F.PA/600/«-8fi/039
December 1986
nFTCBMINATION OF STABLE VALENCE STATES OF CHROMIUM
I?Su3uS AND SOLID WASTE MATRICES - EXPERIMENTAL
IN ^EOT»ERlpFul(3Jruxfl|| OF CHEMICAL BEHAVIOR
J. D. Messman, M. E. Churchwell.
0. Hong, and J. Lathouse
BATTELLE
CoTuoibus Division
Columbus, Ohio 43201-2693
Contract Number 68-03-3224
Work Assignment 1-02
Theodore 0. Martin
Environmental Monitoring and Support Laboratory
Cincinnati, Ohio 45268
EJBO
ARCHIVE
EPA
600-
4-
86-
039
,NVIMONHKNTAL MONITOR INC AND SU??OKT LAIU)^AVORY
oKFic-u OF R;:SCARC;I .\ND DCV.-LOPMF.NT
U.S. ENVIROHMENTAL PROTECTION ACCMCY
CINCINNATI, 01! 45268
REPRODUCED BY
US DEPARTMENT OF COMMERCE
NATIONAL TECHNICAL
INFORMATION SERVICE
SPRINGFIELD. VA 22161
Library
U.S. Environmental Protection Agency
75 Hawthorne Street 13th Floor
San Francisco, California 94105
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F.PA/600/4-86/039
December 1986
EPA
Repositoiy Material
Permanent Collection
DETERMINATION OF STABLE VALENCE STATES OF CHROMIUM
I^QUEOUS AND SOL ID WASTE MATRICES - EXPERIMENTAL
IN A9UEOUV\™FUICATION OF CHEMICAL BEHAVIOR
by
J. D. Messman. M. E. Churchwell,
D. Wong, and J. Lathouse
BATTELLE
Columbus Division
Columbus, Ohio 43201-2693
USEHA
Headquarters and Chemical Librarie
Contract Number 68-03-3224 EpA West B|dg Room 3340
Work Assignment 1-02 MailCOde 3404T
1301 Constitution Ave NW
Washington DC 20004
202-566-0556
t»»'t«|>»»^ nrr•«•»»«•
Theodore 0. Martin
Environmental Monitoring and Support Laboratory
Cincinnati, Ohio 45268
.NVIKONMKNTAL MONITOR INT AND SUPPORT UBOKAVORY
oi-Tici; OF R;:SCARC;I AMD DCVI:LOPMI::IT
U S. EXVIROMMCNTAL PROTECTION ACCNCY
CINCINNATI, Oil 45268
REPRODUCED ^ARTMENT OF COMMERCE
NATIONAL TECHNICAL
INFORMATION SERVICE
SPRINGFIELD. VA 22161
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PB87-140927
Determination of Stable Valence States of
Chromium in Aqueous and Solid Waste
Matrices - Experimental Verification of
Chemical Behavior
Battelle Columbus Div. , OH
Prepared for
Environmental Monitoring and Support Lab.
Cincinnati, OH
Dec 86
w*w»
fttimal
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TECHNICAL REPORT DATA
(flcasr rratl Intlnicliom on Iht rtvenc btjort completing]
REPORT NO
EPA/600/4-86/039
3 RECIPIENT'S ACCESSION HP.^
PB8 7 1 4092ly/AS
TITLE ANOSUBTITLE
Determination of Stable Valence States of Chromium
in Aqueous and Solid Uaste Matrices - Experimental
Verification of Chemical Behavior
S. REPORT DATE
December 1986
6. PERFORMING ORGANIZATION CODE
AUTHOR(S)
J.D. Messman, M.E. Churchwell, D. Wong, and
J. Lathouse
a. PERFOR*
ORGANIZATION REPORT NO
PERFORMING ORGANIZATION NAME AND ADDRESS
Battelle Laboratories
Columbus Division
Columbus, Ohio 43201-2693
10 PROGRAM ELEMEN1
CBSD1A
11 CONTRACT/GRANT NO
68-03-3224
2 SPONSORING AGENCY NAME AND ADDRESS
Environmental Monitoring and Support Laboratory
Offica of Research and Development
U. S. Environmental Protection Agency
Cincinnati, Ohio 45268
13 TYPE OF REPORT AND PERIOD COVERED
Draft Final 2/85 - 9/86
14. SPONSORING AGENCY CODE
EPA 600/6
5 SUPPLEMENTARY NOTES
6 ABSTRACT
The objective of this research effort was to experimentally assess the chemical
behavior of the stable species of chromium during the preparation, chemical
manipulation, and spectrophotometric analyses of simulated and authentic
environmental samples for hexavalent chromium.
The diphenylcarbazide colorimetric method was found to be specific and
sensitive for Cr(VI), as either dichromate or chromate, in simulated aqueous
solutions containing up to 1000-fold ratios of Cr(III). Problems of reduction were
encountered with the method for analyses of simulated samples containing excesses
of both Cr(III) and sulfide. Studies of selected digestion methods for the
analyses of insoluble chromates revealed that the alkaline digestion generally
provided satisfactory recoveries of Cr(VI) however, the nitric acid digestion was
inadequate for the conditions studied. Although Cr(VI) spikes were stable in
alkaline digests of most of the environmental samples studies, Cr(III) spikes were
found to be partially oxidized in the alkaline digests, resulting in positive
errors by as much as 100% in Cr(VI) measurements.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lOENTIFIERS/OPEN ENDED TERMS |c. COSATI I Icld/Croup
IB M5TniauTir.il MA.UULM—
01s tribute to publ< c
>•>•• ...... iifii-i in.. 4.MI »
16
•KitmN it «»i*L»»» 4
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NOTICE
The Information 1n this document has been funded wholly or in part by
the U.S. Environmental Protection Agency under Contract Number 68-03-3224 (Work
Assignment 1-02) to the Battelle Memorial Institute, Battelle Columbus
Division, Columbus, Ohio 43201. It has been subject to the Agency's peer and
administrative review and approved for publication as an EPA document. Mention
of trade names or commercial products does not constitute endorsement or
recommendation for use.
