtBr6], cis-Pt(NH2CH3)2Cl2, (NH4)2[PtCl4], and (NH4)2[PtCl6] .
Passive-Transfer Tests in Man and Monkey
The report13 of positive reactions to passive-transfer tests in a non-
sensitive human recipient, in whom positive reactions were elicited by tests
of the sensitized sites with platinum conjugates, suggests the presence of
specific IgE antibody.
7-15
-------
Passive-transfer tests (W. E. Parish and J. Pepys, personal oatmuriicatLon;
Parish299'300) (with unheated and heated serum of 10 sensitive subjects) for
the presence of specific IgE and short-term specific IgG antibodies were made
in three monkeys that were given intravenous injections of
Na2[PtCl4], and (NH^tPtClg] with Evans blue. All the tests were negative.
The difficulty of demonstrating IgE antibodies specific for the small-
molecule substances such as platinum even when conjugated to carrier proteins
is common in allergy to small-molecule drugs. In the case of penicillin allerg
conjugates of penicilloyl determinants with polylysine and serum proteins have
been effective in PAST for demonstrating specific IgE antibody,461 but this is
an exception, rather than the rule. Because specific IgE antibody, if present,
against platinum conjugates could constitute one of the most sensitive laboratc
tests for sensitization (particularly in atopic subjects) and thus provide evi-
dence of atmospheric contamination with appropriately reactive platinum deriva-
tives, it is urgent that such a test be developed.
Immediate Reactions to Histamine Tliberation by Complex Platinum Salts
Immediate reactions to histamine liberation by complex platinum salts
are relevant to skin and other tests, but are not likely to be playing more
than a subsidiary role in sensitized subjects who can react to very small
doses that are not histamine-liberating. Such an effect could be expected
in all subjects, irrespective of sensitization.
It has been reported that sodium chloroplatinate, in particular, liberate
histamine on intravenous injection into guinea pigs and on addition to guinea
pig ileum.302'379'415 During investigations183 in which 0.1-ml volumes of
10~4 concentrations of complex platinum salts were injected intracutaneously
into guinea pigs previously treated with intravenous injections of Evans blue,
7-16
-------
it was found that the sodium chloroplatinate and chloroplatinite salts caused
the most exudation of dye; arnnonium chloroplatinate and chloroplatinite caused
trace to moderate exudation. Conjugates with human serum albumin of ammonium
chloroplatinite and sodium chloroplatinite and chloroplatinic acid produced
only trace exudation of dye. This demonstration that salts other than sodium
chloroplatinate may cause histamine liberation on introduction into the skin
of guinea pigs makes appropriate controls for such tests necessary.
TYPE II ALLERGY: AUTQAT.T.KRGIC
In Type II allergy, the haptene combines with a cell surface to produce
the complete antigen against which an immunologic response may be mounted.
•this is a form of autoallergic reaction. The capacity of the platinum co-
ordination compounds to combine with ENA on the surface of malignant cells
and T-lymphocyte& makes it necessary to keep this form of allergy in mind
as a possibility with platinum compounds.
TYPE III ALLERGY; PMUNEK3QMPI3SX, CX3MP3JMEINT-DEPENDEIQT
The production of precipitating antibodies and their combination with a
moderate excess of antigen lead to the formation of toxic soluble complexes
that fix and activate the C3 component of the complement. Tissue-damaging
Type III reactions ensue from the effects of these enzymatic aggregates.
The possibility of Type III allergic reactions to platinum salts is
suggested by the findings (W. E. Parish and J. Pepys, personal communication)
of positive passive-transfer reactions in the guinea pig, in which the
heterologous human precipitating antibody sensitizes mast cells, so that
histamine is liberated on allergen challenge.
7-17
-------
In these tests, each guinea pig was passively sensitized by intracutaneous
injections of 0.05-0.1 ml of the serum of the particular group of subjects and
4 h later was given intravenous injections of Evans blue and 1 - 1.5 mg of
the particular platinum salt or conjugate. In the first series of 10 subjects,
the tests were made with Na2[PtCl6], with Na2[PtCl4], and with (NH^tPtClg].
The serum of two of the subjects gave reactions regarded as positive; in one
of these, the two specimens were taken at different times. Reactions were
elicited with serum of both subjects to (M^^tPtCl^ and with serum of one
of them to Na2[PtCl4] as well- In the second series of nine different sub-
jects, the platinum salts or conjugates were injected intracutaneously into
sites passively sensitized 4 h earlier; the guinea pigs had been treated
previously with intravenous injection of Evans blue. In this test, serum of
three had weak or faint, equivocal reactions to 1-mg test doses of
(NE^^tPtd^ and Na2[PtClg]; the ser[m °f one of "them also had a faint
reaction to (NH4)2[PtClg]. Tests with conjugates of Na2[PtCl4] with human
serum albumin and of (NH^) ^PtCl^] with horse serum produced no reactions
at all. The fact that serum of three of the nine had these weak and uncertain
reactions, whereas all the others were negative, suggests that they may have
been positive; but the reactions were not unequivocal, as was the case with
the serum from the two guinea pigs. In a third series of tests with serum of
25 guinea pigs, the guinea pig skin was sensitized with 0.1 ml volume of the
serum undiluted and at a 1/10 dilution, and the tests were made by intravenous
injections of 1 mg of Cs2[Pt(N02)3Cl], Cs2[Pt(N02)Cl3l, an<* ^ne^r conjugates
with human serum albumin; cis-PtCA^Cl^ where (A = CE^N!^' ^[PtOK^)^;
Cs2[Pt(N02)2Cl2J and (JJH4)2[PtCl4J. Mb positive reactions were elicited.
7-18
-------
These tests, albeit with only limited results, suggest that precipitating
antibodies are demonstrable in a small number of the exposed subjects. The
production of precipitins requires, as a rule, more intensive antigenic stimula-
tion and continued exposure for their demonstrable persistence in the serum.
Many of the test specimens of serum were taken at least months after cessation
of exposure, which in any case would probably have been relatively limited.
Further suggestions of the possible presence of precipitating antibodies
were provided by a report on hyposensitization of a chemist who was highly
sensitive to (NH^tPtClgJ. A prick test on this patient with an 8% solution
of (NH^tPtClg] caused a severe anaphylactic reaction. An attempt was made
to hyposensitize him with intracutaneous injections of gradually increasing
amounts. When he reached the 5-yg dose, a reaction resembling serum sickness
appeared, with urticaria! skin eruptions, swelling of joints, and red eruptive
papules that showed, histologically, a vasculitis with eosinophil infiltration,
as did some of the injection sites at which necrosis also appeared. This reac-
tion is very suggestive of Type III allergy, an important part of serum-sickness
reactions. The patient's sensitivity to occupational exposure decreased. Auto-
passive-transfer tests (in which serum taken before, during, and after injection
treatment was mixed with the platinum salt and then, after incubation, injected
intracutaneously into the patient's back) showed decreased reaction to the test
with the posttreatment serum mixture, suggesting that blocking antibodies had
been formed. Repetition of the hyposensitization treatment at a later date
resulted in the same pattern of reactions resembling serum sickness. The pa-
tient's clinical sensitivity to (NH^^tPtClg] was also decreased, although
exposure to (NH^tPtCl^ caused eye, nasal, and skin reactions again, which
suggested allergenic specificity of the different salts.
7-19
-------
Tests for the presence of antibodies showed no response to challenge of
his lymphocytes in vitro with (NH^tPtClg] or to a conjugate of (NH4)2[PtCl4]
in solution. Serologic tests produced negative results for precipitins and
hemagglutinating antibodies. Uniformly negative results have also been found
in precipitin and hemagglutination tests and in tests with erythrocytes and
latex particles linked to a wide range of platinum salts and their conjugates
with protein.
Recent studies12? have shown that a number of extracts of organic substanc
implicated in Type III allergic lung reactions in man and animal s309 can also
activate the C3 component of the complement directly (an alternate pathway).
Such possibilities have now to be considered with all agents that give rise
to reactions that have features of Type III allergy, including platinum com-
pounds.
With well-established allergens, it can often be claimed confidently
on the basis of positive skin-test reactions that the relevant allergen is
of clinical importance in the particular subject. Many workers, however,
consider that provocation tests are necessary. In spite of the reported
hazards of skin-testing with platinum salts, 139,441 it is possible to have
not only safe skin tests by the prick-test method, but also nasal tests with
solutions of the platinum salts and occupational-exposure tests for asthmatic
reactions to the dust that rises when mixtures of small amounts with lactose
powder are poured from one receptacle to another. 3H
Nasal Tests
Nasal tests were made^H with 0.01 - 0.02 ml of the test material intro-
duced (after a control test with diluent) into one nostril at the lowest con-
centration and then, if the results are negative, at increasing concentrations.
7-20
-------
positive reactions consist of itching of the nose, sneezing, rhinorrhea,
and sometimes nasal obstruction. These reactions appear within a minute
or two and run the same time course as the wheal reaction to the prick test.
Seven of 11 subjects had positive reactions to concentrations of 10~5 - 10~3
g/ml.
Bronchial Tests
The complex salts of platinum were prepared in powder form as separate
mixtures containing 40 mg of each salt and 1 kg of lactose powder. The con-
trol lactose powder and the mixtures were dried for 20 h at 105° C and then
kept in a desiccator, because the lactose is hygroscopic and loses its
powdery, dusty consistency.
The test consisted of having the patient shake 250 g of the mixture
repeatedly from one tray to another that was 0.3 m below it31^ and inhale
the dust so created. Initially, daily tests were made with increasing
amounts, in microgram quantities, of the platinum salts mixed with lactose,
until it was found that 40 mg in 1 kg of lactose sufficed to elicit reactions
in sensitive subjects.
The maximal total exposure was 30 min, divided into segments of 5, 10,
and 15 min with 10-min intervals between them. Exposure was terminated at any
time if there was evidence of a clinical reaction or decrease in ventilatory
function. Tests of ventilatory function were made before the test and at each
interval during the exposure, then every 10 min for 1 h after its completion,
and then every hour for 8 h or until any reaction had resolved. No further
exposure was made if the 1-s forced expiratory volume (FEV,) decreased by at
7<-21
-------
least 10%, and only one test was made per day. The patients were exposed to
control lactose alone and to a mixture of platinum salt and lactose.
The 16 subjects investigated had worked in a platinum refinery~--mne
for less than 6 months, five for less than a year, and two for many years—
before showing signs of sensitivity. The majority were either quite well
or much improved on leaving the refinery. Detailed measurements of pulmonary
function on admission for investigation showed 10 to be normal in all respects,
five to have slight airway obstruction, and one (who was not found to be sensi-
tive to platinum and who was a heavy smoker) to have evidence of early re-
strictive ventilatory defect with a decrease in carbon monoxide gas-transfer
factor. All were investigated many months after (some more than a year after)
exposure ceased.
Inhalation tests elicited positive immediate reactions, starting in 10 min
and resolving in 1 h, in eight patients, all of whom had immediate prick-test
reactions. These reactions were blocked by pretest inhalation of cromolyn
sodium, just as immediate asthmatic reactions to test with common allergens
are blocked; this suggests a similar mechanism. In one of the eight there
was also a late reaction. In two other subjects, only late reactions were
elicited: in one, the reaction started after 30 min, reached a maximum at
1.5 h, and resolved in 4 h; in the other, the reaction started after 4 h and
reached a maximum at 7 h, when it was reversed by an injection of epinephrine.
Neither of these patients had immediate prick-test reactions; in one, the
nasal test was also negative. Thus, 10 of the 16 subjects had positive
bronchial reactions. Rhinitis was also provoked in six cases, conjunctivitis
in three, dermatitis in taro, and urticaria in one case; some of these occurred
together.
7-22
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The immediate asthmatic reactions are analogous to the Type I skin-test
reactions in speed of appearance and in duration, and they are like those pro-
duced by common allergens when IgE is involved. The late reactions are like
those to conroon allergens when Type III allergy is thought to be present, and
they are compatible with Type III reactions in speed of appearance and in
duration.
It must be pointed out that, although inhalation tests conducted in this
manner are reasonably reproducible and can establish the relative allergenicity
of various conpounds, they cannot be quantitatively related to airborne concen-
trations in micrograms per cubic meter. For this reason, they are not very
useful in helping to set standards. Furthermore, these tests are not very
sensitive measures of the effect of particle size of the allergens, a character-
istic that is probably important in determining the degree of respirability of
the compounds.
Comparison of Different Platinum Salts
Table 7-3 shows that the salts differ in their capacity to elicit reactions,
with annonium tetrachloroplatinate being the most potent and sodium hexachloro-
platinate the least potent. These results show that, with care, it is possible
to make skin and provocation tests safely for establishing the presence of
allergic sensitization to the platinum salts. They can also be used effectively
for identifying the particular allergen or allergens responsible for sensitizing
individual patients.
7-23
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TABLE 7-3
Comparison of Inhalation Tests with Cotplex Platinum Salts0
Platinum
Salt
(NH4)2lPtClfiJ
Patient
1
2
3
4
5
J 6
j
k
7
8
9
Exposure,
mm
15
4
6
10
30
30
30
15
30
Decrease
in EEVi, %
38
28
31
32
0
0
26
29
17
(NH4)s>[PtCl4J
Exposure Decrease
min
10
5
10
10
30
30
15
20
30
in FEVi %
49
45
38
34
18
15.5
30
29
17
Na2lPtCl6J
Exposure, Decrease
mm
10
30
15
30
30
30
30
20
30
in FEVi, %
35
16
23
0
0
0
0
19
0
aData from Pepys et al.311
-------
TYPE IV £T.T.RT3Gy; TWTJffED TUEERCULIN-TyPE
Contact dermatitis is part of the skin manifestations of sensitivity to
platinum conpounds. Type IV allergy has been proposed as the mechanism to ex-
plain such reactions, and both skin (patch) and in vitro lymphocytes tests can
be used for diagnosis of this form of platinum sensitivity. Care is required
with skin tests to exclude the presence of Type I allergy, so as to avoid un-
desirable reactions, and, with lymphocyte tests, to exclude nonspecific effects
of platinum compounds on lymphocytes.
CONCLUSIONS
Investigations for allergic sensitivity to platinum compounds need to
be based, until laboratory tests for antibodies and cellular sensitivity are
developed, mainly on the use of skin tests for Type I (immediate) allergy.
The exquisitely high degree of this type of allergy in which extremely low
doses elicit reactions in sensitized platinum-refinery workers and chemists
endows it with considerable value as a means of assessing the presence of
similar complex platinum salts in the environment and makes it possible to
study cross-reactivity between such substances. Patch tests for Type IV
(contact dermatitis) allergy, which is likely to be far less cannon, may
also find a more limited role. These investigations are appropriate and
are already being applied to refinery workers in whom preemployment and
serial tests are valuable in detecting the development of sensitivity at an
early stage.
The questions posed here are how these findings can be applied to the
investigation of environmental sensitization to platinum compounds in the
general population and who may be encountering them in one form or another
7-25
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as new atmospheric contaminants. The evidence of the greatly increased
capacity for sensitization (not only to common allergens, but also to
platinum salts) of the particular immunologic group in the population
termed "atopic" means that people in that group can be regarded as "biologic
monitors" of environmental allergenic substances.
In the ordinary clinical investigation of atopic subjects, it is common
practice to include routinely, with the relevant cannon allergens or as
additional preparations, tests with other potential allergens. For example,
investigators have for the last 5 years used extracts of the enzymes of
Bacillus subtilis (used in biologic detergents) for routine testing in patients.
These materials caused Type I allergy in heavily exposed workers when they were
introduced and even in small numbers of so-called consumers. 136,310,312 It
was therefore desirable to determine, in communities where these were being
used, the presence or absence of allergy to them. Tests of nearly 2,500
patients,312 now extended to a total of approximately 5,000 patients (Pepys,
personal conraunication), about two-thirds of whom were atopic, showed no
evidence of sensitivity* thus demonstrating that they are acceptable in this
respect in the form in which they are being used. A similar approach is
appropriate with other new potential environmental allergens to provide in-
formation on whether Type I allergy (at least to agents tested) is present or
is developing in the community with the passage of time. Such tests can be
done routinely as patients present themselves for investigation. Selection
of test products can be guided by information from sensitized workers and
perhaps patients under treatment with platinum coordination compounds. The
skin-prick test, if carefully used, is simple, safe, reproducible, and highly
7-26
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precise; it also minimizes problems of nonspecificity due to test methods
or irritancy of test materials, the introduction of such tests into selected
centers as part of routine investigations could be used as a guide to whether
sensitization is occurring and, if sor whether it can be correlated with measure-
ments of platinum compounds in the environment.
Considering the simplicity, reliability, and safety of these skin-prick
tests, coupled with the possibility of human exposure to low concentrations of
platinum-group metals that may enter the environment through new uses, it
seems that failure to include such tests as a routine part of examinations in
appropriate clinics would mean overlooking the most sensitive method now avail-
able to monitor any changes in human sensitivity to these new environmental
pollutants.
7-27
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CHAPTER 8
ENVIRONMENTAL CONSIDERATIONS
The purpose of this chapter is to assess the potential impact of new
sources of platinum-group metals that may provide pathways for their entry
into the environment. Before such an assessment can be properly made, it
is important that the current distribution of the metals be ascertained.
The first section discusses what little information is available on the
concentrations in various segments of the environment and in tissue. The
major potential contributors of platinum-group metals to the environment
as a result of actions of man can be divided into mobile sources and
stationary sources. A discussion of these two categories makes up the re-
maining sections of this chapter.
PRESENT DISTRIBUTION
Soil, Water, and Air
As mentioned in Chapter 2, the concentrations of the various platinum-
group metals in the earth's crust are estimated to range from 10 ppb for
palladium and 5 ppb for platinum to less than 1 ppb for osmium, iridium,
260
and ruthenium. Such concentrations are far too low to allow economic
extraction. Ores in South Africa, Canada, and the U.S.S.R. contain much
higher concentrations—1-10 ppa—and these provide the bulk of the world's
supply of these materials. Before discovery of the palladium-platinum ore
in Jfontana, there were no areas in the United States (except for some deposits
near Qoodnews Bay, Alaska226) where the concentration was thought to be
sufficiently high to allow economic primary mining of the platinum-group
metals.
