SrEPA
United States
Environmental Protection
Agency
Environmental Monitoring and Support EPA-600/4-78-038
Laboratory July 1978
Cincinnati OH 45268
Research and Development
Arsenic Determination
by the Silver
Diethyldithiocarbamate
Method and the
Elimination of Metal
Ion Interference
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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 MONITORING series.
This series describes research conducted to develop new or improved methods
and instrumentation for the identification and quantification of environmental
pollutants at the lowest conceivably significant concentrations. It also includes
studies to determine the ambient concentrations of pollutants in the environment
and/or the variance of pollutants as a function of time or meteorological factors.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/4-78-038
July 1978
ARSENIC DETERMINATION BY THE
SILVER DIETHYLDITHIOCARBAMATE METHOD
AND
THE ELIMINATION OF METAL ION INTERFERENCE
by
Shingara S. Sandhu
Claflin College
Orangeburg, South Carolina 29115
Grant No. R 804164-01
Project Officer
Morris Gales
Inorganic Analysis Section
Physical and Chemical Methods
Environmental Monitoring and Support Laboratory
Cincinnati, Ohio 45268
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Environmental Monitoring and
support Laboratory, Office of Research and Development, U.S. Environ-
mental Protection Agency, and approved for publication. Mention of
trade names or commercial products does not constitute endorsement
or recommendation for use.
n
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FOREWORD
Environmental measurements are required to determine the quality of
ambient waters and the character of waste effluents. The Environmental
Monitoring and Support Laboratory-Cincinnati conducts research to:
°Develop and evaluate techniques to measure the presence and
concentration of physical, chemical, and radiological
pollutants in water, wastewater, bottom sediments, and
solid waste.
Investigate methods for the concentration, recovery, and
identification of viruses, bacteria, and other microbio-
logical organisms in water. Conduct studies to determine
the responses of aquatic organisms to water quality.
°Conduct an Agency-wide quality assurance program to assure
standardization and quality control of systems for monitor-
ing water and wastewater.
The standard methods for analysis of water and waste samples are
under continual review to assure that the most accurate results possible
are obtained. If a chemical interference in an important analytical
procedure is discovered it must be evaluated and if necessary a procedure
modification made to circumvent the interference. This report investi-
gates the interference of metals in the analysis for total arsenic and
suggests a method for removing these interferences.
Dwight G. Ballinger
Director
Environmental Monitoring and Support Laboratory
iii
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ABSTRACT
Several metal ions—chromium, cobalt, copper, mercury, molybdenum,
and antimony—have been reported to interfere in the determination of
arsenic in water and waste water by the Silver Diethyldithiocarbamate
(SDDC) method, but the limit of interference by each ion has never been
precisely evaluated. Conflicting reports have also appeared in the
literature on the role of chromium and antimony as interfering ions in
this method. The present study was undertaken to resolve the confusion
that surrounds the use of the SDDC method for the determination of
arsenic in water and waste water.
Except for minor deviations, the procedure adopted for this study
is similar to the one given for the standard method. The recovery of
micro amounts of arsenic in the presence of several interfering ions
was studied. It was found that the SDDC method provides reliable data
for arsenic concentrations in fairly polluted waters. Using this method,
the recovery of arsenic up to 0.5 jig (0.01 mgL'1) from polluted waters is
quantitative. The absorbance peak at 410 nm is assigned to the formation
of a hydrogen-SDDC complex rather than a chromium-SDDC complex. Antimony
and mercury interfere with arsenic color development by yielding complexes
with maximum absorbances at 510 and 425 nm respectively.
The recovery of arsenic, released on the digestion of standard
solutions of natural water and cacodylic acid (sodium salt) by potas-
sium permanganate, is found quantitative.
The research was also directed towards developing a technique to
effectively concentrate and isolate arsenic from the interfering metal
ions, generally found in waste water. Two different approaches were
tried in order to solve the problem.
Distillation; Arsenic was quantitatively reduced to arsenic(IH)
by cuprousU) chloride in hydrochloric acid and distilled as arsenic
(III) trichloride before its determination by the SDDC method. The
concentration of arsenic in the distillation mixture was found to be
critical for the effective recovery of arsenic. The optimum recovery
of arsenic seems to be at 0.0909 mgL~l or at higher concentrations of
arsenic. The synthetic as well as the natural water samples containing
0.01-0.05/igL"1 of arsenic were concentrated to bring the arsenic con-
centrations within an appropriate range, but the recovery of arsenic,
especially at lower concentration levels, is unsatisfactory. It appears
that a major loss of arsenic takes place during the reduction of sample
volume. The reduction step is also a very time-consuming and laborious
iv
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step and cannot be recommended for routine analysis of arsenic in waste
water.
Ion Exchange: The synthetic and natural water samples containing
arsenic were oxidized with potassium permanganate under acid conditions.
The excess potassium permanganate was destroyed by the use of hydroxyl-
amine hydrochloride. The arsenic in a water sample was isolated and
concentrated by passing it through a strongly basic anion exchange
(functional group-N-(CH3)3 +C1~) column before its determination by the
SDDC method. The recovery of arsenic at 0.005 mgL'1 is found satisfac-
tory, with an average of 86%.