11
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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, Ohio conducts research to:
• Develop and evaluate methods to measure the
presence and concentration of physical, chemical,
and radiological pollutants 1n water, wastewater,
bottom sediments, and solid waste.
• Investigate methods for the concentration,
recovery, and Identification of viruses, bacteria
and other microbiological organisms 1n water; and,
to determine the responses of aquatic organisms to
water quality.
» Develop and operate an Agency-wide quality
assurance program to assure standardization and
quality control of systems for monitoring water and
wastewater.
» Develop and operate a computerized system for
Instrument automation leading to Improved data
collection, analysis, and quality control.
This report presents the results of the evaluation of U.S. EPA Method
3060, "Alkaline Digestion for Hexavalent Chromium" and Method 7196,
"Spectrophotometric Method for Hexavalervt Chromium".
Robert L. Booth, Director
Environmental Monitoring
and Support Laboratory
Cincinnati, Ohio
111
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ABSTRACT
The objective of this research effort was to experimentally assess the chemical
behavior of the stable species of chromium during the preparation, chemical manipu-
lation, and spectrophotometric analyses of simulated and authentic environmental
samples for hexavalent chromium. The effort for this research was divided into
four experimental phases, addressing specific objectives: (1) characterization and
ruggedness evaluation of the diphenylcarbazlde (DPC) spectrophotometric method for
hexavalent chromium, (2) evaluation of the stability and reactivity of hexavalent
chromium under simulated, but controlled aqueous matrix conditions, (3) evaluation
of alkaline and acidic digestions for the analysis of insoluble chromate standards
and trivalent chromium, and (4) evaluation of alkaline and acidic digestions for
chromium analyses of environment samples.
The diphenylcarbazide colorimetric method was found to be specific and sensi-
tive for Cr(VI), as either dichromate or chromate, in simulated aqueous solutions
containing up to 1000-fold ratios of Cr(III). Problems of reduction were encoun-
tered with the method for analyses of simulated samples containing excesses of both
Cr(III) and sulflde. Studies of selected digestion methods for the analyses of
insoluble chromates revealed that the alkaline digestion generally provided satis-
factory recoveries of Cr(VI) however, the nitric acid digestion was inadequate for
the conditions studied. Although Cr(VX) spikes were stable in alkaline digests of
most of the environmental samples studies, Cr(III) spikes were found to be par-
tially oxidized in the alkaline digests, resulting In positive errors by as much as
100% in Cr(VI) measurements.
1v
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Many of the environmental samples became turbid or colored during the alkaline
digestion which affected the DPC colorimetric measurement of Cr(VI). To overcome
this problem additional filtration and dilution was required. Such manipulations
required measurements to be performed in a lower absorbance region, resulting in
increased imprecision.
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CONTENTS
Foreword .-..
Abstract fv
Figures vi11
Tables 1x
Acknowl edgments x11
1. Introduction 1
2. Conclusions 7
3. Recommendations 9
4. Materials and Methods 10
Instrumentation 10
Reagents -...» 1L
Standard solutions 14
EirvlronmentaT samples ;. 14-
5. Experimental Procedures * 18
Simulated sample analyses 18
Environmental sample analyses 19
Dilution schemes for environmental
sample analyses 22
Chromium spiking schemes.for environmental
sample analyses 22
6. Results and Discussion 28
Phase I - Characterization and ruggedness evaluation.
of dlphenylcarbazlde spectrophotometry 28
Phase II - Analyses of synthentlc aqueous solutions
containing trfvalent chromium and sulffde 40
vl
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CONTENTS (Continued)
Phase III - Digestions and analyses of trlvalent
chromium salt and Insoluble standard chromates
Phase IV - Digestions and analyses of 7g
envl ronmental samples .......................................
.................. 112
References ...........................................