8-1
-------
Because of the very low concentrations, only a few measurements of
these metals have been reported. The most reliable data appear to be those
collected by the Southwest Research Institute (SWRI) on platinum and palladium
in 1974 (before the introduction of cars with catalytic converters) to estab-
lish a baseline for comparison with future measurements.22^ The data they
collected on surface-soil, ambient-air, and water samples are given in
Table 8-1. The measurements were made with a carefully calibrated Perkin-
Elmer 306 atomic-absorption spectrophotometer. The surface-soil samples
analyzed (except for two samples taken in the precious-metals section of
the Canadian mines) were below the detection limits of 0.8 and 0.7 ppb
for platinum and palladium, respectively. The air samples (except those
in the refinery and salts sections of a typical refinery in New Jersey) were
also below the detection limits of 5 x 10~8 and 6 x IQT^ yg/far^ for platinum
and palladium, respectively (California samples), and 3 x 10~3 yg/m3 for
either platinum or palladium (mine, refinery data). It might be noted that
the air samples taken in the Canadian mines were in the presence of operating
fork-lift trucks equipped with catalytic converters that contained noble
metals and were similar to those now on many U.S. automobiles. Apparently,
there was no detectable emission of platinum or palladium from these devices.
Ml these measurements are well within the OSHA standard of 2 yg/m3 (as
soluble salts) described in the Federal Register. Finally, the same group
analyzed tap water in southern California and water in and around the Sudbury,
Ontario, Canadian mines. Again, they found no measurable amounts of the
platinum-group metals in any of the samples.
On the basis of these very sparse data, it is impossible to make a
sweeping generalization about the concentrations of platinum-group metals
8-2
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TABLE 8-1
Baseline Concentrations of Platinum and Palladium
in Surface Soil, Ambient Air, and Water*2
Type of Sample
and Type of Area
Soil;
Near freeway
Near freeway
Mining
Mining
Mining
Concentration
Location
Lancaster, Ca.
Los Angeles, Ca.
Sudbury, Ont., near mine
Sudbury, Ont., precious-
metals section
Sudbury, Ont., Copper
Cliff mine
Platinum
<0.8 ppb
<0.8 ppb
<0.8 ppb
<0.8 ppb
<0.8 ppb
Palladium
<0.7 ppb
<0.7 ppb
<0.7 ppb
4.5 ppb
2.0 ppb
Air;
Near freeway
Near freeway
Mining
Mining
Refinery*3
Refinery0
Refinery0
Refinery0
Lancaster, Ca.
Los Angeles, Ca.
Sudbury, Ont., engineering
building
Sudbury, Ont., south
mines
Precious-metal section
Furnace room
Salts section
Refinery section
<5xlO"8 yg/m3
<5xlO~8 yg/m3
<3xlO~3 yg/m3
<3xlO~3 yg/m3
0.377 yg/m3
<3xlO~3 yg/m3
0.18 yg/in3
0.16 yg/m3
<6xlO~ yg/m2
<6xlO~8 yg/m3
<3xlO yg/m3
<3xlO yg/m3
0.291 yg/m3
<3xlO"3 yg/m3
0.03 yg/m3
0.09 yg/m3
Water;
Near freeway
Near freeway
Mining .
Lancaster, Ca.
Los Angeles, Ca.
Sudbury, Ont.
<0.08 ppb
<0.08 ppb
<0.05 ppb
<0.024 ppb
<0.024 ppb
<0.015 ppb
Data from Johnson et al. Measurements based on atomic-absorption
spectrophotometry.
.,
OSHA standard, 2.0 yg/nr (as soluble salts).
'Refinery "B," Johnson-Matthey, New Jersey.
8-3
-------
in soil, air, or water. However, it is highly unlikely that harmful con-
centrations of these species exist in air, water, or soil anywhere in the
United States (except possibly within the confines of the two noble-metal
refineries on the East Coast). Additional data are needed to test this
prediction.
Data do not appear to be available with respect to coal. As more and
more coal is consumed in the United States, it becomes important to assess
any potential hazards that might be associated with trace ccmponents, including
the platinum-group metals. It is recommended that data be collected to measure
the concentration of these materials in coal and to determine their fate as
coal is consumed in combustion, synthetic-fuel production, and chemical synthesis
Only recently have baseline data begun to appear on the four minor platirim-
group metals (rhodium, ruthenium, osmium, and iridium). Their concentrations
are below the detection limits of all but the most sensitive instruments.
Data on ruthenium, which is a byproduct of nuclear waste, have been reported
by Brown.62 Although no dramatic increase in use of these metals is anticipated
(except possibly rhodium and ruthenium in catalytic converters), it is important
that this type of information be compiled, if for no other reason than to allow
one to assess their role in biologic processes.
CO
Additional data can be nht-jHncrf from the book by Bowen and the report
by Dawson. •"*
Man
With the increased possibility of human exposure to platinum-group metals
through escape of auto emission control catalysts or through use of antitumor
chemotherapy agents containing the metals, there has been some activity recently
8-4
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to establish baseline data for the body burden of these materials. The po-
tential hazard is outlined in a review by EPA researchers.65
As noted in Chapter 6, studies with radioactively labeled platinum,
palladium, iridium, and ruthenium indicated that the quantities of these
metals not secreted in the urine or feces are retained mainly in the kidneys,
spleen, and liver.387 It might therefore be expected that their concentra-
tions are greater in these organs than in other organs.
Two careful studies have now been completed to measure the concentra-
tions of platinum and palladium in various tissues, but the data appear to
be quite contradictory. The SWRI group responsible for the data in Table 8-1
collected autopsy tissue samples from 10 people 12-79 years old who died from
a variety of causes in southern California; the data are given in Table 8-2.
For all samples analyzed, the concentrations were below the limit of detection
by atomic absorption spectrophotometry—i.e., less than about 1-10 ppb.
Similar observations were made on blood, urine, hair, and feces collected
from 282 people living in southern California, as indicated by the data in
Table 8-3. A composite of all the blood samples indicated concentrations of
approximately 0.1 and less than 0.01 yg/100 ml for platinum and palladium,
respectively. However, because a number of uncertainties are associated
with measurements of such large samples (about 750 ml), these results are
probably reliable only to within a factor of 2 or 3. Nevertheless, they
suffice to indicate the extremely low concentrations of platinum and palladium
in the blood of people living in southern California.
The second set of measurements, for platinum only, was made by Stewart
123
Laboratories on autopsy tissue samples from 97 people, 95 from southern
California and two from New York City. Up to 21 tissue samples were collected
8-5
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TABLE 8-2
Platinum and Palladium in Autopsy samples, Los Angeles
Concentration, ppb (wet tissue)
Tissue Platinum Palladium
Liver <0.24 <0.6
Kidney <2.6 <6.7
Spleen <1.3 <3.3
Lung <1.3 <3.3
Muscle <0.9 <2.2
Fat <1.3 <1.6
a 225
Data from Johnson et al. Measurements based on
atomic-absorption spectrophotometry• Number of autopsy
cases, 10: Ages, 12-79 years. Sex, five male and five
female. Causes of death, pancreatic carcinoma, laryngeal
carcinoma, aplastic anemia, urinary bladder carcinoma,
acute lynphocytic leukemia, cervical adenocarcincma, hyper-
tension/hypoproteinemia sepsis, myocardial infarction, and
hypertensive cerebrovascular accident.
8-6
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TABLE 8-3
Platinum and Palladium Concentrations in Southern California
a
Group
Age;
1-16 years
17-34 years
35+ years
Composite:
00
Los Angeles
Lancaster
- 'vfc
•*-&
Concentration in
Blood, yg/100 ml
Platinum Palladium
<3.1 <0o9
<3.1 <0.9
<3.1 <0.9
0.049 <0.01
0.18 <0.01
T
-------
from each cadaver for analysis (1,303 samples in all). Sixty-two samples from
45 cadavers had detectable concentrations (>10 ppb for a 1-g wet sample) of
platinum, the detectable concentration being 0.003-1.43 yg/g of wet tissue
(mean, 0.16 yg/g). Table 8-4 shows the frequency of detection of platinum
in the various tissue samples. The authors contended that the frequency
of occurrence is a measure of the distribution of platinum among the various
body organs.
Why the Stewart Laboratories values are considerably higher than those
obtained by the SWRI researchers is not obvious. However, as a general rule,
when measuring amounts of trace substances where unintentional contamination
is possible, it is usually safer to accept the lower value as the more accurate.
The Subcommittee on Platinum-Group Metals tends to apply this criterion in
the present case and thus put more reliance on the SWRI data. Furthermore,
the statistical occurrence of platinum in the various tissues in Table 8-4
is probably not a significant indication of its distribution in humans.
Autopsy data have been collected on two cancer patients who died at the
Wadley Institute of Molecular Medicine. The earlier set has been published, *"
but the more complete set (R. J. Speer and H. Ridgway, personal camtunication)
is still unpublished; both are contained in Table 8-5. Some of these values
are at least an order of magnitude higher than the highest values reported by
Stewart laboratories and may reflect contamination or treatment of the patient
with chemotherapeutic agents containing platinum complexes (although the re-
searchers claimed that neither patient received these drugs). Part of the
discrepancy lies in the concentrations being reported in terms of wet tissue
by the Stewart and SWRI researchers, whereas the Wadley data refer to dry
8-8
-------
TABLE 8-4
Occurrence Frequency of Tissue Samples
with Detectable Platinum0
Tissue
Subcutaneous fat
Kidney
Pancreas
Liver
Brain
Gonad
Adrenal
Muscle (psoas)
Aorta (descending)
Heart (left ventricle)
Spleen
Prostate/uterus
Thyroid
Lung
Vertebra (lumbar)
Rib (fifth)
Femur
Clavicle
Hair, scalp
Hair, pubic
No. Tissue
Samples
Analyzed
74
91
84
90
9
53
60
97
92
82
52
63
73
95
94
97
57
30
9
1
1,303
Samples with
Detectable
Platinum
NO. %
10
11
10
10
1
5
3
4
3
2
1
1
0
0
0
0
0
0
0
0
14
12
12
11
11
9
5
4
3
2
2
2
0
0
0
0
0
0
0
0
61
a
123
Data from Duffield e_t al^ Measurements based on emission spectrochemistry.
Number of autopsy cases, 97—95 from California and two from New York City.
Average 61.5 years. Sex, 39 male, 52 female, six unknown.
8-9
-------
TABLE 8-5
a
Platinum in Human Tissue from Autopsy Cases
Platinum Concentration, yg/g (dry tissue)
Tissue Published data^ Unpublished datag
Stomach — 97
Spleen 24 87
Liver 83 76
Pancreas 48 65
Lung 95 65
Ovary — 54
Muscle, skeletal — 54
Adrenal — 44
Kidney 83 44
Small intestine — 44
Bone marrow — 39
Fat — 33
Bladder — 33
Gall bladder ~ 33
Lymph node 60 22
Skin — 11
Colon — 0
Brain ~ 0
Thyroid 60
d
Measurements based on atomic-absorption spectrophotometry.
Data from Hill et al. Cause of death, acute granulocytic leukemia,
a
'Data from R. J. Speer and H. Ridgway, personal communication.
8-10
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tissue. The percentage of water varies from one tissue to another, and the
value is somewhat subjective, depending on exactly how dry the researchers
consider "dry." Nevertheless, in general it is a fairly good approximation
to multiply the "dry" concentrations by 0.25, to allow direct comparison with
equivalent "wet" concentrations. Even after this is done, however, the Wadley
data are the highest of all those reported.
Although data in Table 8-5 are probably all much too high in absolute
magnitude, the trend of the values may be significant. Stomach, liver, and
kidney have rather high concentrations, whereas the platinum apparently does
not accumulate in the brain tissue. It is interesting to note that, accord-
ing to these data, fat has a much lower concentration than liver, in apparent
disagreement with the conclusions of the Stewart Laboratories people. Data
on the concentration of platinum in human heart tissue have been given by
Wester.457
In summary, it is probable that the baseline concentrations of platinum
and palladium in tissues of unexposed people are both less than 3 ppb
(0.003 ug/g of vet tissue). It is obvious that additional data from other
parts of the United States need to be analyzed carefully, to test the validity
of this statement.
No baseline data for concentrations of rhodium, ruthenium, osmium, or
iridium in human tissue are available. The concentrations of these metals
are very likely below the sensitivity limits of current analytic instruments.
22S
The SWRI researchers have also analyzed samples of blood, urine,
feces, and hair from employees at the Canadian mining facility near Sudbury
and at a noble-metals refinery in ifew Jersey. In addition, tissue samples
8-11
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fron autopsies of nine deceased mine employees were studied. All samples of
blood, urine, feces, and hair collected from the mineworkers ware below the
limits of detection for both platinum and palladium. Tissue from only three
of the nine autopsies had detectable platinum: fat, 4.5 ppb; lung, 3.7 ppb;
and muscle, 25.0 ppb. Although samples of liver, kidney, spleen, lung, muscle,
and fat from each autopsy were analyzed, these three detectable concentrations
were all for different people, thus suggesting some sample contamination. It
is therefore concluded that people who work in mining areas where platinum-
group metals are extracted do not incorporate significant amounts of these
metals into their bodies.
Similarly, blood samples collected from 61 refinery workers in New Jersey
contained no measurable amounts of platinum (less than 1.4 ppb) or palladium
(less than 0.4 ppb).^25 However, about 10% of the urine samples had measurable
amounts of platinum (maximum observed, 2.6 yg/liter; detectable limit,
0.1 yg/liter), and over half contained measurable palladium (maximum observed,
7.4 yg/liter). No autopsy data from refinery workers are available. In
view of these detectable amounts of platinum and palladium in urine, it is
strongly recommended that autopsy tissue samples from deceased refinery
workers be analyzed, to determine whether such exposure leads to platinum
or palladium incorporation and, if so, where the metals accumulate. Such
data would provide information about the probable fate of platinum-group
metals in humans, if the general population became exposed to them.
Vegetation
There have been only a few attempts to determine the concentration of
the plat±num-group metals in vegetation.58,62 Recently, an indirect
8-12
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method has been used to provide information about the concentration of trace
metals in the environment. Researchers from New York428 analyzed by spark-
source mass spectrometry the concentrations of 47 elements in honey that had
been collected near highway, industrial, and mining areas. These elements
included palladium, rhodium, and ruthenium, with the middle of the range of
values reported being about 9, 12, and 18 ppb, respectively. Admittedly, this
technique may be subject to contamination by containerization of the honey,
and the bees may tend to concentrate or magnify the elemental cotaminants in
the air, water, soil, and plants from which they collect nectar. Neverthe-
less, they forage over about 6.4 km2 and thereby provide samples from a
rather large area. This technique deserves further study, to determine its
suitability for monitoring trace-metal contamination in the environment.
One note of caution regarding interpretation of the data needs to be
offered: a newspaper article4^2 describing the honey analysis^8 reported
that all the contaminants could be traced to gasoline and lubricating-oil
emission fumes, diesel additives, and catalytic converters, all used in auto-
motive vehicles. The analysis itself was submitted to a journal for publication on
November 12, 1974, barely a month after the first cars with catalytic con-
verters hit the market. Furthermore, the fact that no platinum was found
in the honey rules out such converters as a source of the metals, inasmuch as
the converters contain platinum and palladium in a ratio of about 2.5:1. One
vould thus expect some platinum to be observed in the honey, if the bees had
foraged in an area that contained residue from catalytic converters. This
points out some dangers associated with careless interpretation of results.
8-13
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Animals
Only one incomplete set of tissue data appears to have been reported for
four organs of a New Zealand white rabbit.4^- According to the report, spleen,
liver, and lung all contained platinun at 100 yg/g (dry weight) , and the
kidney, 120 ug/g (dry weight) . Sane of the same researchers are now measuring
tissue samples from a rat (R. J. Speer and H. Ridgway, personal communication) .
As mentioned earlier, all these values are probably much higher than would
have been obtained from the ganpi*^ by either the SWKC or Stewart laboratories
researchers.
Because of the wide differences between baseline values ohf-a-ingf! by the
different laboratories, it seems obvious that comparative measurements on the
same tissue sample need to be made by all the laboratories involved in de-
termining baseline data.
AgTOMCBH£ EMISSION OONTHGL DEVICES
The newest and by far the most extensive use of platinum-group metals
is in catalysts for purifying exhaust streams from automobiles.^87'2^'453
Although used for many years to control emission from vehicles operated in
restricted environmental areas (fork-lift trucks, mining equipment, etc.),1
such devices were first installed on general-purpose automobiles in the
United States in October 1974. Almost a~n cars manufactured in the United
States since then, as well as a large fraction of new imparted vehicles,
contain these devices. Haaction by the general p^lic to their use has been
varied and strong, with both proponents and opponents using arguments based
on environmental quality, fuel economy, cost, simplicity, and reliability.
Although the catalytic converters do indeed decrease emission of hydrocarbons
8-14
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and carbon nmnnxirie, they introduce the possibilities of producing sulfur
trioxide (or sulfuric acid after reaction vrith moisture) , of emitting platinum-
group metals and their ocnpcunds into the environment, and of creating a fire
hazard through overheating. The purposes of this section are to describe the
development and operation of catalytic converters and to assess their possible
positive and negative impacts on the environment.
Government decisions have had an overwhelming effect on development of
catalytic converters. Over 15 years ago, the California legislature enacted
a law that would require all new cars sold in the state to meet specified
emission standards as soon as at least two techniques could be perfected for
253
achieving those standards. The potential market being substantial, con-
siderable effort was expended on both catalytic and noncatalytic approaches
to solve the emission problem. Three catalytic devices were actually certi-
453
fied, but they worked only with unleaded fuel. The unavailability of un-
leaded fuel, coupled with the ability to meet the proposed standards through
minor engine mnd-if-ifgHcnj; and carburetion recalibration to lean mixtures,
stifled further development of catalytic devices for several years.
The next government decision that affected catalyst development was the
U.S. Federal Clean Air Act of 1970. 339,436,439 lhis ^ required a 90% re-
duction in Fpyflfx^yix'" * and
-------
cars. Although each of these deadlines for compliance oould be legally de-
layed for a year (provided that it could be demonstrated that the standards
could not be met after a "good-faith" effort had been exerted), this was the
first time that legislation had attempted to enforce implementation of a tech-
nology that had not been perfected.
Because of the oil embargo in the 1970's, the Clean Air Act was amended
by the Energy Supply and Environmental Coordination Act in 1974340 to post-
pone until 1977 (or 1978, as later allowed by EPA Administrator Russell E.