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CONTENTS
Foreword i i i
Abstract i v
Figures vii
Tables viii
Acknowl edgment ix
1. Introduction 1
2. Conclusions......,,,,, 3
3, Recommendations , 4
4. Materi al s and Methods 5
Apparatus 5
Materials 5
5. Experimental Procedures 6
Ionic interference 6
Elimination of ionic interference 6
Distillation'. 6
Ion exchange.*, 7
6. Results and Discussion,,, ,..«.. 8
Ionic interference,., ,,,.,,,.,..,,.,,,,,,,.. 8
Ionic interference elimination,,,,,,, ,.,,.. 17
Di sti 11 ati on .,.,,.,„,. 17
Ion exchange -,..,, ,.,,.,., 19
References ,., 24
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FIGURES
lumber Page
1. Absorbance spectrum of a reagent blank recorded
against a SDDC solution as reference 9
2. Absorbance spectrum of a 2 /jn of arsenic recorded
against a SODC solution as reference 9
3. Absorbance spectrum of 2 /jg of arsenic in combi-
nation with 1 mil"1 each of Cr(VI), Co(II),
Cu(H), Mo(VI), and li(II) recorded against
a SODC solution as reference 13
4. Absorbance spectrum of 2 jjq of arsenic recorded
aqainst a reagent blank as reference 1")
5. Absorbance spectrum of 2 ^q of arsenic in combi-
nation with 1 nigL"' each of Cr(Vl), Co(II),
Cu(II), ^o(VI), and -11(11) recorded aqainst
a reagent blank as reference 11
6. Effect of hydrochloric acid on the absorbance peaks
at 410 nm (a) SODC-hvdroqen complex and 535 nm (b)
SDDC-arsenic complex 11
7. Absorbance spectrum of SDOC-mercury(II) complex 13
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TABLES
Number Page
1. Recovery of Various Forms of Arsenic from
Demineralized and River Hater 15
2. Recovery of Arsenic from Demineralized and River
Water in the Presence of Interfering Ions 16
3. Preliminary Arsenic Recovery by Distillation from
Synthetic Aqueous Samples 18
4. Arsenic Recovery by Distillation from Reduced
Volume of Demi nerali zed and River Mater 20
5. Arsenic Recovery by Ion Exchange from Deminer-
alized and River Water 21
Vlll
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ACKNOWLEDGMENTS
The author is grateful to the United States Environmental
Protection Agency for funding this research project. I am par-
ticularly indebted to Mr. Peter Nelson, chemist, South Carolina
State College, Orangeburg, S. C., for his assistance in dupli-
cating some of the experimental results.
I am also grateful to Dr. H. V. Manning, President;
Dr. B. L. Gore, Academic Dean; and Dr. .-leison Smith, Chair-
man; all at Claflin College, Orangeburg, S. C., for their
encouragement and cooperation in undertaking the studies
reported in this bulletin.
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SECTION I
INTRODUCTION
Arsenic is widely distributed in the human environment and
any sample of water, if analyzed by a suitably sensitive methpcL
will be found to contain at least a small quantity of arsenio 'J.
The U.S. Public Health Service recommends that arsenic concentra-
tion in drinking water should not exceed 0.01 mg L"' and that
water with an arsenic concentration greater than 0.05 mg L~'
should be rejected for human consumption^2'. The arsenic concen-
tration of potable water is generally less than 0.005 mg L"1,
although a concentration as high as 0.1 mg L"' has been reported^3/.
Arsenic is a suspected carcinogen(4); consequently, there is a
growing interest in arsenic contamination of the environment.
Several methods (4-13) are available for the determination of
arsenic, but silver diethyldithiocarbamate(SDDC)(10) is the most
widely used technique for the separation and determination of arsenic
in water samples. This method consists of reducing inorganic arsenic
in a water sample by acid zinc reaction to arsine (AsHs) which is
scrubbed through lead acetate impregnated glass wool and is absorbed
in silver diethyldithiocarbamate dissolved in pyridine. The color
developed due to arsine (AsH3) silver diethyldithiocarbamate reaction
is photometrically measured at 535 nm. Conflicting reports ( '2» '^)
have been published on the role of chromium as an interfering ion in
the standard (SDDC) method. Though chromium suppresses arsine gener-
ation(lO), a color enhancement ascribed to chromium interference in
the determination of arsenic by the SDDC method was observed. The
authors ('4) speculated that chromium SDDC-complex was responsible
for the absorbance peak at 410 nm. Certain other metals — cobalt,
copper, mercury, molybdenum, nickel, platinum and silver—have also
been reported to interfere in the generation of arsine '''"' but fieir
limits of interference have not been thoroughly investigated. It has
been indicated that the presence of antimony in a water sample inter-
feres in the development and photometric measurement of SDDC-arsenic
complex color (*®) but a recent report indicates that antimony con-
centrations up to 0.2 mg L"' do not show any significant interference.
In the early procedure (5) it was stated that antimony and mer-
cury interfered but copper, cobalt and nickel did not interfsre in
the determination of arsenic which was reduced to elemental arsenic
by a hypophosphate solution in hydrochloric acid. The reduced ele-
mental arsenic was boiled for coagulation and separated by filtration
and washed with water to free the filter paper of impurities. The
1
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arsenic in the coagulum was determined by oxidation with dilute iodine
solution, the excess of which was titrated with thiosulfate.
Arsenic from solution was reduced to elemental form^) by calci-
um hypophosphate in hydrochloric acid. The coagulated arsenic was
separated by filtering through a cotton pad and dissolved in measured
quantity of standard eerie sulfate, the excess of which was titrated
with standard arsenic trioxide. This procedure eliminated the inter-
ference from tin, antimony, mercury and copper.
The samples containing 1 to 100 parts per billion of arsenic
were fused v/) \tfth sodium peroxide and leached with water. The
leachate was heated to coagulate cobalt, nickel and platinum which
then were removed by filtration. Chromium(VI) interference was
eliminated^'/ by its oxidation to perchromic acid, using dilute
hydrogen peroxide. The perchromic acid decomposes sp9ntaneously to
chromium(III) which does not interfere in the generation of arsine
(AsH3).
Determination of arsenic in small volume of water samples was
facilitated by its distillation with cuprous chloride and hydro-
chloric acid('6)_ The acid concentration was maintained above its
azeotropic level. This not only concentrated the arsenic from water
sample but also eliminated the ions that interfered in/tbe generation
of arsine and evaluation of arsenic by standard method' '^).