vii
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FIGURES
Number Page
1 Absorption Spectrum for 0.5 tag/I Cr(VI) by
Diphenylcarbazlde Spectrophotometry 4
2 Calibration Curve for Cr(YI) as Dlchromate fn Low
Absorbance Range 31
3 Calibration Curve for Cr(YI) as Otchromate In High
Absorbance Range 32
4 Absorption Spectrum for 500 mg/L Cr(III) by
Diphenylcarbazlde Spectrophotometry 38
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TABLES
Page
Number
1 Summary of ICP-OES Operating Parameters for Chromium 12
2 Summary of Dilution Schemes 'for Environmental Sample
Analyses by DPC Spectrophotometry and ICP-OES Following
Alkaline Digestions
3 Summary of Dilution Schemes for Environmental Sample
Analyses by DPC Spectrophotometry and ICP-OES Following
N1 tr1 c Add 01 gestlons
4 Summary of Pre-01gestlon Chromium Spiking Schemes for ^
Environmental Sample Analyses
5 Summary of Post-Digestion Chromium Spiking Schemes for
EnTronm^ntal Sample Analyses After Alkaline Digestions 27
6 Calibration Data for Cr(VI) as Dlchromate and Chromate 30
7 Repeatability of Measurements for Cr(YI) as Dlchromate ^
at Selected Concentrations
8 Day-to-Oay Variability of Measurements for Cr(YI) as
Dichromate at Selected Concentrations
9 Time Stability of the Cr(III)-DPCO Complex 36
10 Determination of Residual Cr(YI) In Trlvalent
Chromlurn N1trate
11 Measurements of 0.05 mg/L Cr(YI) 1n the
Presence of Cr(III)
12 Measurements of 0.5 mg/L Cr(VI) 1n the ^ 42
Presence of Cr(III) .•
13 Effect of Order of 01phenylcarbazlde Reagent and Sulfurlc
Acid Additions on Cr(YI) Absorbance Measurements «
14 pH Measurements of Simulated Aqueous Samples Containing
Cr(YI), Cr(III) and Sulflde
15 Effect of Holding Time on Acidic Cr(YI) Solutions
Containing Cr(III) and Sulflde 4fi
16 Effect of Holding Time on 0.5 •!/»- Cr(VI) M Dlchromate
In the Presence of Sulflde In Alkaline Solution
17 Stability of 0.05 mg/L Cr(YI) as Chromate In Aqueous ^ ^
Sulf1de Solutlons
1x
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TABLES (Continued)-
Number
18 Stability of Simulated Aqueous Sample Solutions Acidified
to pH 2 with Nitric Add ........................................ 51
•
19 Effect of Organic Filter Membrane on the Stability of
Hexavalent Chromium 1n Add and Alkaline Media .................. 54
20 Results of Dlphenylcarbazlde SpectrophotometHc
Measurements of 0.5 mg/L Cr(VI) Solutions 1n
Varying Concentrations of Nitric Add ........................... 57
21 Results of Analyses of Dlcnromate and Insoluble
Chromates 1n the Presence of Trlvalent Chromium
Following Alkaline Digestions ................................... 58
22 Results of Analyses of Barium Chromate Solutions Following
Alkaline Digestions ............................................. 50
23 Results of Analyses of Barium Chromate Solutions Following
Nitric Add Digestions ........ „ ................................. °*
24 Results of Analyses of Trfvalent Chromium Nitrate Solutions
Following Alkaline Digestions ...... • ............................. 64
25 Results of An?lyses of Chromlum(ril) Nitrate Solutions
Following NUHc Add Digestions ................................ 55
26 Results of Analyses of Barium Chromate Solutions Following
Nitric Acid/Persulfate Digestions ............................... 68
27 Results of Analyses of Chromlum(III) Nitrate Solutions
Following Nitric Acld/Persulfate Digestions ..................... 69
28 Results of Analyses of Chromlum(III) Nitrate/Potassium
Dlchromate Solutions Following Nitric Acld/Persulfate
Digestions Employing Various Nitric Add Concentrations ......... 71
29 Results of Analyses of Room-Temperature Digestions of
Potassium Olchromate 1n Nitric Add/Persulfate Media ............ /3
30 Results of Analyses of Room-Temperature Digestions of -
Chromlum(III) Nitrate In NUHc Acld/Persulfate Media ........... '«
31 Summary of Results for Chromium Analyses of Hexavalent and
Trlvalent Chromium Compounds Using Persulfate Digestions ........
32 Summary of Results for Chromium Analyses of N8S-SRM 1645
(River Sediment) Using Alkaline Digestions ......................
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TABLES (Continued)
Page
Number
35 itt«nEtt3tfs^~* -
of Results for
So -Jle "A" Using Alkaline
37 sugary of Results for Chromium Analyses of Contaminated
Soil "A" Using Acid Digestions
"
Alkaline Digestions ............................
'11 98
Digestion Periods
DPC Spectrophotometry
46 Sugary of Results for Chromium Analyses of River Water
Using Alkaline Digestions .......................
x1
of R«ults_for Chromum^n 1Q
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ACKNOWLEDGMENT
The authors acknowledge the support of Mr. Gerald 0. McKee and Mr. Theodore 0.
Martin of the U. S. Environmental Protection Agency (USEPA) who were helpful with
technical discussions and guidance.
Some of the environmental samples used 1n this research program wen supplied
from the USEPA repository, Stauffer Chemical Company and the Leather Industries of
America Research Laboratory (University of Cincinnati). The contributions of such
a wide variety of environmental samples facilitated extensive evaluations of the
nexavalent chromium methods.
We acknowledge our Battelle colleagues, Mr. Charles T. Litsey and Mr.
Donald L. Sgontz for their technical support; and Dr. John R. Nixon, Dr.
Allison F. Fentiman, Ms. Leslie A. Stanton, Ms. M. Gayle Pakrosnis and Ms. Cindy
Boitse who assisted in preparation and review of the report.
we also gratefully acknowledge the following individuals for helpful reviews
of this report: Mr. Robert L. Booth, Dr. Otis Evans, Mr. John F. Kopp, Mr.
James J. Lichtenberg, Mr. Gerald D. McKee. Mr. Theodore D. Martin of the USEPA,
Environmental Monitoring and Support Laboratory - Cincinnati; Ms. Nancy Ulmer of
the USEPA, water Engineering Research Laboratory - Cincinnati; and Mr. Frank H.
Rutland and Mr. Edward Menden of Leather Industries of America Research Laboratory
(University of Cincinnati).
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SECTION 1
INTRODUCTION
The analyses of solid waste materials for hexavalent chromium
represent formidable challenges to the analytical scientist. A metal
speclatlon scheme to differentiate between tHvalent and hexavalent chromium
species, Cr(III) and Cr(VI), must address: (1) solublllzatlon of chromium
species from solid matrices, while (2) maintaining the Integrity of the
individual chromium species during all sample manipulation phases of the
analytical method. Whereas much research has focused on the separation and
detection of dissolved chromium species In synthetic aqueous mixtures or
relatively clean liquid environmental samples, the chemical solublllzatlon and
determination of Cr(VI) In solid waste materials have not been adequately
addressed. From a recent computerized literature search conducted by Battelle.
only one studyl was identified which addressed factors relevant to extractions
of Cr(VI) in the presence of large excesses of Cr(IIX) In solid materials.