Train^SS) enforcement of the statutory standards and to establish some less
stringent interim standards. The California Air Control Board has estab-
lished even more stringent interim controls that apply to new cars sold in
the state and has enforced these standards recently by levying substantial
fines against some manufacturers for failing to comply. All the standards
are now being reconsidered by both the executive and legislative branches
of the U.S. government; it is very likely that the statutory hydrocarbon
and carbon monoxide standards will be postponed at least until 1981, and
there is a strong possibility that the federal NOX standard will be permanently
relaxed somewhat.188
When automobile emission was first being regulated, it was suggested that
a tailpipe "concentration" standard be established for each pollutant. Of
course, this was unrealistic, in that installation of an air pump to dilute
the pollutants could produce whatever concentration was desired. It was then
suggested that a standard based on "mass emission" per unit distance traveled
be established, and this is the basis of all current regulations. Because
the emission varies widely for a given car, depending on the mode of operation,
8-16
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it is essential that some standard test be established; the federal test
procedure339'436'439 now used in the United States is the "constant-volu.ie-
sampling, cold-hot start" (CVS-CH) test. With a car mounted on a chassis
dynamometer and put through a complex, well-defined 11.5-mile driving cycle
(41.3 min,) including a 10-rain shutdown), constant-volume samples of the
diluted exhaust are collected sequentially in three bags for analysis by
nondispersive infrared techniques (for carbon monoxide and dioxide), flame
ionization (for hydrocarbon), and chemiluminescence (for NO^, x _> 1.
Table 8-6 surtmarizes precontrol, present, and statutory emission, according
to CVS-CH tests. It is apparent that progress has been made in decreasing
emission, but even the present California standards are higher than the
statutory federal requirements by a factor of about 2-5. It is now possible,
through the use of catalytic converters, to meet the statutory requirements
for hydrocarbon and carbon monoxide.
Technology for
meeting the NDX statutory standard for 50,000 miles has not been demonstrated,
and it is doubtful that it can be developed without imposing a substantial
fuel-economy penalty. It is on this aspect that research needs to be concen-
trated.
Japan has laws that will require oxidation catalysts on many cars
beginning in 1976 and NC^ reduction catalysts by the end of 1978.
8-17
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TABLE 8-6
Exhaust Emission from U.S. Cars
FTPa Emission, g/mile
Precontrol cars, before 1968
Present standards in 49 states
(excluding California),
until 1978
Present California standards
Federal Clean Air Act statutory
requirements
Hydrocarbon Carbon monoxide NPy
17.0 125.0 6.0
1.5
0.9
0.41
15.0
9.0
3.4
3.1
2.0
0.4
„ „
Federal test procedure
339,436f439
8-18
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(Characteristics of Catalytic Systems
General Description. Before details of catalyst formulation are given,
it is instructive to examine some of the system characteristics and a few of
the demands that are placed on catalytic converters. A primary character-
istic is transience.453 Even when the fully warmed-up car is operated at
a constant speed, the exhaust gas pulsates as the contents of each cylinder
are dumped into the exhaust manifold. More importantly, as driving mode
changes, the catalyst must be expected to perform under a wide variety of
temperatures, space velocities (flow rates of exhaust gases through the
converter), and mechanical shock conditions. It must also be resistant
to poisons from the fuel, oil, or air that occasionally contact it. In
general, it must be able to withstand considerable mistreatment at the hands
of drivers who have not been educated to appreciate the sensitivities of
catalytic materials. This is a far cry from the usual mode of operation of
catalytic reactors in chemical plants or petroleum refineries, where the
byword is stability.
Two different environments are required to purify the exhaust gases.
For hydrocarbon and carbon monoxide control, an oxidation catalyst in a
fuel-lean (oxidizing) atmosphere must be used. For NO removal, a reduction
J^
catalyst in a fuel-rich (reducing) atmosphere is used to effect reduction
by carbon monoxide, hydrogen, ammonia, or hydrocarbon. Actually, the most
straightforward way of removing NOX is by simple decomposition into nitrogen
and oxygen, a reaction that is tnermodynamically favorable, except at very
high temperatures. However, no effective catalyst for this reaction has
yet been found.
8-19
-------
To accjomrnodate these different environments, it was proposed very
early!87 that taro different catalyst beds be used in series, the first
operating in a reducing atmosphere for NO control, and the second in an
X*
oxidizing atmosphere for hydrocarbon and carbon monoxide control, as shown
in Figure 8-1. The engine would be tuned to run rich to produce a reducing
(oxygen-deficient) atmosphere in the first reactor, and an air pump driven
by a fan belt would inject air at point 2 to produce an oxidizing (oxygen-
rich) atmosphere in the second reactor.
Although such a scheme would theoretically accomplish the objectives,
it has some problems. First, if the mixture is too far on the rich side
of stoichiometric in the first bed, some ammonia is formed, owing to reaction
of hydrogen (produced either in the engine or via the water-gas-shift reaction
in the catalyst bed) with the nitric oxide. The ammonia, not particularly
harmful itself at these concentrations (there is already a measurable amount
of it in the atmosphere), would be converted back into nitric oxide in the
oxidizing atmosphere in the second reactor. Thus, the net nitric oxide
conversion woulo^Be significantly decreased. Second, operating the car in
a fuel-rich mode at all times will cause a substantial fuel-economy penalty.
Third, the first catalyst that will become
effective (or reach its "light-off" temperature—i.e., where 50% conversion
occurs) is the one for NOX control, whereas the oxidation catalyst (further
downstream) will heat more slowly. Unfortunately, it is oxidation activity
that is more needed early in the driving cycle, beginning from a cold start,
because the reducing atmosphere caused by functioning of the choke creates
large amounts of carbon monoxide and hydrocarbon initially. NO^ emission
8-20
-------
*
•
1 v
Air irump p
Engine
(tuned to
run rich)
\
'1 Reduction
Catalyst
For NO*
RemovaT
i
' 2 Oxidation
Catalyst
For HC/CO
Removal
FIGURE 8-1. Dual-catalyst exhaust control scheme. V, switching
valve. 1 and 2, air-injection points.
8-21
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does not become important until the engine becomes hot. To avoid this situa-
tion, it has been suggested that air be injected into the first reactor at
point 1 (Figure 8-1). thus using the first reactor as an oxidation catalyst
until the system becomes hot, at which time the air would be switched to
point 2 for normal operation. This would sacrifice NO control for a short
2£
time initially, but it would increase hydrocarbon and carbon monoxide control,
because a large fraction of the hydrocarbon and carbon monoxide collected
during the federal test procedure comes from "Bag 1," the first of three
samples collected, and includes samples of emission during the cold-start
part of the driving cycle. In addition to being more complex because it
involves a switching valve (V in Figure 8-1), such an approach demands ex-
treme versatility of the first catalyst by requiring both oxidation and
reduction activity. Fourth, introduction of two catalyst beds doubles the
pressure drop in the system; this situation can cause accelerated engine
wear and decreased performance. Finally, the excess air injected into the
oxidation reactor (at point 2, Figure 8-1) will maximize the formation of
sulfuric acid, because the equilibrium formation of sulfur trioxide from
sulfur dioxide and air is clearly dependent on the partial pressure of oxyen,
as seen by the equilibrium conversion curves in Figure 8-2.
Another scheme that has been suggested is a single catalyst bed to
effect removal of all three pollutants simultaneously. This three-jway
catalyst approach stems from the observation that, for mixtures very near
stoichiometric (air:fuel ratio, about 14.7 Ib of air per pound of fuel).
conversion of all three pollutants is high (Figure 8-3). However, if one
shifts more than 0.1 Ib of air away from that point in either direction,
conversion of at least one of the components falls off substantially.
8-22
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100
700
900 1100
Temperature, °F
1300
FIGURE 8-2.
Equilibrium conversion of sulfur dioxide to sulfur trioxide
at total pressure of 1 atm for two oxygen concentrations.
Bashed curve and points refer to equilibrium and observed
conversions in an automobile. (Data provided by Engelhard
Industries, June 10, 1974.)
8-23
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100
STOICHIOMETRI
14.0
A/F Ratio
14.5 15.0
(Pounds Air/Pound Fuel)
FIGURE 8-3,
Performance of three-way catalyst system and oxygen sensor
as a function of carburetion—i.e., airrfuel ratio. Solid
curves indicate new system, dashed curves an "aged" system.
(Data provided by Nissan Company, May 23, 1974.)
8-24
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During normal driving, the carburetion varies over a much wider range than
this. For example, during medium cruise, the ratio normally is on the lean
side of stoichiometric at about 16:1, whereas it may drop to as low as 12:1
for maximal power during rapid acceleration. Thus, it is obvious that sub-
stantial changes in engine control will be required to keep the air:fuel
ratio within the + 0.1 "window." Such control can probably be achieved only
through use of an oxygen sensor and a feedback system to maintain the oxygen
partial pressure exactly correct. High-temperature solid electrolytes, such
as zirconium dioxide, can be used; they develop high voltages when the oxyen
partial pressure approaches zero. An example of how the potential changes
with the air:fuel ratio is shown in Figure 8-3. If a sensor were placed
near the catalyst bed, its output voltage could be fed into a small computer
that will either increase or decrease the air:fuel ratio in the carburetor
as needed. Like the dual-catalyst system, the three-way approach has prob-
lems. First, the air;fuel ratio tolerance is extremely limited. Second, the
sensor does not become effective until it is hot—a problem it shares with
the catalyst. Third, there is a delay between the catalyst's "seeing" some-
thing and the computer's dictating of action at the carburetor. Fourth, both
the catalyst window and the sensor signal may shift as the system ages, as
indicated by the dashed curves in Figure 8-3. If the two curves do not shift
in concert, the device may begin controlling at a point far removed from the
catalyst window. On the positive side, such a system has a lower pressure
drop, requires less catalyst, and minimizes the ammonia and sulfur trdoxide
problems that plague the dual-catalyst approach. Considerable catalytic
research needs to be done to increase the width of the effective window
8-25
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and to stabilize the system against shifts due to aging. If the window can
be widened enough, it may be possible to control the carburetion sufficiently,
in a simple way, without the use of a complex oxygen sensor-feedback system.
The scheme currently being used in U.S. cars involves an oxidation
catalyst to control hydrocarbon and carbon monoxide either with or without
addition of air pumps. Some degree of NOX control is achieved with exhaust-
gas recirculation (EGR) , which minimizes formation of NO, in the engine by
•^ • j± -^ «*
decreasing the combustion temperature. A large fixed recirculation ratio
will result in a substantial fuel-economy penalty, although a "proportional"
recirculation ratio that varies with driving mode can almost eliminate the
penalty. In the best case, EGR can be expected to reduce the NO^ emission
to no less than 1.0 g/mile; this is far in excess of the U.S. statutory
limits. Other engine forms, such as stratified-charge and diesel, also have
NO emission in the same range, even under optimized conditions. 4 It thus
Ji
appears that catalytic converters will be required, if the statutory
0.4-g/mile standard for NOjj is ultimately to be enforced.
Geometry and Physical Properties. The location and geometry of catalytic
converters play quite important roles in determining their overall performance
and influence the health effects that may be associated with these devices.
If they are very near the exhaust manifold for rapid heating, the converters
may be subject to overheating that can irreversibly damage the catalyst.
Cylinder misfiring and sparkplug-wire disconnection are common malfunctions
that cause overheating. In most configurations, the converters are either
under the front seat or just ahead of the front floor panel.
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Although at least six physical forms of catalysts have been proposed/ only
the first taro are currently in use. The six are catalyst pellets (used by
General Motors, American Motors, etc.), ceramic monoliths (placed on Pond and
Chrysler products, as well as several non-American cars), layered expanded
metal screens (tested by Questor^), coiled wire mesh (made by Gould^S) f
alumina-coated wire strands (synthesized by Texaco), and metal sponges (pro-
posed by Clyde Engineering Co.).
The "pelleted" catalysts are in the form of extrudates, spherical parti-
cles, or cylindrical pellets about 1/8 in. (about 0.3 cm) in diameter. The
most popular converter is the "frying-pan" or "turtle" configuration de-
veloped by General Motors, which has a volume of 260 in.3 (about 4.3 liters);
another version used on some smaller cars has a capacity of 160 in.3 (2.6
liters). The catalyst particles are held in a thin bed between two screens
that are almost horizontal, but the exhaust gases enter the converter at
one end, flow down through the bed, and are collected below the bottom screen
and exit at the other end. The screens, which allow for particle expansion
and shrinkage during heating and cooling, maintain some pressure on the
catalyst bed, to minimize loose packing of particles and their collision with
each other; this could cause attrition and loss of catalytic material from
the exhaust system during the violent shaking sometimes encountered on rough
roads. The converter has the advantageous capability of being refilled (if
necessary) with fresh catalyst through a hole in its side without removal
from the car. The large converter has a bed density of about 0.65 and holds
a total of about 2.5 kg of catalyst.
8-27
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The monolithic catalysts are single pieces of ceramic material with
parallel channels running along their length. They are sometimes in what
is called a "honeycomb" structure. Their length varies from about 3 to
6 in (7.6 to 15.2 cm), and their diameter from 4 to 6 in (10.2 to 15.2 cm),
although some are oblong; and there are usually between 10 and 20 channels
per inch (or between four and eight per centimeter). The channels can be
triangular, square, hexagonal, or sinusoidal, depending on the manufacturer.
The volume of a monolithic converter is usually one-fourth to one-half that
of a particulate converter. The open structure minimizes back-pressure effects
for flow rates that can approach 500 standard cubic feet (scf) per minute, or
14 m^/rnin (or give a space velocity—volume of gas per volume of reactor per
hour—of up to 200,000/h). The monolithic catalysts suffer from being difficult
(if not impossible) to replace without complete removal of the converter from
the car. They are also more subject to thermal-stress cracking than are the
pelleted catalysts, but attrition does not seem to be so great with monoliths
as with pelleted material.
Composition of Oxidation Catalysts. The ingredients of oxidation
catalysts can be divided into two parts: active catalytic material (minor
component) and relatively inert support material (major component). In all
cases, the active catalytic components for both oxidation and reduction are
platinum-group metals or base transition metals and their oxides. These
materials are dispersed either atomically or in the form of widely separated
small crystallites on a high-surface-area support, usually gamma alumina.
8-28
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Platinum and palladium (and sometimes rhodium) are the only active in-
gredients that have proved durable for application as oxidation catalysts
in automobile exhaust; in most cases, a loading of about 0.06 troy oz/car is re-
quired. For pellets, the entire support is made of gamma alumina
(100-200 m2/g), sometimes combined with a "stabilizer"—such as magnesium
oxide, MgO; cerium oxide, CeO2' sodium oxide, Na2o" zinc oxide, ZnO; or
titanium dioxide, TiO2—to increase high-temperature stability and to de-
crease shrinkage. The noble metals are added either by a batch-impregnation
technique, in which the pellets are immersed in an aqueous solution of
ILPtClg or PdClj, or by a continuous-flow method, in which the preformed
pellets are sprayed with a solution containing these chemicals. For best
results, the platinum group metals should be concentrated near the outer
surface of the pellets, because metal buried deep inside is prevented
by diffusion resistance from participating effectively in the reaction.
However, some of the metal should be at least a bit below the external sur-
face, to protect it from such poisons as lead and phosphorus that may
periodically find their way into the fuel or lubricants. Wei.454 has con-
trasted the poisoning resistance of the "egg-yolk" catalyst with the reaction
availability of the "eggshell" catalyst; a bit of both seems to be optimal,
with none of the platinum-group metals deposited deep in the center of the
pellets. Once impregnanted, the metal salts are reduced to zerovalent metal
atoms, and it is in this form that they are catalytically active. Sometimes,
hydrogen sulfide is used as the reducing agent and also helps to "fix" the
platinum-group metal atoms in such a way as to prevent crystallite growth and
loss of effective surface area.
8-29
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The ceramic material used as a base in the monolithic oxidation catalysts
is now almost exclusively cordierite (2MgO* 220^03 *5Si02), chosen mainly be-
cause of its very low thermal coefficient of expansion;^ the low coefficient
is to avoid stress cracking under the large thermal gradients that can occur
during some modes of operation. Mast suppliers now use extrusion processes
to prepare the monoliths, although the earliest versions339,436,439 were made
from layered (or wound) corrugated cardboardlike material containing powdered
ceramic that could be calcined to remove the paper binder and fix the geometry.
There are about 300 channels/in.2 (about 46/cm2) in the best configurations.^4
Because the ceramic has an extremely low surface area (<0.1 n»2/g), a thin "wash
coat" of stabilized gamma alumina* is deposited on the insides of the channels,
to serve as a support for the platinum-group metals. This wash coat is usually
applied as a slurry that is forced through the channels; the excess if re-
moved by compressed air. In some cases, the metals are impregnated from
aqueous solution directly on the wash coat after it has been applied to the
ceramic; in other cases, they are mixed directly with the wash-coat slurry
before its application. If the wash coat is too thick or is improperly
applied, it has a tendency to flake off the ceramic support and may be
eliminated, with the platinum-group metals, from the converter. In any
case, the final material is calcined and the crystallites "fixed" by treatment
with hydrogen sulfide.
*Tne exact nature of the stabilizers used is proprietary, but it probably
does not involve alkaline earth oxides.
8-30
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Base transition metals cannot compete with platinum-group metals as
oxidation catalysts, for several reasons. First, the "light-off"
characteristics (activity versus temperature) of base metals are usually
much less desirable than those of the noble metals, as shown in Figure 8-4.
These data were obtained by passing a stream of synthetic exhaust through a
laboratory reactor that contained a small amount of catalyst. As the tempera-
ture was increased, the conversion of carbon monoxide and hydrocarbon was
periodically measured. The platinum-group-metal catalysts typically yield
curves that increase rapidly at a moderate temperature to nearly 100% con-
version (a desirable characteristic), whereas the base transition-metal
catalysts require much higher temperatures to approach 100% conversion.