The ion-exchange behavior of arsenic(III), at relatively high
concentration (40 g l-"\of arsenic III), on Varion anion exchange
resins was evaluated^'') in acid as well as in alkaline solutions.
It was reported that about 90% of arsenic was removed from alkaline
aqueous solutions. Arsenic recovery was comparatively lower in acid
than in alkaline solutions.
This report presents data to resolve the confusion that surrounds
the extent of ionic interference in the determination of arsenic in
water and waste water by silver diethyldithiocarbamate(SDDC) method^ '^/.
The effort has also been made to develop a methodology to eliminate
the ionic interference in the generation of arsine as well asin the
quantitative evaluation of total arsenic by standard method^'0'.
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SECTION 2
CONCLUSIONS
The results of experimental studies on the recovery of micro
amounts of arsenic in the presence of several interfering ions in-
dicate that the silver diethyldithiocarbamate method provides reli-
able data for arsenic evaluation up to 0.01 mgL"', even in fairly
polluted water. The metal ions--chromium(VI), cobalt(II), copper(II),
molybdenum(VI), and nickel(II)— up to a concentration of 5.0 mgL
do not appear to interfere in the generation of arsine. .'lone of the
metal ions in this category are responsible for an absorbance in the
visible region.
Antimony(III) concentrations of 0.3 mgL" or above and mercury(II)
concentrations of 1.5 mgL"' or above show a significant positive inter-
ference in arsenic-SDDC color development and measurements.
The potassium permanganate digestion method is found satisfactory
for the release of organically bonded arsenic before its determination
by SDDC.
The distillation method is found unsatisfactory for the concen-
tration and isolation of arsenic from interfering ions in synthetic
dilute aqueous solutions and natural waters before its determination
by SDDC method.
The anion exchange method reported here is found effective for
eliminating the metal ions interfering in the determination of arsenic
by the SDDC method, but the potential of the anion exchange technique
used to separate arsenic from the interfering ions is not unlimited.
T'IG arsenic recovery decreases in the presence of extremely high ani-
onic concentrations in polluted -vater, but the recovery can be im-
proved by increasing the amount of resin used in the chromatographic
column.
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SECTION 3
RECOMMENDATIONS
The Silver Diethyldithiocarbamate method, in the hands of a
knowledgeable and trained technician, is quantitative and can be used
for routine analysis of inorganic arsenic in fairly polluted waters.
The SDDC method will not provide reliable data for waters con-
taining total metal ion concentration of more than 5.0 mgL .
The potassium permanganate digestion method is satisfactory for
the release of organically bonded arsenic before its determination
by the SDDC method.
The distillation of arsenic as arsenic(III) trichloride does
not appear to provide satisfactory information for arsenic concentra-
tions below 0.0909 mgL~' in the distillation mixture.
The anion exchange technique can be effectively used for the con-
centration and separation of arsenic from the interfering ions found
in polluted natural waters before the determination of arsenic by the
SDDC method.
The anion exchange potential of the resin used in the work repor-
ted here needs further study for establishing a mathematical relation
between the weight of the resin and the extent of anionic pollution of
water.
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SECTION 4
MATERIALS AND METHODS
APPARATUS
The arsine generator and absorber assembly have been previously
described(lO) and were purchased from Fisher Scientific (Cat. No. 1-
405). A Beckman Model 24/25, double beam scanning spectrophotometer
with 1-cm cells, equipped with a digital read out system and a strip
chart recorder was used for spectra studies and absorbance measure-
ments.
MATERIAL
The vertical condenser used during distillation of arsenic as
arsenic(III) trichloride was the-Allihn type, to minimize the hold
up of distillate.
The ion exchange resin, Amberlite IRA-401S C.P., code 3401, was
purchased from Mallinckrodt Chemical Works, St. Louis, Mo.
The stock solutions for arsenic(III) , arsenic(V),. chromium,
cobalt, copper, mercury, molybdenum, nickel, phosphate and antimony
containing 1 g L-l of ionic concentration were prepared from arsenic
trioxide (As203), sodium arsenate (Na2H As04 '.7^20), potassium
chromate (KzCrty), cobalt chloride (CoCl2- 6H20) , copper nitrate
(Cu(N03)2'3H20), mercury(n), chloride(HgCl2) , ammonium molybdate
( GMty J6"1o7c24< 4FbO) , nickel nitrate (NnNOjJz). potassium dihydrogen
phosphate (KK?P04) and antimony trichloride (SbCls) respectively. The
intermediate solutions were prepared by diluting the stock solutions
1:10, and working solutions containing requisite concentrations of
various ions were obtained by diluting the intarmediate solutions.
Analytical grade reagents were used.
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SECTION 5
EXPERIMENTAL PROCEDURES
IONIC INTERFERENCE
The procedure adopted for the present study is primarily similar
to the one given for the standard method^10) except that a 50 ml water
sample is used which requires the use of 7.5 ml of concentrated hydro-
chloric acid, 2.0 ml of potassium iodide and 0.5 ml of stannous chloride
in hydrochloric acid. The reaction is allowed to proceed for 15 minutes
at room temperature, following the addition of 3.0 g of zinc, after which
the Generator is transferred to a water bath at about 50°C for another
15 mvijtes. The solutions from the absorber tubes are poured directly
into a 1-cm cell and scanned for a complete absorbance spectrum (700 nm
- 350 nm), using a SDDC solution and a reagent (SDDC solution treated
in the absorber tube similar to the experimental procedure but without
arsenic) blank as references. Though the concentration of arsenic(III)
as well as arsenic(V) in the prepared standards and river water is main-
tained at 2 >jg or less per 50 ml of sample (.04 mg L~1 or less), the
amount of interfering ions is varied up to 350/jg per 50 ml, (7.0 mg L-l)«
Absorbance calibration curve using 0.0, 1.0, 2.0, 4.0 and 5.0>ig of
arsenic was prepared.