The main research efforts of this study have been directed toward
method evaluation and the study of the analytical chemistry of stable chromium
species during the preparation, chemical manipulation, and Instrumental
analyses of simulated and selected authentic environmental samples. The
present study has focused on an Investigation of three selected digestion
methods for the chemical solubHlzatlon of Cr(VI) In barium chromate test
compounds and In real environmental samples: (1) an alkaline digestion medium,
consisting of an aqueous solution of sodium carbonate and sodium hydroxide. (2)
a nitric add digestion method, and (3) a nitric add/persulfate digestion
method. The relative merits of the digestion methods have been based on the
analytical results for solublUzatlon of Insoluble chromates as well as
stability of Cr(III) and Cr(YI) spikes In various test solutions and authentic
environmental samples. The chromium chemistry encountered during sample
preparation, manipulation and analyses 1s briefly described below.
The two stable chromium oxidation states In natural and wastewaters
are 3* and 6+. Hexavalent chromium In alkaline solution exists In the form of
the chromate Ion. Crttf-. At acidic pH. the dlchromate Ion and protonated
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chro-nate Ion predominates. The equilibrium 1s represented In the following
equations: u
2Cr042- * 2H+ < * Cr2072- * H20 K - 4.2 x 10.1*
HCr04- < > Cr042- + H20 K - 3.2 x 10'7
The distribution of HCr04- and CrjK);2- 1s concentration and pH dependent
according to the equilibrium:
2HCr04- < > Cr2072-+H20 * " «
For example, at 1M total chromium concentration, the predominant species 1n
add solution 1s Cr2072- while CrtU2' predominates at PH higher than 7. At
lower concentrations (l * 30H'; Ksp ' &• •
In the presence of excess base. Cr(OH)3(S) can be resolublllzed by forming a
hydroxy complex: Cr(OH)3 * OT 4-> Or(OH)r; «f - IQ'0-4' ««« aging and
heating, Cr(OH}3 precipitation 1s promoted, presumably through polymerization.
The dlphenylcarbazlde spectrophotometHc method, as described In EPA
Method 7196. was employed to measure concentration changes In hexavalent
chromium for each test sample solution resulting from chromium redox phenomena
occuring during the digestions. Under the present test conditions for DC
spectrophotometry, an excess molar concentration (at least 25 molar rat:lo
excess) of dlphenylcarbazlde was provided for the concentration level of Cr(VI)
expected in the test solutions.
In Phase I testing, the dlphenylcarbazlde spectrophotometrlc method
for Cr(VI) has been evaluated under controlled environmental an* analytical
conditions. The dlphenylcarbazlde spectrophotometrlc method Is the most
established, rapid and economical of the available test methods for hexava ent
chromlum.3 01phenylcarbaz1de 1s regarded as a specific reagent for Cr(YI) In
the presence of Cr(lII). whereas the nonspedfldty of atomic absorption
spectrometry (AAS) in terms of elemental valence state requires chemical
isolation of Cr(VI) prior to the quantification step. However, the
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dlphenylcarbazlde spectrophotometric method suffers from potential
interferences due to matrix components such as Mo(VI). Hg(IX). and V(V) species
which my react to form color with the dlphenylcarbazlde reagent or Fe(III)
which 1s chronophorlc and forms yellow-colored solutions that absorb 540-nm
radiation.
The soluble red-violet species 1s a chelate of Cr(III) (formed by
reduction of Cr(VI) by the DPC reagent) and dlphenylcarbazone (the oxidized
form of DPC). The structural formulae of dlphenylcarbazlde and
dlphenylcarbazone are shown below:
sym-01 pheny 1 carbazl de ( 1 , 5-01 phenyl carbohydrazl de )
0 - C
Dlphenylcarbazone ( Pheny lazoformlc add 2-phenylhydraz1de)
The chelate 1s of the form [CrI"(HL)2] + where H2L Is
dlphenylcarbazone and H4L 1s dlphenylcarbazlde.^ The reaction? may be
written:
2Cr042- «. 3H4L + 8H+ - > [Cr«I(HL)23* * Cr* + H2L + 8H20
where H4L ,s dlphenylcarbazlde (DPCI),
H?L 1s dlphenylcarbazone (DPCOK and
[Cr"I(HL)2]+ 1$ the soluble, red-violet chelate of Cr(XII) and DPCO
formed through a redox reaction as seen 1n Figure 1.
Phase II and Phase III of this study were designed to Investigate the
dynamics of the Cr(IXI)-Cr(YIJ redox couple under various pH conditions. The
electrochemical properties of aqueous chromium Ions are highly pH-dependent.
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310
370
430
490_ .550
Ww«<«ngtii (nm)
610
670
730
Figure 1. Absorption Spectrum for 0.5 mg/L Cr (VI)
by Diphenylcarbazide Spectrophotometry
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In acid solution, chromate and dlchromate are readily reduced as shown in the
following half reactions:
HCr04- + 7H+ * 3e- < - > Cr3* + 4H20 E° - 1.195V
Cr2072- + 14H+ + 6e- < - -» 2Cr3+ + 7H20 1° * ^
The Nernst Equation for the CraOy2"/^* couple Indicate* a large
dependency of this half reaction on [H+], reduction of Cr(VI) 1s therefore
thermodynamlcally favored at low pH. At near neutral and alkaline pH, the
latent or solublllzed Cr(OH)3 can participate 1n an oxidation reaction,
yielding Cr(VI) as CrtU2', as shown below:
Cr(OH)3(hyd) + 50H- < - > Cr042- + 4H20 + 3e~ EO --0.13Y
The electrode potential Indicates that Cr3+ should be easily oxidized and
Cr042- should be stable at alkaline pH, but the rate of oxidation of tHvalent
chromium may be limited by the rate of solubllizatlon of Cr(OH)3(s)-
Irr phase II and phase III testing, analytical results have been
presented on many aspects of the Cr(III)-Cr(YI) redox couple considered
critical for handling, digestions, and measurements of Cr(VI) 1n solid waste
samples. These results were obtained with synthetic aqueous sample solutions
and with digestions of Cr(III) and Cr(YI) compounds. Special emphasis was
placed on documenting Cr(VI) reduction by sulflde under various pH conditions
in simulated aqueous samples. Extensive testing was also performed to
determine the ability of the three digestion methods to (1) solubillze
Insoluble chromates and (2) provide a stable redox environment for Cr(III) and
Cr(YI), 1n the presence of added oxldants and reductants. Simulated aqueous
wastes were also employed to determine potential reduction of Cr(YI) during
sample digest fUtratlons and potential DPC spectrophotometrlc Interferences
due to residual add following nitric add digestions.