Second, the conversion over base metals is a sensitive function of space
velocity/ whereas conversion over noble metals is relatively insensitive
to changes in the flow rate (a desirable feature, because the flow character-
istics vary so widely during different driving conditions). Third, the
platinum-group-metal catalysts are not poisoned significantly by sulfur in
the fuel, whereas the base metals form surface sulfates and become deacti-
vated, until the decomposition temperature for the sulfate is reached
(typically around 700° C). This is generally above the normal operating
temperature for the converters. Fourth, the base metals have a tendency
to react chemically with the support to form inactive spirals, such as
NiAl^O^, but this can be minimized by proper choice of the combination of
transition metal and support. Fifth, the base metals are less active per
unit weight than the platinum-group metals by a factor of 100-1,000, and
this requires a considerable quantity of the former for acceptable performance.
8-31
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100
oo
-------
Such large amounts can lead to a decrease in the surface area by plugging
of the pores of the gamma alumina. Finally, the base metals follow essen-
tially first-order kinetics for carbon monoxide oxidation, whereas
over catalysts containing platinum, the reaction order for carbon monoxide is
inverse first-order in carbon monoxide concentration. This means that, as
the partial pressure of carbon monoxide becomes smaller over platinum cata-
lysts, the oxidation rate increases; this makes such catalysts exceedingly
effective for removing small amounts of that pollutant.
Composition of Reduction Catalysts. There are two catalytic routes
for removal of nitric oxide: direct decomposition and chemical reduction.
No catalyst has been found that will decompose nitric oxide effectively in
an oxidizing atmosphere. 18 This is primarily because the released oxygen
remains strongly attached to the surface and poisons the sites for further
catalytic activity. Therefore, most research on nitric oxide removal has
centered on finding active and stable catalysts for reduction by carbon
monoxide, hydrogen, and hydrocarbon. The problem is more complex for oxida-
tion, because a selectivity factor is involved. Ammonia can be formed if
the mixture is too rich, and in some cases nitrous oxide is formed in
significant amounts. It has been suggested"" that one of the primary routes
by which nitric oxide is reduced is through ammonia as an intermediate, which
is illustrated by the imbalanced reactions a and b. Copper, platinum, and
palladium are good catalysts for reaction a, but they do not catalyze the
NO + H2 -£- NB^-HB-) N2 + H2
8-33
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anitonia decomposition effectively. However, nickel is a good catalyst for
the ammonia decomposition (reaction b). Thus, a combination of the noble
metal with nickel, or a mixture of copper and nickel (e.g., the alloy Monel,
which has copper:nickel ratio of 30:70), is a good catalyst for this reaction.
Considerable work has been done by Amoco, Exxon, and Gould on the Monel systems,
and similar material is still being seriously considered as a catalyst for
nitric oxide reduction in cars. The main problem stems from its suscepti-
bility to physical deterioration during the cycling from oxidizing to reducing
atmospheres that occurs in normal driving. It does, however, have the advantage
that it can be used either as self-supporting homogeneous alloy chips (or wires)
or in a form that can be supported on ceramic or metal supports.
By far the most effective catalyst for nitric oxide reduction is
ruthenium. 237 it is so active that only a very small amount of it (perhaps
as little as one-tenth the amount of platinum in oxidation catalysts) is
required, and it does not form significant amounts of ammonia, even under
extremely rich conditions. Unfortunately, the metal can form the volatile
ruthenium trioxide tetraoxide, RuC>3 and Ru04, under oxidizing conditions,
and this leads to its slow removal from the catalytic converter. The
ruthenium can be "stabilized" through formation of ruthenates, such as
lanthanum ruthenate, LaRuC^; barium ruthenate, BaRuC^; and magnesium
ruthenate, MgRuC^, which is formed by impregnation of magnesium oxide
MgO, with an aqueous solution of ruthenium trichloride, PuCl3.3^3/394
Although there is some dispute about this point, it appears that even the
"stabilization" techniques are not sufficient to guarantee that ruthenium
will not be depleted from the catalyst in actual operation. It is therefore
8-34
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doubtful that ruthenium can be used for this purpose, inasmuch as its
oxides are quite toxic.
Cfte of the earliest catalysts used for nitric oxide reduction was
copper chromite.36^ Although this material has good initial activity,
it is subject to poisoning and physical deterioration. More realistically,
mixtures of copper, cobalt, nickel, and chromium have been electrodeposited
onto thin metal foils by Gould to form active expanded metal mesh catalysts.
This material is a good reduction catalyst, but it also suffers physical
deterioration (called "green rot," a concentration of chromium at the grain
boundaries that leads to flaking of the electrodeposited material) when sub-
jected to rapid oxidation-reduction cycling. It should be noted that both
nickel28** and Gr+^ (as chromate)2^ are regarded as potential carcinogens.
To avoid such cycling, Gould has suggested installing a third catalyst—
another oxidation bed, called a "getter," just upstream of the reduction
catalyst, to remove excess oxygen through reaction with carbon monoxide
and hydrocarbon. Addition of the third catalyst allows the engine to be
operated very near the stoichiometric point (slightly reducing)—a factor
that minimizes the fuel-economy penalty associated with rich operation.
All the catalysts thus far described require a reducing atmosphere
(no excess oxyen). However, workers at Exxon^** recently reported that
iridium supported on alumina, AL^O^, can selectively catalyze the nitric
oxide-carbon monoxide reduction reaction, even in the presence of excess
oxygen. Much more work will be required, to determine whether this effect
can be maintained in a system that is more realistic than the laboratory
reactor and synthetic gas mixture used in their tests.
8-35
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Composition of Three-Way Catalysts. Three-way catalyst systems are de-
signed to operate near the stoichiometric point, so formation of ammonia is
not usually a problem. Furthermore, absence of excess oxygen minimizes the
formation of sulfur trioxide. Finally, the possibility of using only a single
catalyst bed makes this system extremely attractive. The very narrow range
of air:fuel ratio for effective performance is the only factor that detracts
from this approach, but this turns out to be a major disadvantage. The
ordinary platinum-palladium oxidation catalysts simply do not have sufficiently
wide windows to be prime candidates. However, catalysts containing rhodium
have a 90% conversion window (see Figure 8-3) of about +0.3 air:fuel units,
and additional research is under way to stretch this window even more. Per-
haps addition of some iridium will help to extend the window on the oxidizing
side. If the window can be widened, it may be possible to use such a system
without an oxygen sensor and feedback loop by properly designing the carburetor.
There is optimism that this can be achieved, and it would greatly simplify emis-
sion control systems, although the availability of rhodium will probably be a
significant problem.
Another possible three-way catalyst was recently patented by an inventor
at Du Pont2^2 who claims that his material is active for both oxidation and
reduction and, more important, is not poisoned by lead in the fuel! It is
a perovskite type of £8203 material, with ruthenium or platinum substituted
into 1-20% of the 3 sites. The remainder of the B sites contain cobalt, and
the A sites are occupied by a mixture of lanthanide ions. The catalyst is
being tested in automobiles. One characteristic is that it is less active
than the oxidation catalysts and thus operates at considerably higher tempera-
tures than are currently being used.
8-36
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Beneficial Effects of Catalytic Converters
Since their installation on most U.S. cars beginning in the
1975 model year, exhaust catalysts have been used to reduce the
emission of carbon monoxide and hydrocarbons from light-duty ve-
hicles. Relative to the emission of these substances in 1968,
carbon monoxide and hydrocarbon emission has been reduced (in 1975)
by 90% in California and by 83% in the other 49 states.
The reduction in ambient carbon monoxide will increase the
oxygen-carrying ability of the blood, which should substantially
benefit vulnerable subgroups of the population, including people
with vascular disease or severe anemia.27^ There may well be
measurable improvements in the ability of people to perform tasks
that require integration of the sensory, judgmental, and motor
functions of the central nervous system.27-^
The reduction in hydrocarbon emission will reduce the concen-
trations of photochemical oxidants in ambient air, and this should
result in fewer days that have oxidant concentrations high enough
to irritate the eyes and the respiratory tract.27^ This reduction
in oxidants should also enhance the functioning of the physiologic
mechanisms that protect and clean the respiratory systems and should
9*7 ^
reduce the frequency of asthmatic attacks. J
In addition to the benefits associated with reduction in
emission constituents regulated by the Clean Air Act, substantial
reductions in several types of unregulated emission are effected
by the use of catalytic converters, with resulting benefits to
8-37
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human health. Because nonleaded fuel is required, lead emission
into the atmosphere will begin to decrease, and this should reduce
community lead burdens and prevent further buildup of lead in
soils and other components of the ecosystem.273 There should also
be a reduction in emission of fine particulate matter consisting
of a core of lead compounds and a shell of complex organic matter^-
including compounds known to be carcinogenic—with a concurrent de-
crease in the risk of respiratory malignancy. Inasmuch as the
halide scavengers used with tetraethyl lead will also be absent,
the emission of particulate and gaseous halides will be reduced.
Vehicular exhaust contains polycyclic aromatic hydrocarbons,
including such carcinogens as benzo[a]pyrene, benz[a]anthracene,
chrysene, and dibenzopyrenes.163/ 273,297 These compounds are
destroyed with greater than 95% efficiency with catalytic con-
verters. 297 Alkylbenzene pollutants, such as m-xylene and
1,3,5-trimethylbenzene, can form compounds that cause appreciable
eye irritation through nitrogen oxide-induced photooxidation.
Conversion of 91-99% is obtained for these compounds through the
use of catalytic converters.2^? in addition, such compounds as
phenols and aldehydes, which are among the unhealthy products of
incomplete fuel combustion in the engine, are effectively eliminate
by catalytic converters.^3
Probably the most dramatic demonstration of the health bene-
fits likely to be derived from the use of catalytic converters is
215
the recent study in which several species of mammals were ex-
posed continuously to diluted exhaust (airrexhaust ratio, 10:1)
8-38
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from an engine operated on nonleaded fuel. In some cases, the
diluted exhaust was exposed to "synthetic sunlight," in half the
experiments, the exhaust was first passed through a catalytic
converter. In the absence of the catalyst, infant rat mortality
was 77% within 7 days with the diluted exhaust and 100% in 5 days
with exhaust that had been irradiated. When the catalyst was used,
mortality was zero in all cases, including one run in which the
fuel contained 0.1 wt % sulfur (about 3 times the current national
concentration). Similarly, with adult rats and hamsters, signifi-
cant pathologic and hematologic effects were observed only in
animals that had been exposed to exhaust not subjected to catalytic
conversion. These pathologic effects were not due to the high
concentration of carbon monoxide in the exhaust (500 ppm after
\
dilution), as shown by a control experiment with 500-ppm carbon
monoxide in clean air. No platinum metal was found in the tissue
of animals that had been exposed to the catalytically converted
exhaust. Earlier animal studies with non-catalyst-treated ex-
haust have shown that it can produce tumors, *® weight loss,^20
decreased pregnancy rate,206/247 deGreased survival rate,206,247
and increased susceptibility to pulmonary infection.206 Catalytic
converters will have a positive influence on reducing all these
adverse effects that have been attributed (at least in part) to
uncontrolled automobile exhaust.
Another distinct advantage of catalytic converters is their
contribution to improvement in fuel economy,28 as shown in
8-39
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Figure 8-5. Note that the dramatic fuel-economy increase coin-
cided exactly with introduction of the catalytic converters that
effectively decouple14 emission control from engine performance
and allow the latter to be optimized.
Potential Hazards of Catalytic Converters
Although catalytic converters have been remarkably effective
in reducing hydrocarbon and carbon monoxide emission and contributir
to improved fuel economy, they have introduced some health-related
problems that will be considered in this section. The more importer
ones are emission of sulfuric acid mists and possibly emission of
toxic noble-metal particulate matter in the respirable size range.
In addition, the tendency for the converters to become extremely
hot during some engine malfunctions, raises the possibility of
fire inside the vehicle or in dried grass under the car.
The Sulfuric Acid Problem. All gasoline contains sulfur
chemically bound in some of the fuel molecules; the amount varies
according to the origin of the crude, the processing techniques,
and the blending policy.200 For leaded gasoline, the national
average sulfur content is about 0.03 wt % (300 ppm), although it
may vary considerably from one location to another; the sulfur
content tends to be higher on the West Coast than in the rest of
the country. Nonleaded gasoline usually has a lower sulfur content
(around 0.01 wt %), owing to the blending of high-octane refinery
streams that require extremely low-sulfur feedstocks (a few parts
per million) in order to function properly—e.g., streams from
8-40
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00
I
0)
68
FIGURE 8-5.
69
70
71
Model Year
72 73 74
Fixed Model Mix Assumed
75
76
0
Fuel-economy (solid line), and carbon monoxide emission (dashed line) for a fixed model mix
of cars from 1968 to 1975. Catalytic converters were introduced into the 1975 model-year
cars. (Reprinted with permission from Austin et al.28)
-------
the reforming and alkylation units.200 Another low-sulfur blending
stream often used contains n-butane. The only significant source
of sulfur in nonleaded gasoline is the catalytic cracker stream,
which may contain as much as 0.1 wt % sulfur. There are, however,
ways of decreasing the sulfur in this stream by desulfurizing the
feed stream to the catalytic cracker, although this entails sub-
stantially increased capital costs.
Essentially all the sulfur in the fuel comes out of auto-
mobile engines as sulfur dioxide; this source represents approxi-
mately 1% of all the sulfur emitted into the atmosphere in the
United States as a result of man's activities.^29 This sulfur
dioxide is slowly (over a period of hours to days, depending on
the atmospheric conditions280'281'282'283) oxidized to sulfur
trioxide, which reacts with moisture and atmospheric bases—e.g.,
ammonia or metal oxides—to form sulfuric acid and airborne sul-
fate salts. These are eventually purged from the atmosphere by
rain or through other mechanisms.
In the catalytic converters, a part of the sulfur dioxide
is oxidized to sulfur trioxide and comes out of the tai Ijjipe as
aerosol particles 0.1-1.0 gm in diameter. That sulfur
-------
trioxide and sulfuric acid are considered to be more physiologi-
cally harmful than sulfur dioxide, their accumulation near road-
ways could represent a potential health hazard. Indeed, mathemati-
cal models440 have predicted that, under "worst-case" conditions
(crowded 10-lane freeway with adverse meteorology, such as zero
wind velocity), the sulfuric acid concentration could reach
600 yg/m3 10 ft (3 m) above the center of the roadway. Considering
that the threshold135a for irritability in sensitive people based
on 24-h exposure appears to be around 10 yg/m , such a large value
(if it ever occurred) would certainly pose a threat to the health
of people who live near freeways or spend considerable time
traveling on the roads, visiting crowded shopping centers, or
working in walled city-street canyons.
The maximal concentration predicted by the mathematical
models has been the subject of intense debate during the last
several months. Proponents argue that it may be off by no more
than a factor of 2 in either direction, which would mean a range
of predicted "worst-case" values of 300-1,200 yg/m3. Opponents122
have suggested that the model is highly unrealistic and that
several important factors that would tend to decrease the concen-
tration are not being considered. These are illustrated in Figure
8-6 for an altitude of 10 ft (3 m) at various distances from the
freeway. First, the original model assumes a single-line source
(actually, a series of closely spaced points, e.g., exhaust stacks,
for which plume equations are well known) at the center of the
roadway. Curve A is the predicted stationary-state sulfuric acid
8-43
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600
500
W
g 400
•H
4J
Id
H
4J
c
-------
concentration profile. The situation is more accurately described
by 10-line sources (one for each lane); the main effect of this
assumption is to decrease the maximal concentration at the center
of the roadway, as illustrated by curve B. Second, wind plays an
122
extremely important role in determining the concentration profile.
For example, curve B becomes curve C when the wind velocity is in-
creased from zero to 0.2 mph (0.3 km/h) at right angles (left to
right) to the road. This drops the maximal concentration from
about 500 to about 80 vg/m3. At a wind velocity of 1.0 mph
(1.6 km/hr), the maximal concentration drops to about 25 ug/m3
10 ft (3 m) above the downwind edge of the road. Third, the
original model does not account for atmospheric reactions (e.g.,
ammonia or metallic oxide particulate matter) that form less toxic
compounds, nor does it allow for irreversible adsorption of the
sulfur trioxide and sulfuric acid on surfaces that are in the
vicinity of the roadway.^22 j^ is estimated that urban ambient
air normally contains particulate matter at a concentration of
60-200 ug/m3 and ammonia at about 2-200 ug/m3- These values are
large enough to allow significant neutralization of the acidic
species. With modest rate parameters and reasonable values for
collision between an aerosol and either ammonia or particulate
matter, one can calculate curve D for a wind velocity of 0.2 mph
(0.3 km/h). Of course, the physiologic activity of the sulfates
themselves is not zero, although they are usually less toxic than
sulfur trioxide and sulfuric acid. Adsorption on surfaces would
8-45
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reduce the concentration profile even further, but calculations
indicate that adsorption has only small effects on the concen-
tration curves.
Some of the characteristics that were used in these calcula-
tions are:
• fuel consumption, 15 miles/gal (24 km/gal, 6.3 km/liter)
• nominal highway speed, 55 mph (88.5 km/h)
• gasoline density, 6.5 lb/gal(779 kg/m^)
• sulfur in fuel, 150 ppm (0.015 wt %)
• conversion of sulfur to sulfuric acid, 30% (0.027
g sulfuric acid emitted per mile, or 0.017 g/km)
• catalyst-equipped cars, 100%
• 10-lane freeway
• 0.5 car passing a point per second in each lane
This has not been intended as a thorough treatment of the
subject, but it should suffice to indicate some of the enormous
parametric sensitivities involved in these models. Until more
reliable experimental values can be obtained for som«- t,\ tile-
critical quantities, it appears that these models are not Very
useful, in that one can obtain almost any number that will sup-
port one's own view of catalytic converters.
Apart from the stationary-state atmospheric concentrations,
several factors affect the amount of sulfuric acid actual!/ omitted
by cars. First, at relatively low converter temperatures, the
sulfur trioxiue can react with the surface of the alumina support
8-46
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and be retained, or "stored," in the converter as aluminum sul-
fate, A12(SO4}3. The surface area available in a typical con-
verter packed with pellets is around 100 acres (0.4 km2), so the
storage capacity is considerable. When the decomposition tempera-
ture of the sulfate (about 770° C) is reached, all the sulfur
trioxide that has been collected is "dumped." This storage ef-
fect is more pronounced with pelleted catalysts than with mono-
liths, owing to the larger amounts of alumina in the former.