ELIMINATION OF IONIC INTERFERENCE
Distillation
The distillation procedure used here has been reportedU6). The
aliquots of prepared water samples containing not less than 0.01 mg of
arsenic were transferred to the distillation flask which was joined to
the Allihn type vertical condenser which in turn led into a distillate
flask (250 ml) immersed in ice and containing 20.0 ml of dilute hydro-
chloric acid (1:1). The arrangement was made for the escaping hydro-
chloric acid vapors to be absorbed in concentrated (12 M) sodium hyrox-
ide in a flask. The apparatus was closed to the atmosphere except for a
side arm on the sodium hydroxide flask. A flexible tubing from the arm
was led to a sink with running water. Two-tenths g of copper (1) chloride
and enough of hydrochloric acid was added so that the water sample to
acid ratio was 1:10. The volume of distillate collected varied between
45 to 55 ml. After the distillate had been collected, the apparatus was
dismantled and condenser was rinsed into the distillate flash with water.
The distillate was quantitatively transferred to a 100 ml volumetric
flask and the volume made up to the mark with water. The aliquot volume
of the distillate, containing not less than 2.0 /jg of arsenic, was trans-
-------
ferred to the arsine generator and the volume diluted to about 57.0 ml
with water and hydrochloric acid in such a way that the final acid con-
centration in the arsine generator was about 1.6 M. The arsine was
generated, color developed and measurements made according to the pro-
cedure described previously. The maximum desirable amount of arsenic
in drinking water is 0.01 mg L-1 and the maximum safe amount is 0.05 mg
L-l (2) so it was found necessary that the arsenic in aqueous samples be
concentrated. Consequently a 4 liters water sample in a beaker was re-
duced by heating on a water bath at about 50°C, to a volume so as the
distillation mixture (water - acid) contained arsenic at not less than
0.0939 mg L-'. The reduced sample was quantitatively transferred to four
distillation flasks, (15.0 ml per flask). The beaker was washed,
thoroughly, four times using 50.0 ml of concentrated hydrochloric acid
per washing, which was equally divided four ways and added to the distil-
lation flasks.
Ionic Exchange
Appropriate amount (2.0-5.0 grams) of Amberlite IRA-401S C.P.,
resin (depending on the extent of ions in waste water) was packed in
10.0 ml. burettes and conditioned with 100.00 ml. of 9.0 M hydrochloric
acid and washed with 250.0 ml. of distilled water.
The synthetic, as well as the natural water samples, containing
interfering ions and not less than 0.005 mg L-1 of arsenic, were
digested and then eluted through the ion exchange columns. The arsenic
retained by the resin was leached with 30.0 ml. of 9.0M. hydrochloric
acid, followed by sufficient water to yield 100.0 ml. of effluent. An
aliquot of effluent containing not less than 2.0 yg of arsenic was
transferred to the arsenic generator and the volume diluted to 57.0 ml.
with water, and hydrochloric acid, (if needed to maintain the acid
concentration around 1.6 M). The arsenic in the effluent was deter-
mined according to the procedure described previously.
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SECTION 6
RESULTS AND DISCUSSION
IONIC INTERFERENCE
Complete absorbance spectra for a reagent blank, arsenic(III),
and arsenic(III) in combination with chromium(VI), cobalt(II), copper
(II), molybdenum(VI) and nickel(II), each at 1.0 mg L-l level (total
interfering ionic concentration 5.0 mg L-1) , against a SDDC solution
as reference, are given in Figures 1, 2 and 3 respectively. Each
spectrum shows an absorbance peak around 410 nm which disappears, or
is considerably reduced, Figures 4 and 5, when the same absorbance is
recorded against a reagent blank.
An absorbance peak at 410 nm, similar to Figures 1, 2 and 3, is
always observed no matter which one of the numerous interfering ions
is used in the generator, either alone or in combination with arsenic
(III), if the absorbance of the SDDC complex developed in the absorber
tube is read against a SDDC solution rather than against a reagent
blank. Although no mechanism for the transport of chromium from the
arsine generator to the absorber, which contains SDDC reagent, was
suggested, the absorbance peak at 410 nm was speculatively due to the
formation of a chromium diethyldithiocarbamate complex(14;. The in-
formation presented here, Figures 1-5, does not substantiate this
report, rather demonstrates that none of the interfering ions is
responsible for the absorbance at 410 nm. The absorbance peak at 410
nm is observed even when the generator has nothing in it except the
deionized water, hydrochloric acid and zinc. The addition of stannous
chloride alone or in combination with potassium iodide does not change
the nature of this absorbance peak in any way.
Accordingly, it is suggested that the absorbance at 410 nm is due
to the reaction of hydrogen (generated by the acid zinc interaction)
with SDDC reagent. It is further observed that the presence of arsenic
in the generator, alone or in combination with the other ions, remark-
ably increases the peak height at 410 nm over reagent blank or inter-
fering ions alone (Figures 1, 2 and 3). It is speculated that during
the reaction of arsine (AsH3J with SDDC, certain amount of additional
atomic hydrogen becomes available for simultaneous combination with
SDDC complex responsible for increased absorbance at 410 nm.
SDDC + AsH3—^- SDDC - Arsenic complex + SDDC - Hydrogen complex
The peak at 410 nm is not precisely reproducible (^30%) and is
8
-------
.10
o
z
<
m
-------
.10
.08
UJ
o
m
(E
o
w
CD
.06
.04
.02
400
500 600
WAVELENGTH, nm
700
400 500 600
WAVELENGTH, nm
7OO
Figure 3. Absorbance spectrum of 2 ug of arsenic in
combination with 1 mg L-l each of Cr(VI),
Co(ll), Cu(ll), Mo(VI), and Ni(ll) recorded
against a SDDC solution as reference.