The nitric add/persulf.ate digestion method was eliminated from
further consideration during the phase III testing. The two remaining
digestion methods, an alkaline digestion and a nitric add digestion, were
applied to authentic environmental samples 1n phase IV.
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For the alkaline digestion method, the- percent recovery of aqueous
Cr(VI) spikes and the percent oxidation of aqueous Cr(III) splices were
determined by DPC spectrophotometry for each sample. The total concentration
of chromium 1n the sample digest solutions was determined Independently by
Inductively coupled plasma-optical emission.spectrometry (ICP-OES). For each
sample carried through the alkaline digestion procedure, the chemistry of
endogenous chromium 1n the sample was Inferred from the results based on
aqueous Cr(III) and Cr(VI) spikes.
Results are presented for three environmental samples carried through
the nitric add digestion procedure. Samples were spiked and analyzed as with
the alkaline digestion procedure. Similar Inferences about the chemistry of
endogenous chromium 1n the sample were based on results for aqueous Cr(III) and
Cr(VI) spikes.
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SECTION 2
CONCLUSIONS
Phase I research activities demonstrated that the EPA protocol for
SN-846 Method 7196 (D1phenylcarbaz1de Spectrophotometry) provides a sensitive
method for Cr(VI) determinations 1n aqueous solution. Instrument response was
linear over two orders of magnitude of Cr(YI) concentration (0.01 to 1.0 mg/L).
The method was specific for Cr(VI) 1n the presence of at least 1000-fold excess
of Cr(III).
Phase II research activities addressed the stability of Cr(VI) In
aqueous solutions containing Cr(III) and sulflde as a function of pH. In
alkaline solutions, reduction of Cr(VI) to Cr(III) by sulflde was slow. As
predicted by standard electrochemical potentials, the reduction of Cr(VI) was
Increased 1n acidic solutions. These results have two significant
ramifications: (1) environmental samples for Cr(VI) analyses should not be
preserved by acidification to pH 2 and (2) dlphenylcarbazlde reagent should be
added to an alkaline sample before pH adjustment to 2 with sulfurlc add to
minimize Cr(YI) reduction "in the quantification step; this verifies proper
order of addition of the two reagents as described 1rr Method 7196.
Phase III research activities demonstrated that a digestion medium
consisting of 50 percent (v/v) nitric acid and 5 percent (w/v) potassium
persulfate is not satisfactory for digestions of solid samples prior to Cr(VI)
determinations. Hexavalent chromium was reduced by the digestion medium
despite the presence of potassium persulfate, generally considered a strong
oxidizing compound. At nitric add concentrations of 20 percent or less, the
nitric acid/persulfate medium demonstrated oxidizing properties.
The 50 percent nitric add medium (without persulfate) and the
alkaline medium (2. percent sodium hydroxide/3 percent sodium carbonate) both
successfully solubllized Insoluble barium cnromate. Furthermore, the valence
states of trlvalent and hexavalent chromium were maintained for standard
solutions carried through either digestion procedure. However, 1n the presence
of oxidizing or reducing agents, the valence states of chromium species were
not maintained in either digestion medium; the extent of valence state
conversion was dependent on the concentration of the specific oxidant or
reductant added.
Phase IV research activities focused on evaluating the ability of the
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50 percent nitric acid medium and the alkaline medium to maintain the valence
states of chromium during digestions of authentic environmental samples. Eight
solid samples were analyzed for Cr(VI) by DPC spectrophotometry following
alkaline digestions. Complete recoveries of Cr(VI) spikes were obtained 1n
alkaline media by the DPC method for most samples (5 out of 8); Cr(VI) spikes
were reduced in Municipal Digested Sludge (organic matrix), Tannery Sludge "B"
(organic/sulflde matrix), and NBS-SRH 1646 Estuarlne Sediment. Partial
oxidation of Cr(IIIj spikes was obtained 1n alkaline media for many of the
samples (4 out of 8); Cr(III) spikes were stable 1n Electroplating Sludge,
Tannery Sludge "B" (organk/sulflde matrix), NBS-SRH 1646 Estuarlne Sediment,
and Municipal Digested Sludge. Partial oxidation of Cr(III) spikes 1n alkaline
media produced measurement errors 1n Cr(VI) concentration by as much as 100
percent (positive bias).
Of the eight solid samples, only the electroplating sludge sample was
successfully analyzed for Cr(Vl) using an alkaline digestion and DPC
spectrophotometry. Although unconfirmed by a collaborative method, endogenous
Cr(YI) in the electroplating sludge was measured as determined by spike
recovery data; Cr(VI) spikes were completely recovered and no measurable
oxidation was observed.
Three solid samples were digested 1n the 50 percent nitric acid
medium and analyzed by DPC spectrophotometry. Although no oxidation of Cr(III)
spikes was observed in any of the samples, Cr(VI) spikes were completely
reduced in all three samples.