Such an effect makes measurements of the sulfur trioxide and
sulfuric acid emitted from the tailpipe heavily dependent on the
driving cycle and previous history of the converter; many opera-
tional modes do not cause the decomposition temperature to be
reached. Another factor is the amount of oxygen present in the
converter. Figure 8-2 shows the effect of partial pressure of
oxygen on the equilibrium fractional conversion of sulfur dioxide
as a function of temperature. Obviously, the more oxygen present,
the greater the amount of sulfur trioxide that can be formed at
a given temperature. The data points collected from an actual
automobile test indicate that equilibrium conversion (compare
with dashed curve) is not quite achieved in practice. Systems
that have essentially no excess oxygen (e.g., the three-way
catalyst system) are thermodynamically constrained from producing
much sulfur trioxide and sulfuric acid. Cars running just on the
lean side of stoichiometric and without air pumps will, of course,
be favored (i.e., will emit smaller amounts), although they will
8-47
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be less effective for the oxidation of hydrocarbon and carbon
monoxide, because of the limited supply of oxygen.382
Another problem related to sulfur occurs when the catalyst
is operated under reducing conditions. When the atmosphere is
oxidizing, sulfur trioxide and sulfuric acid are formed; under
stoichiometric conditions the sulfur remains as sulfur dioxide;
but, under reducing conditions, some of the sulfur is converted
into hydrogen sulfide, a far more toxic substance than the sulfur
oxides. This phenomenon accounts for the "rotten egg" odor oc-
casionally reported by an owner of a new car when the car is
started. When the choke is activated, a rich mixture is produced.
Fortunately, the catalyst is also cold under these conditions and
is not active. However, if it reaches its "light-off" temperature
before the choke is released, hydrogen sulfide can be produced.
Any system designed to operate very close to the stoichiometric
point (e.g., a three-way system) runs the risk of forming either
sulfur trioxide or hydrogen sulfide if the carburetion shift
just slightly to the lean or rich side of stoichiometric. Fre-
quently, when the engine is turned off, an odor due to hydrogen
sulfide is observed.
Finally, attempts have been made to develop effective oxida-
tion catalysts for hydrocarbon and carbon monoxide that will not
oxidize sulfur dioxide. Although there have been reports that
catalysts containing rhodium partially satisfy this goal, the
subcommittee doubts that such catalysts can be developed.
8-48
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Recently, General Motors conducted a series of tests to
determine experimentally the concentration profiles of sulfuric
acid-sulfates that could build up around a freeway.69 A mixture
of 352 cars—including General Motors, Ford, Chrysler, and American
Motors vehicles—were run under carefully controlled driving cycles
on'the General Motors proving grounds near Detroit during October
1975. All the cars had catalytic converters, and the nonleaded
fuel contained about 0.03 wt % sulfur. During the test, samples
of airborne particulate matter were collected at various altitudes
and at several distances from the test track; samples were also
taken inside some of the cars. The road included four lanes, two
in each direction. An inert tracer compound, silicon hexafluoride,
was also emitted at a known rate from some test cars; measurements
of this compound served as a reference against which the sulfate
data were compared. Background sulfate concentrations were also
measured and subtracted from the test data.
The results of the test can be summarized as follows:
• The average sulfate emission rate was 0.037 g/mile, or
.023 g/km (measured as sulfuric acid), when the vehicles
were fully warm. Because of the storage effect, the
observed rate was lower than this at the start of each
test.
• The maximal sulfate concentrations were observed at an
altitude of 0.5 m in stations at the edge of the road
Con the downwind side) or in the median, the average
8-49
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maximal increase above background being 5.2 yg/m
for all runs. •»*?
• The average maximal increase in sulfate concentration
inside the cars was 4.0 yg/m ; opening or closing
windows made very little difference in this value.
• Adsorption of sulfate on surfaces appeared to play a
minimal role as an atmospheric removal mechanism.
• The vertical dispersion of sulfate was much greater
than had been predicted by the models. This may have
been because of failure to account adequately for the
effects of turbulence created by the cars or for the
thermal updraft due to emission of hot exhaust gases.
A contour map of the lines of constant sulfate concen-
tration observed in one of the runs is shown in Figure
8-7.
The values obtained in the sulfate-dispersion experiment
can probably be multiplied by a factor of about 7, to allow
comparison with the "worst-case" predictions of the models shown
in Figure 8-6. Obviously, the observed values are far below those
predicted by the original models.
Finally, several stations have been set up to monitor con-
tinuously the sulfate concentrations near freeways, especially
in California. With 15-20% of the cars on the road now having
catalytic converters, most of these stations have found only very
slight (if any) increases in measured sulfate concentrations.
8-50
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00
I
Ul
Concentrations
in ug/m
0.5
-30
-2 H I G H W A Y 4 20 30
Distance, meters
100
FIGURE 8-7. Contour map showing total sulfate (including background) reached at various heights above
and distances from a four-lane freeway during one of the General Motors sulfate-dispersion
experiments. (Reprinted with permission from Cadle et al.69)
-------
It thus appears that atmospheric sulfuric acid accumulation
due to catalytic converters will not pose a serious threat to
human health. However, it is recommended that the concentrations
around typical freeways continue to be carefully monitored, to
make certain that they do not become excessive. If they begin to
increase toward a potentially dangerous point (which is unlikely),
one possible solution would be to limit the amount of sulfur allowec
in fuels. However, there appears to be no justification for such
action at present.
Noble-Metal Emission. The active platinum metals are present
in automobile catalysts as small crystallites widely separated
on the high-surface-area alumina support. It is possible that,
through attrition or malfunction modes that lead to overheating
of the catalyst, pieces of the support or active ingredient can
be emitted from the tailpipe. Attrition is greater for pelleted
catalysts than for monolithic configurations, owing to the relative
grinding motion of particles in the first case. Emission of cata-
lyst particulate matter can be divided into two categories: large
chunks that are usually collected in the exhaust muffler down-
stream from the converter, and very fine particles that come out
of the tailpipe. With most pelleted catalysts, the first category
predominates; such material does not enter the environment. At-
tempts to collect this material on fine filters and analyze it
have indicated (R. F. Hill, personal communication) that this
mode of loss amounts to 1-3 yg of platinum metal per mile
8-52
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(0.6-1.9 yg/km), which means that less than 10% of the metal is
lost during 50,000 miles (80, 500 km) of operation.
The Subcommittee arranged to have samples of the two types
of particulate matter, obtained from General Motors, analyzed by
the Southwest Research Institute (D. E. Johnson, personal communi-
cation) . The large particles obtained from sweepings of the down-
stream exhaust system contained platinum and palladium at 3.58
and 2.45 yg/g, respectively. One sample of the fine material
contained no measurable platinum or palladium; a second sample
contained platinum at 0.107 yg/g and no detectable palladium.
With an assumed emission rate of 3 yg/mile (1.9 yg/km) and
with the assumption that these small particles behave in the same
way as sulfuric acid aerosol particles, one can calculate a
"worst-case" concentration maximum that might accumulate in the
atmosphere. The maximal possible concentration under these con-
ditions is about 0.06 yg/m3 (compared with the 600 yg/m^ pre-
dicted for sulfuric acid; see Figure 8-6), which is only 3% of
the exposure maximum now in effect for the soluble platinum salts.
As discussed earlier, the actual value would be at least an order
of magnitude less than this under any kind of realistic meteorologic
conditions.
Perhaps the most important question concerns the chemical
nature of the emitted platinum. As shown in Chapter 7, platinum
metal and platinum oxides (the most probable chemical forms emitted
from the converters) have little or no physiologic activity when
8-53
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inhaled. There has never been any evidence of physiologically
active complexes of platinum among collected exhaust samples.
The most definitive work in this subject was conducted by General
Motors researchers who used radioactively labeled platinum to
enhance analytic sensitivity (JR. F. Hill, personal communication).
The radioactive metal was produced by irradiating the catalytic
material with neutrons. Several screens of different mesh were
used to collect samples of the emitted material. About 80% was
collected on a 120-mesh screen (which could trap particles greater
than 125 ym in diameter), and 20% was collected on a glass-fiber
filter. The platinum loss varied from 1 to 3 yg/mile (0.6-1.9 ug/km
depending on the driving cycle used. In all cases, the platinum:
support ratio in the collected material was about the same as that
in the original catalyst; this indicated that the platinum metal
was not selectively lost. To obtain information about the chemi-
cal nature of the platinum, attempts were made to dissolve the
material in different solvents. No radioactive platinum could
be detected in the aqueous fractions; thus, less than 1% (the
lower limit of sensitivity) of the platinum was present in the
form of soluble salts.
It is interesting to note that, in the General Motors sulfate-
dispersion test described earlier, no platinum metals could be
found in any collected samples.
It is concluded that emission of noble metals from catalytic
converters does not pose a significant threat to human health
through inhalation of the exhausted material.
8-54
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As noted above, most of the emitted material leaves the con-
verters as relatively large particles (>100 vim) . This material
would be deposited along the roadside, where it could accumulate
and be subjected to action by the environment. Wood468 has sug-
gested that platinum might be solubilized through methylation by
specific microorganisms in much the same way that mercury is
methylated. If this occurs, it could conceivably become incorporated
into the food chain and become concentrated in its progress up the
biologic ladder to humans. Taylor and Hanna422 have reported
recently that compounds containing platinum(IV), such as K^PtClg,
can be methylated, but there is no evidence that platinum metal
can be methylated unless it is first converted to platinum(IV).
Furthermore, there is no evidence that compounds containing
platinum(IV) are emitted from the converters or that they are
formed in the environment after being deposited along the road-
side. Even if methylation or some other form of solubilization
occurs, the very small quantities of the materials involved make
it remote that this could cause a significant problem in the
foreseeable future. For example, to determine the extent of this
possible problem, one can calculate a concentration of platinum
that would be obtained after 10 years, assuming heavy vehicular
traffic (50,000 cars/day, all with catalytic converters), typical
emission of noble metals (3 yg/mile, or 1.875 pg/km), and accumula-
tion in the topsoil (uniformly distributed 12 in, or 30.5 cm,
deep over a width of 300 ft, or 91.4 m, with a soil density of
1.5 g/cm3). In a test area 1 mile, or 1.6 km, long, 548 g of the
8-55
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noble metal would be emitted in about 7 x lo!0 g of soil. This
would result in a concentration of 0.008 ppm, which is at least
2 orders of magnitude less than the concentration in ores that
are considered to be economically minable. Under these assump-
tions, more than 1,000 years would be required to accumulate
enough material to be considered significant. Thus, it is con-
cluded that this should not pose a significant problem. Never-
theless, it is recommended that the topsoil near a few heavily
traveled freeways be monitored periodically for accumulation of
platinum-group metals.
Other Nonregulated Emission. Workers at the Bell Telephone
445
Laboratories have shown recently that it is possible to form
hydrogen cyanide by passing synthetic exhaust streams over platinum
catalysts in laboratory reactors under reducing conditions. How-
ever, they showed that the presence of sulfur and water in the
stream completely poisons this reaction. Because both are normally
present, it is highly unlikely that detectable amounts of hydrogen
cyanide can be found under ordinary conditions.
As mentioned early in this chapter, ammonia is formed over
many proposed NOX catalysts when the carburetion is set to give
a strongly reducing atmosphere. However, in the dual-bed con-
figuration (see Figure 8-1), the ammonia formed in the reducing
reactor would be converted back into nitric oxide in the oxidizing
reactor; hence, ammonia would not be emitted from the overall
system.
8-56
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Overheating. The two most frequent modes of catalyst de-
activation are poisoning by lead in the fuel and engine malfunc-
tion that causes overheating. The first mode is somewhat re-
versible, in that most oxidation catalysts can tolerate about one
tankful of leaded gasoline in 10 without severe damage.6 However,
if the temperature becomes high enough to melt the catalyst or its
container, the damage is irreversible.
The operating temperature in the oxidizing catalytic con-
verters is a sensitive diagnostic measure of engine performance.
One person has suggested that the converters serve well as "rectal
thermometers." With misfiring spark plugs, missing wires, or
severe carburetion maladjustment, the unburned fuel is burned in
the catalytic converter, where neither cooling water nor mechani-
cal energy can remove the heat. In cars without catalytic con-
verters, this extra oxidation process does not occur, and un-
burned fuel is vented into the atmosphere. Table 8-7 compares
the temperatures observed in cars with and without catalysts
and operated under various malfunction modes. Note that the
exhaust temperature actually decreases with the degree of mal-
function with catalyst cars.
There has been considerable discussion recently about the
fire hazards associated with these devices. Catalyst cars have
been banned from many petroleum refineries, and consideration has
been given to banning them from some national parks. Without
question, the possibility of fire does exist, and grass fires
8-57
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TABLE 8-7
Temperatures of Exhaust systems with and without Catalytic Converters^
Temperature, °F (°C)
oo
i
01
00
Driving Mode
Idle
Idle and soak
City traffic
City traffic, soak
55 mph
55 mph and soak
70 mph
70 mph and soak
Two plugs not firing;
Idle
Idle and soak
Noncatalyst System
Exhaust
Manifold
820
805
712
678
858
818
966
903
519
513
(438)
(429)
(378)
(359)
(459)
(433)
(519)
(484)
(271)
(267)
Exhaust-Pipe
Converter
Location
482
428
389
389
371
374
369
474
528
523
(250)
(220)
(198)
(198)
(188)
(190)
(187)
(246)
(276)
(273)
Muffler
488
439
331
349
307
381
324
446
395
390
(253)
(226)
(166)
(176)
(153)
(194)
(162)
(230)
(202)
(199)
Catalyst System
Exhaust
Manifold
802
773
675
661
822
788
943
883
538
534
(428)
(412)
(357)
(349)
(439)
(420)
(506)
(473)
(281)
(279)
Converter
528
543
314
345
227
495
256
556
875
860
(276)
(284)
(157)
(174)
(108)
(257)
(124)
(291)
(468)
(460)
Muffler
490
492
313
325
292
389
325
451
735
709
(254)
(256)
(156)
(163)
(144)
(198)
(163)
(233)
(391)
(376)
a 149
Data from General Motors.
Additional data from V. Haensel (personal communication): Temperature to
ignite pine needles: in 1 min, 760° F (404° C) ; in 4 min, 660° F .(3$gPC|. Temperature to
ignite dried grass: in 1 min, 840° F (449°C1; in 4 min, 750° F C39
-------
have reportedly been caused by cars with catalytic converters.
However, what is not usually appreciated is that noncatalyst
cars, particularly those produced since 1972, also have very
hot exhaust and can themselves start grass fires. This was
dramatically demonstrated in a recent test in California planned
by Los Angeles County Supervisor Kenneth Hahn. 47° Two American
Motors Hornets, a 1974 model without a catalytic converter and a
1975 model with a converter, were allowed to idle for 30 min and
were then driven onto a patch of dried grass. With the engines
still running, the catalyst-equipped car ignited the grass in
55 s, but the noncatalyst car caused a fire in only 35 si Another
test, in which two spark-plug wires were removed from the catalyst
car, caused the grass to ignite after 14 s; other tests to confirm
this result were unsuccessful.
The Department of Transportation (DOT) has been keeping a
file (Docket 75-13)* on all reported cases of overheating. As
of March 1975, General Motors had reported 327 cases of overheating
out of 2.2 million cars sold; of these, almost half were discovered
by dealers before the cars were turned over to purchasers. Ford
had reported 78 cases out of 1.2 million cars sold during the same
period. The vast majority of these problems were traced to the
ignition system's being improperly assembled or not working properly.
A few cases were attributed to improper application or the wrong
kind of undercoating material directly onto the converters. In
several cases, floor carpets have been scorched; but fewer than
* The National Highway Traffic Safety Administration Docket 75-13 regarding
the risk to the public was closed February 24, 1977, without any rule-making
(Federal Register 42:12284-12285, 1977).
8-59
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a dozen actual fires have been reported. According to the DOT
file, there have been no reports of human injury caused directly
or indirectly by problems associated with catalytic converters.
The number of problems reported is remarkably low, consider-
ing the newness of the technology, and reflects a highly successful
introduction into the market. This is reflected in a statement
A"3O
by the EPA administrator in his decision to postpone enforcement
of the more stringent 1977 standards for a year:
In many ways, catalysts have performed far better
than some predicted when the 1975 interim standards
were first established two years ago. Contrary to
many predictions, both the production of catalysts
and their installation on automobiles is proceeding
without difficulty.
Material Supply. As noted earlier, the amount of platinum
required in catalytic converters each year is approximately equal
to the total amount of platinum imported into the United States
each year for the last several years. This means a doubling of
our requirements for this strategic material. Although world
production (mainly from South African mines) has been expanded
to meet this demand, the total dependence on foreign sources is
undesirable. It has been suggested that the noble metals might
be recycled to a significant degree.239 However, the small
amounts of noble metal in the converters (about 0.06 troy oz,
or 1.87 g, worth about $10 at current prices) and the low
8-60
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concentration Cabout 0.05 wt %). make it unlikely that this will
ever be accomplished. It has been argued that refinery catalysts
containing platinum can be almost 100% reclaimed and that the
same thing could be achieved with automobile catalysts. There
are two significant differences: large quantities of the re-
forming catalysts are present in a single location, and the con-
centration in these catalysts (about 0.3 wt %) is several times
higher than it is in the automobile catalysts. The logistics of
removing the converters, emptying the contents into containers,
and shipping them back to the refinery will hardly be economical,
unless the price of platinum becomes dramatically higher. However,
favoring recycling is the fact that the stainless-steel converter
material in which the catalysts are housed contains relatively
large amounts of nickel. For the nickel content alone, it may be-
come economical to remove the converters from junked automobiles
and to recycle that material separately. In Japan, a law mandates
this recycling to conserve nickel. Even if such removal and sepa-
rate recycling become widespread in the United States, it is doubt-
ful that recovery of the catalyst components would be economical,
because of the very low concentrations of the platinum-group metals.
It is hoped that the recently discovered palladium-platinum
ore in Montana can help to relieve our dependence on nondomestic
sources of these materials. Because the catalytic activity is a
rather insensitive function of the platinum:palladium ratio, it
is likely that the catalysts can be made richer in palladium with-
out sacrificing much in the way of activity. It has been suggested^O
8-61
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that, for some gas scrubbing applications, palladium catalysts
are better than those containing platinum.