Figure 4. Absorbance spectrum of 2 /ug of arsenic
recorded against a reagant blank as reference.
-------
.10
0.2
.08
.06
o
z
<
m
cr
o
OT
CD
.04
.02
CQ
o:
o
O.I
400
500
600
700
0.0
0.2
0.4
0.6
WAVELENGTH, nm
HYDROCHLORIC ACID CONCENTRATION, M
Figure 5. Absorbance spectrum of 2 A*g of arsenic Figure 6. Effect of hydrochloric acid on the absorbance
in combination with 1 mg L-l each of peaks at 410 nm (a) SDDC-hydroqen comolex
Cr(VI), Co(ll), Cu(ll), Mo(VI) and Ni(ll) and 535 nm (b) SDDC-arsenic complex
recorded against a reagent blank as reference.
-------
seen to be preferentially destroyed over the arsenic - SDDC complex
peak of 535 nm. Arsenic - SDDC complex is developed as described in
the procedure and the absorbance spectrum is recorded. The solution
is poured back into the absorber tube and concentrated hydrochloric
acid is added, one drop at a time, to the solution in the absorber
tube and the absorbance spectrum is taken again. This is repeated
until the peak at 410 nm disappears. Molarity of hydrochloric acid,
calculated from the volume of acid added, is plotted against the
absorbance at 410 nm, Figure 6. This figure also shows the effect of
acid on arsenic - SDDC complex at 535 nm. It is apparent that the
absorbance at 410 nm decreases in proportion to the volume of acid
added, while the absorbance at 535 nm, except for a minor dilution
effect, remains the same. The instability of the complex, responsi-
ble for absorbance at 410 nm, is an additional indication that hydro-
gen-SDDC complex rather than metal - SDDC complex is responsible for
the absorbance peak at 410 nm.
The single ion absorbance spectra for Cr(VI), Co(II), Cu(II),
Mo(VI) and Ni(II) are not different from the reagent blank spectrum
and do not show any noticeable peak within 700 to 350 nm if the
readings are taken against a reagent blank. Therefore, it appears
that these ions should not show any positive interference, as sugges-
ted previously(14), -jn the evaluation of arsenic by the standard
method. However, if the absorbance spectrum of the complex formed in
the absorber tube is recorded, using an SDDC Solution instead of a
reagent blank as reference, a large absorbance peak at about 410 nm is
noticed, which causes the elevation of the base line (Figures 1, 2 and 3),
leading to an inaccurately high arsenic content (about 10%) for a given
water sample.
The water samples containing 0.30 mg L.-1 of antimony and 2.0 mg
L-l of mercury show absorbance peaks at 510 nm (average absorbance
.007) and 425 nm (average absorbance .005) resoectively and thus
these ions are expected to interfere positively in the determination
of arsenic by the standard method. Possible positive interference by
antimony has been suggested in the past(lO) but similar interference
by mercury has not been reported. Ionic mercury reacts with stannous
chloride in hydrochloric acid to produce metal mercury(18),
2Hg2+ + SnCI2 + 2CI" — ». Hg|+ + SnCI
4
Hg + SnCI2 + 2CI- — »- 2Hg + SnCI4
There is a distinct possibility that mercury vapors are carried over
from the generator to the absorber tube containing SDDC reagent, re-
sulting in the formation of a mercury - SDDC complex, responsible for
an absorbance peak at 425 nm (Figure 7).
The amount of arsenic recovered from demineralized water is
quantitative up to an arsenic concentration of 0.5 jjg or .01 mg L-1
12
-------
0.08H
0.06r-
LU
O
CO
cr
o
CO
CD
0.02r-
400 500 600 700
WAVELENGTH, nm
Figure 7. Absorbance spectrum of mercury
recorded against a reagent blank.
13
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(Table 1). Arsenic recovery from demineralized water in the presence
of various interfering single ions is presented in Table 2. This in-
formation suggests that the recovery of arsenic is not affected in the
presence of chromium(VI), cobalt(II), copper(II), molybdenum(VI),
nickel(II), nitrate and phosphate, up to an individual ion concentra-
tion of 5.0 mg L'1. But when the single metal ion concentration is in-
creased beyond 5.0 mg L~l, a decrease in arsenic recovery from the stan-
dard solutions is observed and at 7.0 mg L~l level the arsenic recovery
decreases approximately 10%. Nitrate and phosphate concentrations up
to 100.0 mg L'1 do not show any observable effect on arsenic recovery.
It has been reported(12) that antimony concentrations up to 0,2
mg L'1 did not produce any observable interference in determining ar-
senic by the SDDC method. The present study suggests that antimony
concentration of 0.2 mg L~* does not show any significant change in the
recovery of arsenic from standard solutions; however, when antimony con-
centration is raised to 0.3 mg L'1, the apparent rate of arsenic recovery
increases by about 10 percent. The location of arsenic absorbance peak
shifts toward 510 nm in the presence of antimony. This shift seems pro-
portional to the amount of antimony present in a solution containing
0.04 mg L~l of arsenic. The mercury concentrations at 1.5 mg L~l and
above show a significant positive interference in the recovery of arsenic.
Interference by several ions in combination was studied by preparing
standard solutions, containing 0.04 mg L"1 of arsenic and varying the
concentrations of each ion, chromium(VI), cobalt(II), copper(II),
molybdenum(VI), nickel (II), phosphate and nitrate, up to 1.0 mg L"1.
The results on the recovery of arsenic in the presence of a combination
of ions are also given in Table 2. There is no significant interference
by these metal ions up to a combined concentration of 5.6 mg L~l (0.8
mg L-l each ion) but when the combined concentration is increased above
this level there is an observable decrease in the recovery of arsenic
from demineralized water. A combined concentration of 7,0 mg L-l (1.0
nig L~l each ion) decreases the arsenic recovery from standard solutions
by about 10%.