DPC spectrophotometry was -limited by color Interferences encountered
in many environmental samples. The interferences ranged from turbidity and
color formation before the addition of DPC to turbidity and color formation
after DPC addition. Turbidity and color Interference was minimized whenever
possible by dilution but this often led to high Imprecision.
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RECOMMENDATIONS
From the compiled data and observations made during this research task,
certain recommendations are suggested for future research work and to the
feasibility of speciation analysis of Insoluble chromium compounds. These
recommendations are as follows:
1. Use of the alkaline digestion, Method 3060, for chromium speciation
analysis of solid samples 1s not recommended. Slightly soluble
trivalent chromium can be partially oxidized while hexavalent
chromium can be slowly reduced.
2. Discontinue further research into developing a digestion procedure
for chromium speciation of solid samples. The stability of the
chromium oxidation state once solubilized in either acid or base
media is matrice dependent and cannot be predicted In environmental
samples.
3. Modify Method 7196 for improved sensitivity for the analysis of
hexavalent chromium in the dissolved fraction of ground and surface
waters.
4. Develop a hybrid technique involving ion chromatographic
separations of hexavalent and trivalent chromium combined with
on-line ultra-sensitive detection of the individual chromium
species. Recent advances in ion chromatography should provide a
low cost analytical means for spedating the dissolved fraction of
chromium in ground and surface waters.
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SECTION 4
MATERIALS AND METHODS
INSTRUMENTATION
D1phenylcarbaz1de Spectroohotometrv - Method 7196
Hexavalent chromium measurements were performed by DPC
spectrophotometry (SH-846 Method 7196) using either a Beckman Model DU-2 or a
Gary Model 14M spectrophotometer. Absorbances were measured at 540 nm using a
matched set of 1-cm quartz rectangular cells. The Cary Model 14M
spectrophotometer was also used for spectral scanning purposes.
In the DPC spectrophotometrlc method, a standard or sample aliquot
was typically added to a 100-mL volumetric flask. Two ml of OPC reagent were
added and mixed. Alkaline sample digest solutions were acidified w.1th sulfurlc
acid to a pH of 2 +/- 0-5 and then diluted to calibrated volume with delonlzed
water for color development. Addle sample digest solutions with a pH less
than 2 required no pH adjustment. Standard and sample absorbances were
measured 10 to 15 minutes after Initial color development except where stated
otherwise 1n specific applications.
Delonlzed water served as the reference solution except where stated
otherwise 1n specified applications. Small blanK readings for delonlzed water,
generally less than 0.006 absorbance unit, due to a slight difference 1n
transmission properties and positioning of the sample and reference cells were
subtracted from each of the measured absorbances. No detectable blank
contribution due to residual Cr(VI) contamination 1n the spectrophotometrlc
reagents was measured.
A linear calibration curve was constructed each day that test samples
were analyzed. A reagent blank and three aqueous standard solutions
containing 0.05, 0.50 and 0.75 mg/L of Cr(VI) were typically used for
calibration. Calibration check standards were also analyzed periodically
throughout the course of analyses to verify stability of the calibration curve.
10
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Inductively Coupled Plasma - Optical Emission Spectrometry
Total chromium measurements on environmental samples were performed
by Inductively coupled plasma - optical emission spectrometry (ICP-OES) using a
Job1n-Yvon Model 70P combination system. Emission Intensities were measured
under computer control using a fixed channel for chromium (205.55-nm Ion line)
on the Model 32 polychromator of the combined polychromator/monochromator
optical system. The Model 32 polychromator, consisting of a 0.5-ra focal length
optical configuration with a 3600 grooves/mm holographic grating, was operated
in an Independent mode. The ICP-OES operating parameters are summarized In
TABLE 1.
A linear calibration curve was constructed from a standard blank and
an appropriate chromium standard solution within the linear range of the
Instrument. The aqueous calibration solutions were prepared using reagents to.
simulate the digest matrix of the diluted'sample. Calibration check standards
were analyzed periodically throughout the course of analyses to verify
stability of the calibration curve.
pH and Redox Potentlometry
A standard pH meter (Orion Research, Inc.) and hydrogen electrode
system was used for pH measurements. An apparatus for measuring redox
potentials of simulated aqueous samples was assembled according to the
manufacturer's (Orion Research, Inc.) Instructions. The electrode unit was a
Model 96-78 platinum redox electrode combined with a silver/silver chloride
reference electrode In a single body which could be directly connected to the
digital "lonanalyzer" meter. Proper operation of the electrode assembly was
verified by performing potential measurements on two I^FetCJOg-S^O/KsFefCNjg
solutions of different concentrations and of known potentials.
REAGENTS
Deionized water with a minimum electrical resistivity of 15 megaohm-
cm (The Barnstead Company, Division of Sybron Corporation, Boston, MA) or
equivalent; deionlzed water with a irinlmum electrical resistivity of 1 megaohm-
11
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TABLE 1. SUMMARY OF ICP-OES OPERATING PARAMETERS FOR CHROMIUM
Radio frequency (RF) Generator (PlasmaTherm) - 27.12 MHz
Forward Power (watts) - 1100
Reflected Power (watts) - 0
Torch (Fassel-type, 3 concentric glass tubes)
Argon Flow Rates (L/nrin)
Plasma (Coolant) Gas - 12
Auxiliary (Sheath) Gas - 0.8
Aerosol (Carrier) Gas - 0.7
Sample Delivery Rate
Peristaltic Pump (mL/nrfn) - 1
Nebulizer (Meinhard-type, glass concentric)
Pressure (ps1) - 3Z
Mass FTov* Controller - 684
Observation Height (mm) - 7-10, above load coll
Signal Integration Period (sec) - 10
Number of Integrations - 3
12
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cm (Peck Water Systems Corporation, Canton, OH) .or equivalent served as the
Inlet xource for tft* S.vtoron/8*rnjt««d 4«fonf*atfon unft.