EMISSION FROM STATIONARY SOURCES
Smelters
Because the platinum metals are so valuable, great care is
taken to avoid losing significant amounts of them in refining
processes. Thus, economic factors automatically tend to minimize
health effects that might occur if compounds of these metals were
to become distributed into the environment. Osmium appears to be
the only platinum-group metal of which appreciable quantities
are lost. In the refining of copper sulfide ores, osmium is con-
verted to the volatile tetroxide.^4 it has been estimated^"* that
1,000-3,000 troy oz (31-93 kg) of osmium are lost in this process
each year, but this relatively small amount does not appear to pose
significant environmental hazards at present. It is recommended,
however, that plants, animals, and humans living around copper
ii
smelters be examined for possible ill effects that might be attrib-
uted to accumulation of osmium in tissue.
Catalytic Uses
The only important stationary use of catalysts in which
appreciable amounts of noble metals are lost in the reaction
product is ammonia oxidation over a platinum-rhodium (90:10)
gauze to make nitric acid. It is estimated that the amount of
catalyst lost is usually in the range of 0.15-0.35 g/ton of
8-62
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nitrogen oxidized, although it may run as high as 0.5-1.8 g/ton
in plants operating at higher pressures (e.g., up to 9 atm) .
Assuming an annual production rate of 40 million U.S. tons of
nitric acid, one calculates that about 1.3 x 105 troy ounces of
platinum would be consumed in this process each year. Not all
this material is allowed to escape into the atmosphere. Various
filters and "gettering" devices now introduced into most of the
plants allow recovery of perhaps half the lost metal. Of that
which escapes, most is probably trapped in the absorption towers
and ends up in the nitric acid. Through use of the nitric acid
in fertilizers, in pickling baths, etc., the lost noble metals in
the broadest sense are being "dumped" into the environment.
Platinum-metal catalysts have long been used in paint-drying
n "\"Jfi
' '
ovens, wire-enameling ovens, and self -cleaning cookers.
In the earliest types, platinum metals were electrodeposited
onto base-metal supports. These devices were designed to last
10 years with only semiannual washings required, if they were not
chemically poisoned. Emission of the noble metals is negligible
and poses no environmental hazard.
Plameless catalytic space-heaters have been labeled^** an
indirect threat to life, because of the dangerous concentrations
of carbon monoxide that can be emitted from them. When they are
used in unventilated spaces, the high carbon dioxide concentration
and decreased oxygen concentration can combine with what would
•
normally be an innocuous carbon monoxide concentration to produce
8-63
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harmful and even fatal results. Care must be exercised in using
these devices. It does not appear that any catalytic material is
emitted from them.
In addition to mining areas and noble-metal purification
plants, the only other places where noble metals have been observed
to reach physiologically significant concentrations are catalyst
manufacturing plants. Although detailed information about concen-
trations in the air, soil, and tissues of employees is not avail-
able, some statistics accumulated during the last 23 years of
operation from a single such plant in Louisiana are given in Append:
A. Allergic reactions to platinum salts have been noted in a total
of 15 people in the plant that now employs 250 people. Comparable
allergic reactions would probably be observed in other plants that
synthesize noble-metal catalysts.
8-64
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CHAPTER 9
SUMMARY
Some new and potentially extensive uses of the platinum-
group metals (platinum, palladium, rhodium, ruthenium, iridium,
and osmium) have increased the possibility of environmental con-
tamination with compounds containing these metals. Two of the
new uses are in catalytic converters to control emission of
pollutants in automobile exhaust and in drugs to arrest the
spread of some types of cancer. The purposes of this report are
to summarize what is known about the sources, properties, and uses
of these metals and their compounds and the methods for analyzing them; to
describe their toxicology; and to assess the environmental Impact and possible
health effects of their new uses.
Most of the platinum-group metals are produced from ores of
ultrabasic rock formations mined in South Africa, Canada, and the
U.S.S.R. Essentially none are currently produced from ores mined
in the United States. Average concentrations of the six metals
in the earth's crust are estimated to range from 0.001 to 0.01
ppm (1 ppm = 10 wt %), although they may be greater than 10 ppm
in some commercially minable ore. Platinum and palladium are the
most abundant of the six metals (>85%), and the ratio of the two
varies from 0.3:1 to 2.5:1 in the various ores. Of approximately
4 million troy ounces of the metals produced annually, about 1.6
million ounces (40%) are consumed in the United States. The re-
serves outside the United States are estimated to be about 400
9-1
-------
million ounces, enough to last about 100 years at current consump-
tion rates.
Within the last 2 years, an extensive deposit of relatively
concentrated ore (noble metals at about 20 ppm) rich in palladium
(Pt:Pd ratio, about 0.3:1) has been discovered in Montana. The
owners estimate that the deposit contains up to 500 million ounces
of platinum-group metals, which is approximately equal to all
the other known world reserves combined. This is the first poten-
tially minable ore to be discovered in the United States, and the
extent of its commercial possibilities is being studied.
After it is mined, the ore is treated in two distinct steps.
The first stage (carried out near the mines) involves extraction
of a concentrate of the precious metals from a large body of ore;
the second stage (usually carried out in refineries at other loca-
tions) involves separation and purification of the individual
metals. Two of these refineries are on the east coast of the
United States. Because of its intrinsic value, a large fraction
of the used metal is recycled, or "toll refined," for further use.
The most common analytic techniques used to determine trace
quantities of platinum-group metals are emission spectroscopy
(preferred in Europe) and atomic absorption (preferred in the
United States and Japan). The latter is more sensitive (platinum
detection limit, about 0.1 ppm vs. about 10 ppm), but the former
is somewhat less susceptible to physical and chemical interferences
Neutron-activation analysis can detect as little as 10*~9 g of
9-2
-------
platinum and 10 g of iridium, which correspond to 0.001 and
0.00001 ppra, respectively, in a 1-g sample. Sample preparation
can have a significant effect on the analysis, because the platinum-
group metals are often not distributed homogeneously throughout a
sample.
In a variety of ways, the chemical, petroleum, and electric industries
account for about 80% of the platinum-group metals used in the United States.
These metals are among the most versatile catalysts known. Before the intro-
duction of catalytic converters on cars in 1975, the largest use of platinum
was as a catalyst for the reforming of naphtha to make high-octane gasoline.
Platinum, palladium, and rhodium are used as catalysts for oxidizing exhaust-
gas pollutants, synthesizing nitric acid through ammonia oxidation, and
hydrogenating a wide range of unsaturated hydrocarbons. Osmium and ruthenium
are catalysts for stereospecific hydroxylation of olefins, and ruthenium has
high catalytic activity for formation of polymethylenes from carbon monoxide
and hydrogen at high pressure. Portable space heaters using platinum catalysts
are also becoming popular.
Because of its resistance to oxidation, palladium (sometimes
alloyed with other platinum-group metals) is used extensively in
the electric industry for contacts in relays and switchgear, in
resistors and capacitors, as electrochemical and spark electrodes,
and in fuel cells. Platinum (also frequently alloyed with other
metals) has many high-temperature applications, such as in pre-
cision resistance thermometers, thermocouples, strain gauges, and
9-3
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laboratory ware. Palladium has the remarkable ability to dissolve
large quantities of hydrogen, a property that makes it useful in
devices to purify hydrogen. Because of their nontarnishing re-
flective properties, the platinum-group metals are used as reflectoi
surfaces and in jewelry.
Other properties that make the platinum-group metals useful ,
are their resistance to oxidation, ductility, high melting points,
good electric conductivity, and high density. They form a variety
of alloys with each other and with other metals that can greatly
affect their hardness.
In the metallic state, the platinum-group metals are normally
unresponsive to direct chemical attack by oxygen, halogens, acids,
etc., under mild conditions; however, at high temperatures (about
1000° C), volatile oxides and halides can be formed. The metals
are more susceptible to attack by alkaline fusion, especially
in the presence of oxidizing agents. The state of subdivision of
the metal particles can have a profound effect on their reactivity,
the smaller particles normally being the more reactive.
All the platinum-group metals form water-soluble organo-
metallic coordination compounds. Some of these complexes are now
being used as highly selective homogeneous catalysts in commercial
applications (e.g., rhodium halides for converting methanol and
carbon monoxide into acetic acid with selectivity greater than
99%). Others, such as the neutral square planar cis-dichloro-
diammineplatinum(II), have physiologic activity and are being
9-4
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developed as anticancer drugs. Cluster compounds containing two
or more of the platinum-group metal atoms per molecule bound
together with bridging species, such as carbonyl groups, can also
be synthesized. These compounds permit variation of the metal-
metal bond distance, a factor that may have a significant in-
fluence on catalytic behavior.
In their metallic states, the platinum-group metals have
extremely low toxicities. Some alloys containing these metals
are actually used as dentifrices and prostheses. However, many
of the soluble compounds of these metals (e.g., chlorides, amino-
chlorides, and volatile oxides) are highly toxic and can elicit
physiologic responses at concentrations as low as 10"^ g/ml.
Generally, the chlorides are more toxic than the oxides. Within
a given class of compounds, the toxicity appears to follow the
water solubility, although lipid solubility is also necessary to
effect transport into the cells. The route of administration
affects a compound's toxicity and decreases in the order: intra-
venous>intratracheal""intraperitoneal>inhalation»oral. For ex-
ample, the LD50 for PdCl2 administered intravenously in rats and
rabbits is about 5 mg/kg of body weight, but 200 mg/kg when it is
given orally. Neither PtO nor PtCl2 causes ocular irritation,
although a few milligrams of PdCl^ will cause corrosive lesions
of the conjunctiva and severe inflammation of the cornea.
Several ionic derivatives of the platinum-group metals have
the capacity to become attached selectively to specific chemical
9-5
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aitea in proteins. For example, PtCl42" will Attack such func-
tional groups as dlaulfide bonds, terminal NHj groups, and
methionines. liomorphio replacement with the heavy metal a1Iowa
the structure of protein* to be determined more readily by X-ray
crystallography. PdCl2 selectively combine! with and renders
inactive the enzymes trypsin and ohymotrypsin, although it does
not inhibit oatalaae, lysoiyme, peroxidase, and rlbonuolease.
Platinum complexes also Interact with functional groups (e.g.,
-Nil?* -COj", and -80113) on amino aoids and with many viruses,
They are highly effective in deactivating bacteria, although
neuromuscular toxicity prevents their general clinical use in
combating bacterial infections.
Of particular significance IB the ability of platinum
complexes to bind strongly with receptor sites in nucleic acids,
such as DNA. It is thought to be this ability that makes the
complexes effective as antioancer drugs. qis-Diohlorodiammlne-
platlnum(II), as well as other similar g_is-dlohloro complexes,
is highly effective in causing regression of tumors in animals.
Now in Phase II clinical trials, these drugs also have shown
effectiveness in treating cervical, teatlcular, ovarian, proa-
tat lo, bladder, and head and neck cancer in humans. When used
by themselves, the drugs have very severe side effects on kidneysi
however, these effects can be lessened by proper selection of the
Uganda in the platinum complex and by pharmaoologic treatment of
the patient. cJj^Diohlorodiammineplatlnum(XZ) is a useful drug
in the treatment of some types of neoplasm; it appears most
9-6
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effective when uaed in combination with other antineoplaetio
agents. It is anticipated that additional anaieguee with higher
therapeutic ratloa will ba developed.
Compound• of platinum-group metals are quickly eliminated
from the body (mainly in tha urina and feoea), exoapt if Intro-
duoad intravenouaiy, in whioh caaa elimination ia muoh alowar.
Tha matala ratainad ara apparently concentrated in tha kidney•,
liver, and aplean.
Tha effect! of inhalation have become of interest, baoauae
that ia tha moat likely mode of axpoaura of the general population
if aignifioant quantities of tha matala ara emitted from exhauat
ayatema of oara equipped with catalytic convertera. Preaent ex-
poaure ia limited mainly to employeea working in platinum-metal
refinerlea who inhale duat partlolea of aalta or aoida of theae
metals, fuoh people often become "sensitized" and develop asthmatic
or dermatologlo allergiea (sometimes called "platinoala") that
dlaappear when the expoaure la discontinued, Theae reactions are
thought to be caused by hiatamlne releaaa triggered by the com-
pounds, only ionio water-aoluble platinum aalta cause theae
reactlona,
A very aenaitive and reliable skin-prick teat has been de-
vised to aeparate people into two distinct olaaaeai atopie
(sensitive to common environmental allergens—about 30% of the
population) and nonatopio (the remaining 701, who are leaa sus-
ceptible to allergens). After prolonged expoaure by inhalation
9-7
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of dusts containing the soluble platinum salts, a significant
fraction (over 50%) of people from both classifications can be-
come "sensitized," i.e., show clinical symptoms of allergic
reactions; the atopic group become sensitized more quickly than
the nonatopic. As with the toxic behavior, the allergenicity of
the ionic platinum complexes is apparently determined by the number
of chlorine atoms in the molecules; ammonium chloroplatinate,
(NH4)2PtCl6, and chloroplatinic acid, I^PtClg, are the most aller-
genic of the compounds tested. Exactly what concentrations are
required for sensitization and for eliciting clinical responses in
sensitized people is not known; however, it is clear that much
lower concentrations can cause responses in sensitized people than
are required to cause the initial sensitization. Once sensitized,
a person apparently retains that tendency for the rest of his life.
Environmental exposure to soluble salts of platinum-group
metals is currently confined primarily to mining areas, platinum-
metal refineries, and catalyst synthesis plants. Measurements
of soil near freeways and tissue samples from autopsies of un-
__ -t
exposed people show noble metals at less than 1 ppb (1 ppb = 10
wt %). Air samples taken near freeways in California typically
contain platinum or palladium at less than 10~7 ug/m3 (the detection
limit of the analytic techniques used). Autopsy samples of people
occupationally exposed to the platinum-group metals through em-
ployment in mining areas also show negligible concentrations of
the metals in body tissue. However, several urine samples from
9-8
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refinery workers have shown measurable concentrations of platinum
and palladium, although concentrations in the blood of such workers
were below the detection limits of the analytic equipment. Ap-
parently, the inhaled salts are rapidly excreted, primarily in
the urine, and are not accumulated in the body tissue, although
measurements of tissue samples from autopsies of refinery workers
need to be made.
The extremely low concentrations of the platinum-group metals
make it difficult to obtain reliable baseline data. Sample con-
tamination must be rigorously avoided. The difficulties are
readily illustrated by the inconsistencies in concentrations re-
ported by the various laboratories, which can vary by much more
than an order of magnitude. Standardized procedures need to be
developed, with frequent cross-checks on the same sample by various
laboratories involved in making the measurements.
Catalysts containing small amounts of platinum, palladium, and rhodium
to remove pollutants from automobile exhaust were introduced in most
1975 cars sold in the United States; these devices were necessary
to meet the stringent limits on maximal emission of carbon monoxide,
hydrocarbon, and NOX mandated by the Federal Clean Air Act of 1970.
In the converters, platinum and palladium (in a ratio of about
2.5:1, which is typical of the ratio obtained in the South African
mines) at about 0.06 oz/car are deposited as small crystallites
on the surface of spherical pellets (or along the channel walls
of ceramic monolithic supports) made of a refractory oxide, such
as alumina.
9-9
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Carbon monoxide and hydrocarbon (the only pollutants now
being catalytically controlled) require an ozidizing atmosphere,
which is achieved by running the engine "fuel-lean" or by adding
excess air to the exhaust stream via a fanbelt-driven air pump.
Catalytic removal of NOX, the most difficult pollutant to con-
trol, requires a reducing atmosphere (accomplished by running
the engine "fuel-rich"). Normally, the conversion of all three
pollutants would require at least two separate catalyst beds,
each having a unique chemical composition and involving a differ-
ent type of atmosphere. However, if the fuel composition can be
maintained within about +0.2 air:fuel ratio units of the stoi-
chiometric value necessary for complete combustion, all three
pollutants can be controlled in a single three-way catalyst bed.
It is likely that, as the NOX standards become increasingly
stringent, this will be the approach taken to comply.
The use of oxidizing catalytic converters has contributed
greatly to decreasing the emission of carbon monoxide and hydro-
carbon, including such species as benzo[a]pyrene, which are known
to be carcinogenic. That control of such emission improves air
quality has been dramatically illustrated by a comparison of the
effects of diluted exhaust from catalyst and noncatalyst cars on
the health of several species of mammals. Survival rates of infan
rats and the general health of animals exposed to noncatalyst ex-
haust were poor, whereas those exposed to catalyst exhaust behaved
the same as control groups subjected to normal air. Moreover, the
9-10
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catalytic converters permit increased fuel economy, because they
effectively decouple emission control from engine performance
and allow the latter to be optimized.
On the negative side, oxidizing catalytic converters have
a tendency to convert part of the fuel sulfur into sulfuric acid
aerosol mists with particles in the respirable size range. Even
though the vehicles are responsible for only 1% of the total sulfur
oxides released by human activity in the United States, mathematical
models developed by the EPA have predicted that, under "worst-case"
conditions (heavy vehicular traffic and adverse meteorology),
locally high concentrations (up to 600 yg/m3) might occur in the
vicinity of freeways. The threshold for irritability in sensitive
people (on the basis of 24-h exposure) is thought to be around
10 yg/m3. These mathematical models, however, are not very reliable
and tend to overestimate the problem. Wind, even at velocities as
low as 0.2 mph, can dramtically decrease the maximal concentration,
and chemical reactions to form sulfates (which are less toxic than
sulfuric acid) lower the steady-state acid concentration further.
Under any realistic conditions, it is doubtful that the "worst-case"
concentrations of sulfuric acid will exceed about 50 yg/m3.
Recently, experimental tests with 352 cars operated under
simulated freeway conditions gave observed sulfuric acid-sulfate
concentrations much lower (by at least a factor of 7) than would
have been predicted by some of the early EPA models. Particularly
significant was the observed vertical displacement of the
9-11
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acid-sulfate plume, which resulted in concentrations lower than
the models predicted near the ground.
Although concentrations of sulfuric acid and sulfates around
freeways are indeed increased because of the presence of cata-
lytic converters on cars, it appears that the hazard is not
nearly so great as originally feared.
It is generally accepted that minute quantities of noble
metals are emitted from catalytic converters, perhaps 1-3 yg/mile,
There is no evidence that any of this emitted material is in the
form of physiologically active soluble platinum-metal salts.
Moreover, even if all the emitted material remained suspended
in the air and behaved like other aerosol particles, the maximal
possible steady-state concentration (according to the EPA "worst-
case" model) would not exceed 0.06 yg/m3; the OSHA standard for
exposure to soluble platinum salts is 30 times larger than that.