A combined concentration of two ions, chromium(VI) and cobalt(II),
each at 3.5 mg L~^> decreases the arsenic recovery by about 10 percent.
A molybdenum(VI) and copper(II) combination, as well as the combination
of any other'two metal ions in this category at a total concentration of
7.0 mg L"1, show similar results (Table 2). It appears that, under the
present set of experimental conditions, the ionic specificity of the
elements that interfere in arsine generation is not as important as their
total concentration in inhibiting the recovery of arsenic from the stan-
dard solutions.
The studies are also carried out using natural water from the Edisto
River, Orangeburg, S. C., which was spiked with arsenic as well as with
interfering ions. Because only inorganic arsenic is reduced to arsineU°)
a digestion step, as suggested in the literature, using nitric and sul-
fric acids\12) Was tried for the release of organically bonded arsenic.-
14
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TABLE 1. RECOVERY OF VARIOUS FORMS OF ARSENIC FROM DEMORALIZED AND RIVER WATERS
Concentration mq L~*
Demi nerali zed water
Theoretical
0.01
0.02
0.04
0.01
0.02
0.04
0.01
0,02
0.04
Determined1
.0102
.0196
,0410
0.0100
0.0208
0.0395
0.0095
0.0201
0,0405
Recovery
%
102
98
102
100
104
99
As(III)
95
100
101
Standard
deviation %
As(IIl)
5,7
4.3
3.4
As(V)
5,8
4.9
4.0
+ As(V) (50:50)
5,6
3.1
3.5
River water
P
Determined^ Recovery Standard
% deviation %
w
0.0214 107 5.6
0,043 108 4.5
— — —
„-_ — —
_ « — «._« ». __
—
— „„- _„_
0.044 109 5.8
Mean of 35 determinations
p
Mean of 20 determinations
-------
TABLE 2. RECOVERY OF ARSENIC FROM DEMORALIZED AND
RIVER WATER IN THE PRESENCE OF INTERFERING IONS
Concentration mg L
Demi nerali zed water
lons^ added Theoretical
Individually
5
5
7
Sb
0.2
0.3
iia
1.5
5.0
Collectively
5.6, 0.8 each
7.0, 1.0 each
Cr(VI) + Co(II)
3.5 each
Mo(VI)* + Cu(II)
3.5 each
0.02
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
Recovery2
v
a
99.6
98.9
89.5
99.6
109.3
107.5
134.2
96.2
89.4
88.2
90.2
Standard
deviation
%
5.9
5.7
4.3
4.7
4.9
4.5
4.4
4.7
4.0
3.5
4.2
River water
Recovery3 Standard
% deviation
%
107 6.6
108 6.5
97 8.3
103.3 7.2
97.1 9.3
1 Ions used Co(II), Cr(VI), Cu(II), Mo(VI), NOs, N1(II) and
Mean of 12 determinations for each system
Mean of 6 determinations for each system
* Results for other combinations, not reported here
16
73
-------
Data on the recovery of arsenic using this digestion method are accept-
able for standard solutions but the recovery of arsenic, added to the
river water, is inconsistent and reproducibility is very poor. Conse-
quently, potassium permanganate method^} with minor modification is
adopted for the digestion of water samples. One-tenth ml of 5.0% po-
tassium permanganate followed by 5.0 ml of concentrated sulfuric and
0.3 ml of nitric adic is added to 50 ml of water sample, containing at
least 1 ;jg of arsenic. The water sample is placed in a water bath for
an hour at 35°C and the excess of potassium permanganate is destroyed
by adding 1.5% hydroxylamine hydrochloride. Hydroxylamine hydrochloride
is added a drop at a time to avoid its excess in the generator. The
reliability of the potassium permanganate method was further investi-
gated using known concentrations (2 jug and 1 jjg of arsenic per 50 ml)
of dimethylarsenic acid (cacodylic acid, sodium salt). This method not
only gives an acceptable recovery and reproducibility for arsenic in
natural water as well as in organoarsenicals (88.8% arsenic recovery
from cacodylic acid), but is less laborious than the acid digestion
method previously used(12). The recovery of arsenic from the river water
is greater than the amount added (Tables 1 and 2). The difference
approximately equals the amount of arsenic found in the Edisto River.
IONIC INTEREFERENCE ELIMINATION
Distillation
It has been suggested in the literature^' that trivalent arsenic
can be quantitatively removed as chloride from an aqueous solution con-
taining hydrochloric acid corresponding to the azeotropic or constant
boiling mixture at 110°C. With the exception of germanium, no other
element distills under these conditions. Various workers(16, 21, 22)
have attempted to apply this technique for the concentration of arsenic
from aqueous solutions before its determination by standard method(lO).
Distillation was regarded as a processU^} in which the percentage of
arsenic recovered from aqueous solutions depends on the sample volume
(V), the amount (micrograms) of arsenic present in the solution and
amount of hydrochloric acid and copper(I) chloride used. Some of these
variables were studied"°/ but the role of changing sample volume (V)
at various arsenic concentrations was not investigated. The distil-
lation system was not tried for the determination of arsenic at levels
contained in drinking water supplies or surface water samples.