Barium chromate. 8aCr04. FW 253.33; "Certified" reagent grade (Fisher
Scientific (TMTpdnj*, Ftir ld»n, HJ) or *qufv«?«nC.
Lead chromate, PbCr04, FW- 323.18; ACS reagent grade (6. Frederick
Smith Chemical Company, Columbus. OH) or equivalent.
Sodium carbonate, Na2C03, anhydrous, FW 105.99; Analytical reagent
grade (MalUnckrodt, Inc., Paris, KY) or equivalent.
Sodium hydroxide, NaOH, FW 40.00; "Baker Analyzed" reagent grade, (J.
T. Baker Chemical Company, Phllllpsburg, NJ) or equivalent.
Alkaline digestion solution, 2 percent (w/v) sodium hydroxide and 3
percent (w/v) sodium carbonate: Prepared by dissolving 20 g sodium hydroxide
and 30 g sodium carbonate 1n 1 L of deIonized water.
Sodium sulflde, Na2$'9H20, FW 240.2; "Baker Analyzed" reagent grade,
(J. T. Baker Chemical Company, PhWipsburg, NJ) or equivalent.
Potassium permanganate, KMnO&, FW 158.04; "Baker Analyzed" ACS
reagent grade (J. T. Baker Chemical Company, PhilHpsburg, NJ) or equivalent.
Potassium persulfate, I^Os. FW 270.32; "Baker Instra-Analyzed" ACS
reagent grade (J. T. Baker Chemical Company, PhUHpsburg, NJ) or equivalent.
Manganese d1ox1de» MnQz, FH 86.94; 99+ percent (Aldrlch Chemical
Company, Inc., Milwaukee, WI) or equivalent.
L-Ascorbic add, CgHgOs, FW 176.12; Certified ACS reagent grade
(Fisher Scientific Company, Fair Lawn, NJ) or equivalent.
Sym-d1phenylcarbazide (2,2'-Q1phenylcarbon1c dlhydrazlde), FW 242.3
(Sigma Chemical Company, St. Louis, HO or equivalent); refrigerated when not in
use.
Acetone, (CHs^CO, FW 58.08; "Baker Analyzed" ACS reagent grade (J.
T. Baker Chemical Company, PhilHpsburg, NJ) or equivalent.
Olphenylcarbazide solution, 0.5 percent (w/v): Prepared by dissolving
250 mg sym-d1phenylcarbaz1de 1n 50 mL acetone.
SuIfuric acid, ^$04, concentrated (96 percent, 36 normal), FW 98.08;
Analytical reagent grade (MalHnckrodt, Inc., Paris, KY) or equivalent.
SulfuMc acid, 10 percent (v/v): Prepared by diluting 10 mL of
concentrated sulfuric acid to 100 mL with delon1zed water.
13
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Nitric add, HN03, concentrated (70 percent, 16 normal), FW 63.0;
"Baker Instra-analyzed" reagent grade (J. T. Baker Chemical Company,
PhllUpsburg, NJ) or equivalent.
Perchloric add, HC104, concentrated (60 percent, FW 100.46; ACS
reagent grade (G. Frederick Smith Chemical Company, Columbus, OH) or
equivalent.
STANDARD SOLUTIONS
Hexavalent chromium: (1) commercial 1000 mg/L atomic absorption stock
standard, potassium chromate, <2Cr°4 (MCB Manufacturing Chemists, Inc.,
Cincinnati, OH) or equivalent, (2) commercial 1000 mg/L atomic absorption stock
standard, potassium dlchromate, I^Cr^y (Fisher Scientific Company, Fair Lawn,
NJ) or equivalent, and (3) potassium dlchromate, I^Cr^O;, FW 294.19; "Baker
Analyzed" reagent grade (J. T. Baker Chemical Company, PhllUpsburg, NJ) or
equivalent: a 1000 mg/L stock standard solution was prepared by dissolving
potassium dlchromate 1n delom'zed water.
Trlvalent chromium: chromium nitrate, Cr(N03)3«9H20, FW 400.15;
"Baker Analyzed* reagent grade (J. T. Baker Chemical Company, PhllUpsburg, NJ)
or equivalent: a 1000 mg/L stock standard solution was prepared by dissolving
chromium nitrate 1n delonlzed water.
ENVIRONMENTAL SAMPLES
River Sediment
The river sediment sample (NBS-SRM 1645) Is a freeze-drfed sediment
prepared from material dredged from the bottom of the Indiana Harbor Canal near
Gary, Indiana. The certified concentration for chromium In NBS-SRM 1645 Is
2.96 percent with an uncertainty of 0.28 percent. The uncertainty represents
the 95 percent tolerance limits for an Individual sub-sample; I.e.. 95 percent
of the sub-samples from a unit of this SRM would be expected to have the
certified chromium concentration within the Indicated range of values 95
percent of the time. The chemical form of chromium in NBS-SRM 1645 is unknown.
The certified concentration of Iron Is 11.J percent. The following
values, although not certified, for additional matrix components were also
14
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provided: S102 - 51 percent, MgO - 4 percent, -AlgOs - 4 percent, CaO - 4
percent, and phosphorus - 0.05 percent. These Inorganic constituents represent
approximately 75 weight percent of the total sediment material.
Municipal Digested Sludge
The municipal digested sludge sample Is a freeze-dried water
pollution quality control sludge material supplied from the Inventory of the
Environmental Protection Agency. The reference concentration for chromium In
Municipal Digested Sludge (MDS). as determined by EPA reference laboratories,
Is 0.204 mg/g with an uncertainty of 0.090 mg/g at the 95 percent confidence
level. The chemical form of chromium In Municipal Digested Sludge Is unknown.