However, if all the emitted material were localized in the top-
soil near the freeways, it would take over 1000 years for it to
reach the concentrations
observed in the ore in the South African
mines. Although platinum(IV) compounds can be methylated by
microorganisms, there is no evidence to suggest that platinum
metal (the probable chemical form of the emitted material) can
be methylated in such a way. It is thus concluded that emission
of platinum-group metals from catalytic converters in cars will
not pose a significant health problem. However, it would be
appropriate for the concentration in soil near busy freeways to
be monitored during the next several years.
9-12
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Overheating of catalytic converters due to engine malfunc-
tion sometimes occurs and has led to a small number of fires
both inside vehicles and in grass beneath the cars. However,
this problem is not peculiar to cars with catalytic converters;
many late-model cars without catalytic converters also have ex-
haust systems that are hot enough to cause grass fires. This
tendency makes it important to keep the engines of automobiles
in good working condition and to exercise extreme care when
operating cars in such areas as fields and national parks, where
there is dry grass.
The use of platinum-group metals in catalytic converters
has increased by nearly 50% the demand for these imported ma-
terials in the United States. Production from foreign mines
has been expanded to meet this demand, but installation of such
devices on cars in western Europe and Japan could place a burden
on the traditional suppliers. It is hoped that the newly dis-
covered deposits in the United States can be developed to meet
some of this demand.
It is concluded that at present the only adverse health
effects directly attributed to the platinum-group metals involve
workers in refineries that purify the metals and plants that syn-
thesize noble-metal catalysts. However, it is suggested that
vigilance be maintained to determine whether problems arise as
a result of emission of harmful compounds into the environment
from catalytic converters or from anticancer drugs that contain
these metals.
9-13
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CHAPTER 10
CONCLUSIONS
TOXICITY CLASSIFICATION
Some of the water-soluble complex salts (e.g., chlorides and
amino chlorides) and volatile oxides (e.g., 0304, RuO4, and IrO2)
of the platinum-group metals have physiologic activity; chlorides
are more toxic than oxides. Within a given class of compounds,
the toxicity apparently follows the water solubility to some
degree. Although water solubility is necessary for toxicity,
it is not by itself a sufficient condition; lipid solubility is
also necessary, to effect transport into the cell. Nonvolatile
oxides (e.g., PtO and Pt02) have very little toxicity or aller-
genicity. None of the platinum-group metals is toxic or aller-
genic in the metallic state.
ROUTE OF ADMINISTRATION
For a given compound, toxicity varies with route of administra-
tion. Usually (e.g., for PdCl2), the order of toxicity is:
intravenous>intratracheal~intraperitoneal>inhalation»ingestion,
although in some cases (e.g., RhCl3) intraperitoneal administration
produces more toxicity than does intravenous. Absorption into
the bloodstream may be the most important factor in determining
toxicity.
10-1
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MODE OF CIRCULATION
Almost all (greater than 99%) of toxic platinum compounds
circulates as protein-bound material in the serum; almost none
is absorbed by the tissues. Although large amounts of the com-
pounds enter the placenta, only small amounts enter the fetus in
rats. Furthermore, almost none enters the brain.
ELIMINATION
Elimination of metal salts administered orally in trace or
pharmacologic quantities is rapid (>99% in 3 days; more in the
feces than in the urine). When they are administered intravenously,
elimination is much slower; only about half the material is ex-
creted in 14 days. The retained material is concentrated mainly
in the liver, kidneys, and spleen.
CAUSES OF ALLERGENICITY
Several soluble platinum compounds can cause allergic re-
sponses in sensitized people. These allergic reactions are ap-
parently caused by histamine release triggered by the platinum
compounds. Antihistamines can protect animals (e.g., guinea
pigs) and humans from anaphylactic shock resulting from sodium
chloroplatinate in either the +2 or the +4 oxidation state.
However, there is apparently no relationship between histamine
release and acute toxicity.
10-2
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SKIN-PRICK TESTS
Skin-prick tests with dilute concentrations of soluble
platinum complexes appear to provide reproducible, reliable,
and highly sensitive biologic monitors of allergenicity. After
sensitization through previous exposure, administration of as
little as 3 x 10~15 g of the allergenic compound will produce a
wheal in highly susceptible people. There are no suitable animal
models by which to measure human response to these compounds.
•
CANCER CHEMOTHERAPY
Platinum-group-metal complexes—e.g., cis-dichlorodiammine-
platinum(II), l,2-dinitratodiamminecyclohexaneplatinum(II), and
rhodium(II) carboxylates—have remarkably high therapeutic indexes
(some as high as 500 in selected animals) as chemotherapeutic
agents for arresting some types of cancer. The biggest problem
with these compounds in humans has been their harmful effect on
the kidneys. However, this difficulty has been substantially de-
creased by selection of appropriate ligands in the complex and by
applying the complexes in combination with other drugs. With these
successes, some of the most promising compounds are now being tested
in Phase II clinical trials, and it is anticipated that they will
become widely used as anticancer agents in the near future.
REFINERIES
The only places where platinum-group metals reach physiologi-
cally noticeable concentrations are in refineries that purify the
metals, mines, and plants that synthesize catalysts containing
10-3
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the metals. A significant fraction (around half) of the people
exposed in these refineries to some water-soluble salts of plat-
inum develop allergic reactions, with symptoms similar to those
of hay fever, and dermatologic problems. Although the mani-
festations are not usually so severe that they require corrective
action, in a few cases the exposed people must be moved to job
locations where such exposure does not occur. When this is done,
the allergic manifestations disappear.
CATALYTIC CONVERTERS
The newest and most extensive use of the platinum-group
metals is in catalysts for purifying exhaust from automobiles.
Minute quantities (about 1-3 yg/mile, or 0.6-1.9 yg/km) of plat-
inum and palladium are emitted from the exhaust systems of auto-
mobiles equipped with catalytic converters; much of this material
may accumulate alongside roadways. However, this material is in
a chemical form that is physiologically innocuous (no detectable
soluble salts), and it is concluded that such emission poses no
threat to the environment. Because there is no evidence that
platinum metal can be methylated by microorganisms and solubilized
in the same way the mercury is methylated, this deposited material
should not have an adverse effect on the environment.
SULFURIC ACID
Health effects due to emission of sulfuric acid from auto-
mobiles equipped with catalytic converters that contain platinum-
group metals will probably not be very serious. Experiments
10-4
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conducted under simulated freeway-driving conditions have demon-
strated, that the concentrations of sulfuric acid and sulfate are
much lower (perhaps by more than an order of magnitude) than those
originally predicted on the basis of some mathematical models. It
is concluded that available information about the effects of
turbulence, dispersion, thermal drafts, atmospheric reactions,
surface adsorption, and wind velocity is insufficient to permit
valid predictions from such models.
PLATINUM RECYCLING
Recycling of platinum-group metals used as catalysts in
the petroleum and chemical industries is substantial. Recovery
of metal returned to the refiner usually exceeds 99%. It is
doubtful, however, that there will ever be a significant reclama-
tion of used metals from automobile emission control catalysts,
because of the relatively low value of the metals (<$10 in each
converter), the low concentrations (<0.1 wt %), the high cost of
dissolving the refractory oxide support, and the logistics of re-
turning the material to the central refinery.
MATERIAL AVAILABILITY
Material availability may become a problem, because almost
none of the platinum-group metals is produced from domestic
sources. Most of it comes to the United States from mines in
South Africa, the U.S.S.R., and Canada. The recent introduction of
catalytic converters in automobiles has almost doubled our use
of these materials. However, the recent discovery of a new
10-5
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deposit of palladium-rich ore in Montana could help to relieve
the situation. It is estimated that the worldwide proven re-
serves (excluding the new U.S. discovery) should be sufficient
to last about 100 years at current rates of consumption, although
the consumption will probably increase substantially as other
countries (e.g., Japan) begin to require catalytic converters
for control of emission from automobiles.
10-6
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CHAPTER 11
RECOMMENDATIONS
TESTING OF COMPOUNDS
Newly synthesized compounds containing platinum-group metals
should be routinely tested for acute and chronic toxicity, for
allergenicity, and for therapeutic potential as possible agents
for cancer chemotherapy. In addition, new compounds (as well as
several known compounds, particularly those which could enter the
biosphere) should be subjected to long-term physiologic testing
for carcinogenicity and teratogenicity. Bioassays should be estab*
lished to determine whether accumulation of these compounds occurs.
BASELINE TISSUE DATA
As platinum-group compounds become commonly used as cancer
chemotherapy agents, the materials will increase in the tissues
of treated patients. For this reason, it will become important
to monitor their concentrations continuously and to seek concen-
tration trends in the overall population. Current baseline tissue
data are contradictory, and observed concentrations of these
metals vary over two orders of magnitude. Reliable, standardized
methods for ascertaining tissue concentrations need to be developed,
and the same material should be measured by different groups, to
ensure consistency. At the very low concentrations in question,
contamination takes on critical importance. Baseline data need
11-1
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to be collected on workers in platinum-metal refineries and in
catalyst-synthesis plants,
ALLERGENICITY VS. ATMOSPHERIC CONCENTRATION
The inhalation tests developed to examine sensitivity to air-
borne particulate matter, although reasonably reproducible, cannot
be related easily to concentrations of the same materials in the
air. Because inhalation of particulate matter is an important
route of exposure of people in refineries (and possibly of the
general population, if significant quantities of metal are emitted
from catalytic converters), it is important to determine the ef-
fects of concentration, chemical composition, and particle size.
More refined tests to examine these effects need to be developed.
This information is critical in establishing useful concentration
standards.
LEADED GASOLINE AND CATALYTIC CONVERTERS
It has been concluded that the amounts of platinum and
palladium normally emitted in exhaust from cars equipped with
catalytic converters are very small. However, the measurements
were made with fuels that contained no lead. In practice, leaded
gasoline is occasionally introduced (whether inadvertently or
deliberately) into these vehicles, and the consequences of such
introduction have not been fully investigated. Although the
poisoning of the activity by lead has been well documented, the
effects of the halide scavengers (ethylenedichloride and ethyl-
enedibromide) on the volatility of the noble metals is not known.
11-2
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It is possible that the halides react with the metals to form
volatile species that can lead to increased removal of the metals
from the converters. This possibility needs to be tested.
ALLERGENICITY AND CATALYTIC CONVERTERS
People who have been "sensitized"—i.e., who exhibit allergic
responses when exposed to very small quantities of some soluble
platinum compounds—are extremely sensitive biologic monitors of
trace amounts of these materials. It is recommended that carefully
conceived tests be made with such hypersensitive people, to seek
evidence of emission of these compounds from cars equipped with
catalytic converters. Although almost all of the small amount of
platinum-group metals is emitted in an insoluble form, these tests
might provide the most sensitive diagnostic tool for measuring
emission of soluble material. This may become important to the
relatively few people who are hypersensitive.
NEW SOURCES
The claim of an extensive deposit of palladium-rich ore in
Montana needs to be confirmed, and its market potential evaluated.
The search for additional deposits of ore in the United States
needs to be encouraged.
11-3
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APPENDIX
CATALYTIC REFORMING IN REFINING OF PETROLEUM
GENERAL OVERVIEW
The use of platinum catalysts in petroleum refining has been
discussed in the National Materials Advisory Board Report (NMAB-297,
March, 1973), entitled "Substitute Catalysts for Platinum in Auto-
287
mobile Emission Control Devices and Petroleum Refining." Consider-
able changes have since taken place in the refining operation,
particularly with the introduction of the mandatory use of unleaded
gasoline in 1974. The changes in gasoline production can be seen
in Figure A-l, which shows the production of total gasoline and
premium gasoline from 1968 to 1974. It is interesting to note
that the discontinuity in gasoline production in 1971 is coincident
with the impact of the initial noncatalytic attempts at pollution
control—lower compression ratios, higher air:fuel ratios (engines
run with leaner carburetion, or with excess air), and greater spark-
advance retard. As the demand for premium gasoline diminished, a
portion of the capacity for making the premium gasoline went into
production of unleaded gasoline, when the need for unleaded gasoline
became effective in 1974. Thus, the total refinery balance, with
respect to a pool octane, has not been greatly affected. With the
introduction of catalytic converters on 1975 model cars, a fuel-
economy benefit has been realized, and there are now economic
pressures to improve this further. Although the direction toward
A-l
-------
IV)
2.5
'o 2.4
f*n
x
I.
>!2.3
o> •
M
*g 2.2
I
8
04
(0
c 2.1
••H
rH
10
5 2.0
(fl
1.9
67
68
69
70 71
Year
72
73
74
900
\o
800
O
r-l
X
id
s.
H
s
•o
0)
•O
2
Oi
(0
700 g
•H
O
(0
M
CU
600
FIGURE A-l. Total production of gasoline and premium gasoline in the United States, 1968-1974. (Data from
Universal Oil Products Company.)
-------
lighter vehicles is obvious, the sacrifice in car size and comfort
may be more difficult to sell in a competitive automotive market,
so other methods of increasing economy must be examined. The
most obvious one is a higher compression ratio, which provides
for a higher cycle efficiency and some reduction in engine weight;
both factors are beneficial with respect to fuel economy (see
Figure A-2).
Higher compression ratios will lead to a greater emission
of nitrogen oxides; however, within the period involved, it is
quite likely that better carburetion devices for controlling the
air:fuel ratio will be developed for use in conjunction with the
catalytic converters. Thus, a truly three-way control system for
carbon monoxide, hydrocarbon and NOV will have a positive impact
•o.
on both fuel economy and environmental quality-
What will be the impact on the refining industry? Within
the next 5 years, the industry will probably be heading in the
direction of producing a high-octane unleaded fuel as a major
!
refinery product. The gain in fuel economy will more than off-
set the crude-requirement penalty associated with making an un-
leaded, high-octane pool gasoline. From Figure A-3, it will be
observed that, for a given number of miles driven, the crude re-
quirement is at a minimum when the pool octane number is about
96.5. It should be noted that the crude-requirement change de-
picted in Figure A-4 merely indicates an additional crude require-
ment for a given volume of gasoline; in reality, because some of
A-3
-------
0>
c
41
40
(D
U
H
(D
c
-------
100
98 •
96
94
92
90
88 90 92 94 96 98
Unleaded Research Octane Number of Gasoline Pool
FIGURE A-3.
Relative crude oil requirement to drive a given car a given
distance under same driving conditions with unleaded gasolines
of research octanes above 89.5. (Data from Universal Oil
Products Company.)
A-5
-------
115
91
93 95 97
Pool Octane Number
99
FIGURE A-4.
Estimated crude-oil requirements for various pool octane
gasolines. Data based on constant product distribution. East
Coast location, 100,000-barrel/day capacity, typical refinery.
(Data from Universal Oil Products Company.)
A- 6
-------
that additional crude is converted into other products (such as
methane and liquid petroleum gas), the byproducts are salable or
useful for internal refinery energy needs. The net additional
refinery energy need to finance the higher pool octane number is
around 1%.
In the petroleum refining industry, the major use of platinum
catalysts is in catalytic reforming. The last 5 years have wit-
nessed a switch to bimetallic catalysts for reforming. A number
of combinations are in commercial use; of these, platinum-rhenium
is one that is widely used. The use of bimetallic catalysts repre-
sents a great improvement in platinum-based catalytic reforming
systems, from the standpoint of efficient and stable operation.
In addition, there has been a reduction of around 20% in the
concentration of the platinum component in bimetallic catalysts.
It is estimated that the platinum used in catalytic reforming is
now over 1,100,000 troy oz. (34,200 kg) and increases by about
200,000 troy oz. (6,200 kg) each year.
With the increased use of bimetallic catalysts and the more
sophisticated performance requirements, the chances of substitution
for these catalyst systems in catalytic reforming appear to be
even more remote than they were 2 years ago, at the time of the
issuance of the NMAB report on substitute catalysts.
Some comments on precious-metal recovery and allergic reac-
tions to platinum salts are appropriate.
A-7
-------
Because the platinum-group metals are so valuable, great
care is exercised in monitoring all streams entering and leaving
the catalyst manufacturing sites, to ensure that no significant
amounts of materials are lost. Yearly overall materials balances
of about 99.75% are attainable.
Regarding allergic reactions to platinum salts, an example
can be drawn from records about employees maintained since the
commissioning of the platinum-catalyst manufacturing plant by
the UOP Company in Shreveport, Louisiana, in 1952. The work
force was 100 persons at that time and has grown to 250 persons.
During 23 years of operation, a total of 15 persons developed
allergies to platinum salts, with various degrees of sensitivity.
In a number of instances, removal to a different location or job
in the same plant area solved the problem. Those with the greatest
sensitivity have moved to jobs outside the company. There have
been no reports of allergic reactions when the catalyst in its
final form was handled at the refinery site.
The NMAB report (p. 2 of "Summary of Conclusions and Recom-
mendations") indicated that "the use of platinum catalysts in
other petroleum processes also will increase, but until 1980 the
amount of platinum required for these processes will be less than
25% of that needed for reforming.11 This statement has been re-
examined in the light of the present trend toward clear gasoline
production. The other processes that use platinum and palladium
are hydrocracking, isomerization, and hydrogenation (benzene to
A-8
-------
cyclohexane)- The balance of the various refinery processes will
depend to a great extent on the characteristics of the crude-oil
supply and the changing needs for various products, as well as on
improvements in refining. Nevertheless, the emerging patterns are
such that isomerization will be playing a more important role in
gasoline production, in view of the remaining need to balance out
the relationship between boiling point and octane number. Hydro-
cracking is playing an important role in the production of clean
middle oils and may assume greater interest for gasoline produc-
tion; however, the advances in catalytic cracking have a bearing
on the selection of the process for gasoline manufacture. With
respect to the use of noble metals, these appear to be much more
entrenched in isomerization than in hydrocracking, where the trend
has been away from noble metals. Thus, it remains safe to indi-
cate that, although the process split has been changing, noble-
metal use in nonreforming processes will remain below 25% of re-
forming use for a number of years,
CATALYTIC REFORMING
In the petroleum industry, the word "reforming" is used to
designate a process by which the molecular structure of naphthas
is changed, or "reformed," with the intent of lessening the knock-
ing tendency (or raising the octane number, which describes the
ability of the fuel to burn smoothly under conditions of high
pressure) of those naphthas in internal-combustion engines. The
reforming process is also used to synthesize aromatics—particularly
A-9
-------
benzene, toluene, and Cg aromatics—from selected naphtha
fractions.