The recovery of arsenic at different concentration levels is shown
in Table 3. The arsenic recovery decreases sharply as the concentration
of arsenic in the distillation mixture decreases beyond 0.0909 mg L~l-
It appears that the concentration of arsenic rather than its total amount
in the distillation mixture plays a critical role in determining the
efficiency of the distillation process. The lower limit of arsenic in
distillation mixture for its maximum recovery seems to be around 0.0909
mg L'1. Consequently, the distillation process would not be very effec-
tive for the determination of arsenic at levels in drinking water unless
the sample volume is reduced to bring the arsenic concentration within
17
-------
TABLE 3. PRELIMINARY ARSENIC RECOVERY BY DISTILLATION FROM SYNTHETIC AQUEOUS SAMPLES
oo
Sample
vol . ml
15.0
30.0
50.0
15.0
30.0
50.0
15.0
30.0
50.0
Arsenic 1n
sample mg x 10"^
10.0
10.0
10,0
15.0
15.0
15.0
50,0
50,0
50.0
Acid added
ml
150,0
300.0
500.0
150.0
300.0
500.0
150.0
300.0
500.0
Arsenic concentration
in distillation
mixture mg L~*
0.0606
0.0303
0,0182
0,0909
0.0455
0,0273
0.3030
0.1515
0.0909
Arsenic*
Recovery
%
90.7
80.0
56.0
94.2
86.0
78.0
100.5
99.6
96.0
Standard
deviation
%
6.6
10.0
20.0
6.0
10.6
12.7
3.6
4.3
4.8
Volume of distillate collected 45-55 ml
Amount of copper (I) chloride used 0.2 grams per distillation
* Mean of 15 determinations
* Readings corrected for reagent arsenic, based on the label values
-------
the range of its optimum recovery. For the same reason, bubbling of
hydrochloric acid gas into a one liter water sample^16' until the
hydrochlorice acid concentration reaches the desired level does not
seem to be practical .
The recovery of arsenic from the synthetic as well as the aqueous
samples drawn from the Edisto River, Orangeburg, S. C., is not uniform
(Table 4). These samples were concentrated to bring the arsenic concen-
tration within the range of its maximum recovery (0.0909 mg L"1). The
natural water samples from the Edisto River were spiked with arsenic as
well as with interfering ions (1.0 mg L~l) each of Co(II), Cr(VI), Cu
(II), Hg(II), Mo(VI),Ni(II), and Sb(III), and digested, using potassium
permanganate, before they were subjected to the volume reduction step.
It is speculated that this concentration step is responsible for the
loss of arsenic and thus results in its low recovery. The loss of
arsenic appears to be about the same irrespective of the original amount
of arsenic in a liter of aqueous sample. The recovery of arsenic from
the river water is given in Table 4.
Ion Exchange
The resin used in the present study is in the basic anion form with
-N-(CH3)| Cl~ as the functional group and is effective in separating the
anions from the cations. Hydroxylamine hydrochloride used to destroy the
excess of potassium permanganate during the digestion of aqueous samples
also reduces the chromate (CrOzj") and dichromate (Cr^J to cationic form
of chromium (Cr3+). Excess hydroxylamine hydrochlonde was carefully
avoided.
"
8H+ + 5e — *- Mn2+ + 4H2°
14H+ + 6e — *• 2 Cr3+ + 7H20 (E° = 1.33V)
«
Cr04"+ 8H+ + 3e — .- Cr3+ + 4H20 (E° = 1.19V)
o_
As04'+ 2H+ + 2e -~ AsO^ + Ho (E° = 0.56V)
2NH3OH — N2 + 4H+ + 2H2o + 2e (E° = 1.87V)
N2 + H20 — *- N20 + 2H+ + e (E° = 1.77V)
Even if the part of arsenate (As04~) may have been reduced to
arsenite (As03~) it still remains in the anionic form. The amount of
resin used for preparing ion exchange columns depends on the extent of
impurities expected for removal from the aqueous samples. Generally,
higher amounts of impurities require more resin.
The results on the recovery of arsenic by ion exchange method is
shown in Table 5. It appears that arsenic in water samples can be
quantitatively concentrated by use of ion exchange before its determi-
19
-------
TABLE 4. ARSENIC RECOVERY BY DISTILLATION FROM REDUCED AQUEOUS SAMPLE VOLUME
OF DEMORALIZED AND RIVER WATER
Concentration mg L
Theoretical
0.010
0.015
0.020
0.05
Demi nerali zed
Recovery*
%
78.2
80.5
79.0
86.0
water
Standard
deviation
%
10,7
9.0
10.8
8.6
River
Recovery*
%
70,8
72.0
71.8
80.0
water
Standard
deviation
%
12.2
13.4
13.5
11.6
* Mean of 20 determinations
Readings corrected for arsenic in river water (0.07 mg L )
* Readings corrected for reagent arsenic, based on label values
-------
Interfering
ions added
TABLE 5. ARSENIC RECOVERY BY ION EXCHANGE FROM DEMORALIZED AND RIVER WATER
Concentration mg L'1
Demineralized water River water
Theoretical Recovery!
Standard
deviation
Recovery^ Standard
% deviation
N)
0
0
0
0.020
0.010
0,005
Individuallyj
10.0 0.005
Sb
1.0 0.005
Hg
5.0 0.035
Collectively^
9.0 (1.5 each) 0.005
18.0 (3.0 each) 0.005
Resin used, 2.0 grams
96.7 4,6
92.0 5.0
88.6 5.8
87.0
88.3
88.4
6.0
5.8
5.8
86.8 6.3
67,8 7.8
Resin used, 5,0 grams
86.0
5.4
(continued)
-------
TABLE 5. (continued)
Interfering Theoretical
ions added
18.0 (3.0 each) 0.005
Demi nerali zed water
Recovery* Standard
% deviation
81.0 7.5
River water
Recovery^ Standard
% deviation
V
h
Mean of 12 determinations, readings corrected for reagent arsenic based on label values
2
Mean of 8 determinations, readings corrected for river water and reagent arsenic
3 Ions used Co(II), Cr(VI), and Ni(II)
Ions used Co(II), Cr(VI); Cu(II), Hg(II), Ni(II) and Sb(III)
-------
nation by the standard method'^). The maximum desirable amount of
arsenic in drinking water is 0.01 mg L and the maximum safe amount
is 0.05 mg L~* (*l. The ion exchange method used here can be effec-
tively applied for the concentration of arsenic from drinking water
even below the desirable limit to facilitate its determination by the
standard method^10'
23
-------
REFERENCES
1. Lee, H. K. D. Metallic Contamination and Human Health. Academic
Press, New York, pp. 118-123 (1972).
2. Public Health Service, U. S. Department of Health, Education and
Welfare. Drinking Water Standards. United States Publication
No. 956, United States Government Printing Office, Washington,
D.C., (1962).
3. Subcommittee on Air and Water Pollution of the Committee on Public
Works, United States Senate. Water Pollution, Part 4. United States
Senate, Ninety-first Congress, Second Session. U. S. Government Prin-
ting Office, Washington, D.C., (1970).
4. Caldwell, J. C., R. Lishka, and E. McFarren. Evaluation of a Low Cost
Arsenic and Selenium Determination at Microgram-per-liter Levels.
JAWWA, 65, 731 (1973).
5. Chaney, A. L. and H. J. Magnuson. Coloimetric Microdetermination
of Arsenic. Ind. Eng. Chem. Anal. Ed., 12, 691 (1940).
6. Kolthof, I. M. and E. Andur. Cerimetric Determination of Small
Amounts of Arsenic. Eng. Chem. Anal. Ed., 12, 177 (1940).
7. Liederman, D., J. E. Bowen, and 0. I. Milner. Determination of
Arsenic in Petroleum Fractions and Reforming Catalysts. Anal.
Chem., 30, 1543 (1958).
8. Powers, G. W., R. L. Martin, F. J. Piehl and J. M. Griffin. Arsenic
in Naphthas. Anal. Chem., 31, 1590 (1959).
9. Ballinger, D., R. J. Lishka, and M. E. Gales. Application of Silver
Diethyldithiocarbamate Method to Determination of Arsenic. JAWWA,
54, 1424 (1964).
10. American Public Health Association, American Water Works Association,
and Water Pollution Control Federation. Standard Methods for the
Examination of Water and Waste Water, 13th Ed. American Public Health
Association, Washington, D.C., pp. 62-67 (1971).
11. Tarn, K. C. Arsenic in Water by Flame!ess Atomic Absorption
Spectrophotometry. Env. Sci. Tech. 8, 734 (1974).
12. Kopp, J. F. Ephedrine in Chloroform as a Solvent for Silver
24
-------
Diethyldithiocarbamate in the Determination of Arsenic. Anal. Chem.,
45, 1736 (1973).
13. Sandhu, S. S. Colorimetric "athod for the Determination of Arsenic(III)
in Potable Nater. Analyst 101, 856 (1975).
14. !/hitnack, G. C. and H. H. Martens. Arsenic in Potable Desert Ground-
water: An Analysis Problem. Science 171, 383 (1971).
15. Evans, 8. S. An Improved Method of Titrating Arsenic Precipitated by
Hypophosphorous Acid. Analyst 57, 494 (1932).
16. Farkas, E. J., R. C. Griesbach, D. Schachter, and '-1. Mutton. Concen-
tration of Arsenic from '-later Samples by Distillation. Environ. Sc.
Tech., 6, 1116 (1972).
17. Salint-Ambro, J. The Ion Exchange Behavior of Arsenic(III) on
Various Anion Exchange Psesins. J. Chromatography, 102, 475 (1974).
13. Garret, A. B., II. H. Sisler, J. Bonk, and R. C. Stoufer. Semimicro
Qualitative Analysis, 3rd Ed. Blaisdell Publishing Company, 'Jaltham;
Mass., pp. 38-53 (1957).
19. Perkin-Elmer Corporation. Analystical Method for Atomic Absorption
Spectrophotometer, Mercury Analysis System, 303-3119, Norwalk, Conn.,
(1972).
20. Skoog, D. A. and D. '•]. '-lest. Fundamentals of Analytical Chemistry,
2d Ed. Holt, Rhinehart and '-Jinston, Inc., -Jew York, p. 785, (1967).
21. Baumhardt, G. C. Arsenic Trichloride for Scientific Use. Eng. Quim.
(Rio de Janeiro), 5, 10 (1953).
22. Small, F. D. and C. T. Small. Colorimetric Methods of Analysis, 3rd
Ed., Vol. 2. Van 'lostrand, New York, pp. 380-385 (1967).
25
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/4-78-038
2.
3. RECIPIENT'S ACCESSION" NO.
4. TITLE AND SUBTITLE
Arsenic Determination by the Silver Diethyldithio-
carbamate Method and the Elimination of Metal Ion
Interference
5. REPORT DATE
July 1978 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Shingara S. Sandhu
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Olafin College
Orangeburg, South Carolina
10. PROGRAM ELEMENT NO.
1HD622
29115
11. CONTRACT/GRANT NO.
R-8041640-01
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
Environmental Monitoring § Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
14. SPONSORING AGENCY CODE
EPA/600/06
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The interference of metals with the determination of arsenic by the silver
diethyldithiocarbamate (SDDC) Method was investigated. Low recoveries of arsenic
are obtained when cobalt, chromium, molybdenum, nitrate, nickel or phosphate are
at concentrations of 7 mg/1 or above (individually or collectively). A positive
interference is obtained when the concentration of antimony is 0.3 mg/1 or above
and the concentration of mercury is 1.5 mg/1 or above. Prevention of antimony
and mercury interference was found to be possible by removal with an ion exchange
resin. Potassium permanganate digestion at 35°C was found to be reliable.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Arsenic, water analysis,
arsenic inorganic compounds,
arsenic organic compounds
metals,
Distillation
Interference
Digestion,
ion exchange
99A
18. DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
35
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
26
U.S. GOVERNMENT PRINTING OFFICE: 1978- 767-140/1413
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