The antiknock quality of unleaded gasolines is related to
the chemical structure of their constituent hydrocarbons. Paraf-
fins, olefins, naphthenes, and aromatics are the four main hydro-
carbon types of which gasolines are composed. Normal paraffins
have the lowest octane numbers among the hydrocarbons, and the
isomerized or branched paraffins have much higher octane ratings.
It is well known that the octane number scale has been defined
by ascribing a zero rating to n-heptane, which is particularly
prone to knocking, and a rating of 100 to isooctane (2,2,4-
trimethylpentane), one of the more highly branched octanes.
Generally, monoolefins have higher octane numbers than corres-
ponding paraffins. Naphthenes, or cycloparaffins, have very high
octane numbers. The aromatics have exceptionally high octane
numbers, generally over 100. Although the relationship between
hydrocarbon structure and knock rating is highly involved, these
broad generalizations indicate the structural changes that reform-
ing processes are intended to accomplish, to raise the octane
number of gasolines.
Native hydrocarbon types vary widely in the proportions—
in which they occur in petroleum from different fields; therefore,
the octane ratings of "straight-run" gasolines vary. Most straight-
run gasolines, obtainable by simple distillation from crude oil,
contain only paraffins, naphthenes, and aromatics, and they have
A-10
-------
octane numbers of 50 or less. As examples, the hydrocarbon types
and octane numbers of a typical domestic midcontinent (produced
from the center of the United States) and of a Kuwaiti depentanized
naphtha are shown in Table A-l. The octane number of straight-run
naphthas, even with the addition of lead alkyls, is too low to
permit their inclusion in commercial gasolines. Therefore, the
chemical composition of these straight-run naphthas needs to be
changed, or reformed, so that they can be used in modern internal-
combustion engines without knocking.
THERMAL REFORMING
The first refining process used to change the composition of
native naphthas to improve their octane rating was thermal reform-
ing, introduced in 1930. In this process, which was conducted at
just over 1000° F (538° C) and at pressures of 500-1000 psig
2
(3,450-6,900 kN/nr), olefins were produced from paraffins, high-
molecular-weight paraffins were cracked to produce low-molecular-
weight paraffins with higher octane numbers and olefins, and
native arpmatics were concentrated by the cracking of paraffins
into much smaller gaseous fragments. Although other reactions
were involved, those named resulted in an increase in the research
octane number up to about 85, but the liquid product reformate
yields would have been too low to be economical today. The crea-
tion of aromatics with very high octane numbers was insignificant
in thermal reforming; they were concentrated by the destruction of
paraffins to gaseous hydrocarbons.
A-11
-------
TABLE A-l
Compositions and Octane Numbers
of Straight-Run Naphthas?
Source
a 416
Data from Sterba.
Mid-Continent Kuwait
Hydrocarbon com—
position, vol %;
Paraffins 48 67
Naphthenes 42 22
Aromatics 10 11
Research octane
number;
Clear 47 39
With tetraethyl lead 73 67
at 3 ml/gal
A-12
-------
The inefficiency resulting from this destruction of paraf-
fins by the thermal process and the inability to synthesize aro-
matics constituted an incentive to develop a catalytic reforming
process that could be more selective in promoting the desired re-
actions and in minimizing the unwanted reactions. The first suc-
cessful catalytic process that arose from this effort made its
commercial appearance in 1949 under the name of the "UOP Plat-
forming process"; it used a catalyst containing a noble metal.
PROCESS DESCRIPTION
Principal reactions (to be described later) that character-
ize reforming processes that use noble-metal catalysts are:
• dehydrogenation of naphthenes to aromatics, to
near completion, with very little ring rupture
• isomerization of paraffins to more highly branched
forms
• dehydrocyclization of paraffins to aromatics
• hydrocracking to paraffins to lower molecular
weights, but with minimal production of light
gaseous hydrocarbons
Because these reactions are conducted in an atmosphere of hydrogen
under pressure, no olefins are produced; catalytic reformates are
therefore much less susceptible to oxidative reactions and are
more stable in storage than thermal reformates. The hydrogen en-
vironment in the catalytic reaction zone is created deliberately
A-13
-------
to minimize fouling and deactivation of the catalyst by the
formation of carbon on its surface.
The superiority of catalytic over thermal reforming was
demonstrated experimentally in a comparison-^8 that showed a
reformate yield of 85 vol % for the catalytic process and a yield
of only 55 vol % for the thermal process, in the reforming of a
midcontinent naphtha to a product with a research octane number of
85.
Although a variety of catalytic reforming processes are in
use, the simplified flowsheet in Figure A-5 describes the Plat-
forming process, which was the original of the family using noble-
metal catalysts and is still the most extensively used process of
this group. The flowsheet shows the reforming unit to be composed
of five principal sections:
• reactors that contain the particulate catalyst in
fixed beds
• heaters to bring the hydrotreated naphtha charge and
hydrogen recycle gas to reaction temperature and to
supply the heat of the reaction
• product cooling section and a gas-liquid separator
• hydrogen gas recycling system
• fractionation to separate light hydrocarbons dis-
solved in the separator liquid
The particulate catalyst is contained as a fixed bed in
three (.sometimes more) separate adiabatic reactor vessels, with
combined-feed preheating before the first and reheating between
A-14
-------
Heaters
Reactors
Separator
Stabilizer
(Jt
Net
Hydrogen
Light Ends
To
Recovery
" 1 <
. Charge" »
FIGURE A-5. Universal Oil Products Company conventional Platforming process.
Plotformote
-------
subsequent reactions. Because of the rather large endothermic
heat of the dehydrogenation reactions, there is a substantial
drop in temperature of the flowing reacting stream, particularly
in the first reactor, in which the rapid naphthene dehydrogena-
tion reaction occurs. Therefore, the effluent from the first and
second reactors is reheated to the required inlet temperature for
the subsequent reactor. Usually, the charge heater and the inter-
heaters are in the same furnace housing.
Effluent from the last reactor in the train is cooled to
ambient temperature and led to a receiver in which the product
mixture is separated into liquid and gas streams. Most of the
. separated gas stream, which is largely hydrogen, is compressed and
recycled to the reactors, to provide the protective hydrogen partial
pressure in the reaction environment. A net hydrogen product stream
is withdrawn from the system by pressure control on the reaction
system.
The receiver liquid, containing dissolved light hydrocarbons,
is routed to a fractionator to produce a stabilized reformate suit-
able for blending into finished gasoline pools and generally free
of hydrocarbons lighter than C$.
Most variations in the flow-diagram arrangement of the several
catalytic reforming processes in commercial use involve the concept
of catalyst regeneration frequency. At relatively high pressures
and high ratios of hydrogen recycle gas to naphtha feed, the cata-
lyst is not rapidly fouled and deactivates rather slowly, so that
continuous processing runs of a few months to more than a year are
A-16
-------
attainable. At the end of this uninterrupted processing, the re-
actor inlet-temperature requirements to maintain the desired
reformate octane number may reach the limit of heater capabilities,
or the catalyst selectivity may diminish to a point where it is
economical to terminate the run and restore the activity and
selectivity of the catalyst by in situ regeneration.
Regeneration is usually performed in place by burning the
carbonaceous deposit accumulated on the catalyst with air diluted
with combustion-product gases; the gas recycle compressor is used
to circulate the mixture of air and combustion gas through the
reactor system at a controlled burning temperature. This regenera-
tion procedure, with proprietary additional steps, can restore the
performance of the catalyst to what it was at the beginning of the
preceding processing cycle. In an alternative procedure, which
avoids the downtime involved in in situ regeneration, the spent
catalyst is unloaded and the reactors are reloaded with fresh
material. The spent catalyst is generally returned to the supplier
for recovery of its precious-metal content.
At the other extreme is the concept of regeneration frequency
in the process designed to operate at distinctly lower pressures
and lower ratios of hydrogen recycle gas to naphtha. Higher
reformate yields of a given octane number are obtainable; but,
under these conditions, the catalyst fouls and deteriorates much
more rapidly, so frequent catalyst regeneration is necessary.
These plants are provided with an additional reactor, so mani-
folded to the other reactors and appropriately valved that any
A-17
-------
one reactor can be taken off-line and regenerated, while the
others continue to process naphtha feed. A reactor can be iso-
lated from processing service for regeneration as often as once
a day. Although higher reformate yields are obtainable with
this "swing reactor" design, the units tend to be more expensive,
because of the additional equipment required.
Between these extremes are other designs and operating tech-
niques that can perform at any intermediate regeneration fre-
quency; the choice of either extreme or an intermediate depends
on the economics of the particular refining situation.
In the last 3 years, a new concept in catalytic reforming
has been developed. It involves continuous catalyst regeneration
and, by proper integration of the operation of the two sections,
permits truly continuous reforming. These units are generally
operated at lower pressures, to take advantage of the higher
yields attained under such conditions, and usually have bimetallic
catalysts. In this continuous separation, the UOP design involves
a slow but continuous withdrawal of the catalyst from the lowest
reactor in the stacked configuration; the catalyst is continuously
•I
regenerated in an external system and then returned to the top re-
actor. There are nine continuous platformers in operation and 40
in various stages of design and construction.
WORLDWIDE CATALYTIC REFORMING
The extent to which catalytic reforming is applied in the
petroleum refining industry is shown in Figure A-6 as daily
A-18
-------
id
*
01
H
9
B
a
w _.
C 5
o
•rl
•H
g
4
V
•H
0
10
&
3 3
9
•H
-------
capacity for the world and for each hemisphere for each year from
1960 to 1973. Capacities for the eastern hemisphere and the world
exclude the U.S.S.R. and its bloc, for which data were not avail-
able. During the last 14 years, the catalytic reforming capacity
of the world has increased by a factor of 2,5, having risen from
2.8 to 7 million barrels per day (445,000 to 1,110,000 m3/day).
Most of this increase has occurred in the eastern hemisphere,
where the application of catalytic reforming has been catching
up, so to speak, with that in the other hemisphere. Along with
the general slowdown in overall refinery expansion since 1971,
the reforming capacity (Figure A-6) has followed this trend in
all sectors of the world.
Another way of regarding the application of catalytic reforming
is by indicating its capacity as a percentage of crude oil processed
in a given area (Figure A-7)- About 50 million barrels (7,950,000 m
crude oil were being processed per day early in 1973; therefore,
the 7-million-barrel/day (I,110,000-m3/day) worldwide catalytic
reforming capacity represented about 14% of the crude oil. As
shown in the figure, this percentage has not changed appreciably
on a worldwide basis since 1960. The western hemisphere catalyti-
cally reforms a greater proportion (about 18% in recent years) of its
processed crude oil than the eastern hemisphere (about 121). As
is also shown in Figure A-7, the proportion reformed in the western
hemisphere has not changed notably since 1960, but there has been
a sharp increase in the eastern hemisphere, from 8% to 13% (in 1970)
and 12% (in 1971 and 1972).
A-20
-------
24
** 00
•rl 22
0
a
o
C 20
•H
w
ID
rl
18
U
0
16
•rl
U
0,14
id
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-------
The plot in Figure A-7 shows that the United States was re-
forming about 24% of its processed crude oil in 1973; this pro-
portion represents very nearly the entire native C7 400° F (200° C)
content of the average crude oil refined in the United States.
However, a small amount of U.S. reforming capacity is occupied
with the processing of thermal and hydrocracked naphthas.
Although nearly all the available naphtha in western hemisphere
crude oils is catalytically reformed, only about two-thirds of
the estimated 19% of potentially reformable naphtha in the eastern
hemisphere oils is being reformed. This suggests that there is a
large potential for expanding catalytic reforming capacity in the
eastern hemisphere.
CONTRIBUTION OF CATALYTIC REFORMING TO THE U.S. GASOLINE POOL
The nation's gasoline pool is composed of five broad classes
of gasolines, defined on the basis of their origin: straight-run,
thermal, catalytically cracked, catalytically reformed, and
alkylate and polymer. Although the national gasoline pool can be
resolved into premium and regular grades as actually marketed (the
proportions have changed with time), it will be considered as a
single commodity in this discussion. Even though there are several
methods that could be used for characterizing the quality of gasoline
for simplicity only the research-method octane numbers of the pool
and its components will be mentioned here.
The changing composition and octane numbers of the nation's
gasoline pool are shown in Table A-2. In this table, the "unleaded"
A-22
-------
TABLE A-2
a
Composition and Quality of U.S. Gasoline Pool
Composition, vol %;
Straight-run
Thermal
Catalytically cracked
Catalytically reformed
Alkylate and polymer
Research octane number;
Unleaded
With lead
Lead content of pool, g/gal (U.S.)
Year
1940
50
46
2
0
2
100
1950
40
32
20
1
7
100
1960
19
10
31
30
10
100
1972
12
4
38
33
13
100
64.3 75.3 86.2
89.3
74.6 85.1 94.0 96.8
1.5
2.2
2.0
2.3
Data from Sterba.
A-23
-------
octane number of the pool represents the gasoline quality as pro-
duced by the refiner's processing units, and the "leaded" octane
number represents what is dispensed at the filling station. Of
interest is the dramatic increase of 25 in the octane number of
the unleaded gasoline during the last three decades. This has
been made possible by the increasing application of catalytic
processing during that period. In 1940, catalytic processing was
just beginning, and the nation's gasoline pool was essentially a
blend of equal amounts of low-octane-number straight-run and
thermally cracked gasoline having a moderate octane rating. By
1950, the pool gasoline had risen in unleaded octane number from
64 to 75, with the aid of catalytic cracking, alkylation, and
polymerization.
As Table A-2 shows, the unleaded pool octane number rose
another 11 units in the next decade, from 1950 to 1960, largely
through the extensive use of catalytic reforming. During that
decade, the proportion of catalytic reformate in the nation's
gasoline pool rose sharply from a nominal 1% to just over 30%,
with much smaller increases in the percentages of alkylate and
catalytically cracked gasolines. This represents an impressive
contribution of catalytic reforming to improving the antiknock
quality of the gasoline pool. Also to be noted in Table A-2 is
the slight drop in the use of lead alkyls during this decade.
This suggests that it was more economical to increase octane
numbers by catalytic reforming than to use lead alkyls to achieve
octane ratings required by the high-compression automobile engines
A-24
-------
During the 12-year period from 1960 to 1972, the octane
ratings have risen by about 3 units, as shown in the table, with
all catalytic processes contributing to the increase. As pointed
out earlier, nearly all the available naphtha in the nation's
processed crude oil is being catalytically reformed. However,
catalytic reforming can continue to contribute to raising the anti-
knock rating of the nation's gasoline pool, because, by modifying
the conditions of operation in the reforming reactors, the octane
number of the reformate can be increased substantially. By con-
trast, the octane numbers of the gasoline products from other
catalytic processes are relatively constant. Thus, the unleaded
research octane number of catalytically cracked gasolines is in
the range of 90-93, and that of alkylates and polymer gasoline
ranges typically from 92 to 95. However, the catalytic reforming
process is versatile in this respect. In the early 1950's, when
the process was being commercialized, the national average un-
leaded octane number of reformates was just under 85; it is now
in the vicinity of 95, and there are some sustained commercial
operations producing reformates with octane numbers of over 100.
AROMATICS
In addition to its importance in helping to provide the world
with high-octane-number motor fuels, the catalytic reforming process
has been the primary instrument in the synthesis and supply of the
basic aromatic building blocks-'-benzene, toluene, and the Cg aro-
ma tics.
A-25
-------
Of the total benzene consumed about 25 years ago, the portion
made by the petroleum industry was around 5%; this figure has
since risen to well over 90%. During this span of years, the
production growth rate of each of these primary aromatics has been
about 10%/year. These aromatics are produced in high purities by
extraction of selected boiling-range catalytic reformates, largely
with sulfolane and polyethylene glycol solvents.
USEFUL BYPRODUCTS
In the manufacture of high-quality motor fuel and aromatics,
the simultaneous production of hydrogen should be considered a
product, rather than a byproduct, of catalytic reforming, and it
is generally profitable to maximize its yield. For the United
States, this production of hydrogen from catalytic reforming
amounts to an estimated 2.5 billion cubic feet (about 71 million
cubic meters) per day. This hydrogen is useful in the catalytic
hydrotreatment of over 4 million barrels (about 635,000 m3) of a
wide variety of stocks per day. In addition, some of the hydro-
gen is used for the 750,000 barrels (120,000 m3) required for
hydrocracking per day-
Of the light hydrocarbons made in catalytic reforming, pro-
pane finds its way into liquid petroleum gas, isobutane goes to
alkylation units, and n-butane is used to adjust the vapor pressure
of finished gasoline. Only small amounts of methane and ethane
are directed to refinery fuel.
A-26
-------
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TECHNICAL REPORT DATA
;I'U-asc read Instructions on the reverse before complttint;;
1. REPORT \O.
EPA-600/1-77-040
2.
4. TITLE ANDSUBTITLE
PLATINUM-GROUP METALS
3. RECIPIENT'S ACCESSION-NO.
5. REPORT DATE _.._
September 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHORlS)
Subcommittee on Platinum-Group Metals
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Committee on Medical and Biologic Effects of
Environmental Pollutants
National Academy of Sciences
Washington. D.C. 20460
10. PROGRAM ELEMENT NO.
1AA601
11. CONTRACT/GRANT NO.
68-02-1226
12. SPONSORING AGENCY NAME AND ADDRESS
Health Effects Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park. N.C. 27711
13. TYPE OF REPORT AND PERIOD COVERED
RTP,NC
14. SPONSORING AGENCY CODE
EPA-600/11
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This document assembles, organizes, and evaluates all pertinent information
(up to April 1976) about the effects on man and his environment that result either
directly or indirectly from pollution by platinum-group metals: iridium (Ir),
osmium (Os)., palladium (Pd)., platinum (Pt), rhodium (Rh) and ruthenium (Ru).
The document describes physical and chemical properties, sources, measurement,
and effects on plants, animals, and humans. The information presented is
supported by references to the scientific literature, and the summary, conclusions
and recommendations represent a consensus of the members of the Subcommittee.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
iridium
palladium
rhodium
air pollution
health
chemical analysis
osmium
platinum
ruthenium
toxicity
ecology
06 F, H, T
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report,
UNCLASSIFIED
21. NO. OF PAGES
352
20. SECURITY CLASS (This pagej
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
347
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