The sludge matrix Is relatively high In organlcs Including
approximately 7 percent petroleum hydrocarbons and approximately 23 percent
total organic carbon (TOC). The principal Inorganic matrix components Include
approximately 0.5 percent aluminum, 0.1 percent copper, 2 percent Iron, 1
percent zinc, and 2 percent titanium-
Contaminated Soils "A" and "B"
Contaminated soil samples "A" and "B* are milled soil samples. Both
soil samples appeared to be freeze-dried. Soil samples "A" and "B" contain
approximately 0.1 percent chromium and 1 percent chromium, respectively, as
indicated from independent analyses records. The chemical form of chromium In
Contaminated Soils "A" and "B" 1s unknown. Historical information on the
sample matrices was not provided.
Electroplating Sludge
The electroplating sludge sample is a freeze-dried, quality control
sludge material (WP-286) supplied from the inventory of the Environmental
Protection Agency. The reference concentration for chromium In Electroplating
Sludge, as determined by EPA reference laboratories, is approximately 7 mg/g on
a dry-weight basis. The chemical form of chromium in Electroplating Sludge is
unknown. Minimal information on the history of the sample was available
although reference concentrations for selected metals were provided. The two
15
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principal quantified Inorganic constituents, In addition to chromium. Include
aluminum, approximately 3.6 mg/g, and zinc, approximately 3.5 mg/g.
Estuarlne Sediment
The estuarlne sediment sample (NBS-SRM 1646) Is a freeze-drled
sediment dredged from the Chesapeake Bay. The certified concentration for
chromium in NBS-SRM 1646 1s 76 jig/g with an uncertainty of 3 jig/g. The
estimated uncertainty represents an evaluation of the combined effects of
method Imprecision, possible systematic errors among methods, and material
variability for sample sizes of 500 mg or more. The chemical form of chromium
in NBS-SRM 1646 is unknown.
The certified values for aluminum and iron are 6.25 percent and 3.35
percent, respectively. The following values, although not certified, for
additional matrix components were provided: silicon - 31 percent, sodium - 2.0
percent, potassium - 1.4 percent, sulfur - 0.96 percent and titanium - 0.51
percent. These inorganic constituents represent approximately 45 weight
percent of the total sediment material.
Tannery Sludges "A" and "B"
The tannery sludge samples represent chrome tanning sludges of low-
sulfide content ("A") which had not undergone a hair removal process (no
beamhouse) and of high-sulfide content ("B") which had undergone the full
beamhouse process. The beamhouse process (hair removal) yields a lime-protein
rich sludge with high sulfide from sodium sulflde additions. Based on
analytical information submitted with the tanning sludge samples, the low-
sulfide sludge was characterized as- approximately 30-40 percent solids with a
chromium concentration of approximately 25 mg/g. The high-sulfide sludge
sample was a sludge of high moisture content reported to contain 15 percent
solids and a chromium level of 39 mg/g. It was not verified whether the
reported chromium concentration was based on a "dry-weight" or "wet-weight"
basis. The chemical form of the chromium In these tannery sludge samples is
unknown. Information on sulflde concentrations was not provided with the
samples. Quantitative determinations of sulflde were not performed by Battelle
to obtain this Information.
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River Water
Approximately 500 ml quantities of two river water samples were
collected along the east bank of the Sdoto River between the Marina and the
Grlggs Reservoir Dam on August 7, 1986, 8:30 a.m. The pH of the water samples
was 8.3 within an hour of collection. . The river water samples appeared
slightly yellow In color with slight turbidity. The unflltered water samples
were digested and analyzed within 24 hours of collection. The chemical form of
any endogenous chromium in the river water Is unknown. No other sample history
was available. The river water samples were collected 1n two acid-cleaned
polyethylene bottles. Prior to collection of the samples used for analyses,
the same polyethylene containers were rinsed with river water from the same
location for conditioning purposes.
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SECTION 5
EXPERIMENTAL PROCEDURES
SIMULATED SAMPLE ANALYSES
Selected digestion methods (alkaline, nitric add and nitric
acld/persulfate) were tested on simulated samples of barium chromate and
trivalent chromium nitrate prior to their evaluations on authentic
environmental samples. The digestion methods were tested on the simulated
samples alone and 1n the presence of selected reducing and oxidizing species
commonly encountered 1n the environment.
Alkaline Digestions (Method 3060)
Appropriate masses of simulated solid samples were digested In the
alkaline medium (aqueous mixture of 2 percent sodium hydroxide and 3 percent
sodium carbonate) according to procedure. Because the pressure-filtration step
was very lengthy, the procedure was slightly modified after Initial trials by
replacing the pressure-filtration apparatus with a MllUpore glass vacuum-
filtration system. The vacuum-filtration apparatus Including a 47-mra diameter
filter membranes having average pore porosities of 0.45 urn.
Nitric Acid Digestions
Appropriate masses of simulated solid samples were digested In a 50
percent nitric acid medium. One hundred-mL-of deionized water were first added
to test portions of the solid samples prior to the addition of 100 mL of
concentrated nitric acid. The concentrated nitric acid was slowly added in
small volumetric increments under controlled stirring and heating conditions.
The test samples were digested on a hot plate at low heat for approximately 2
hours and then vacuum-filtered through 0.45-um filter membranes according to
the procedure of Method 3060. The filtrates were then transferred to 1-L
volumetric flasks and diluted to calibrated volume with deionlzed water.
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Nitric Ac1d/Persu1fate Digestions
Appropriate masses of simulated samples were digested 1n a nitric
acld/persulfate medium consisting of 50 percent (v/v) nitric add and 5 percent
(w/v) potassium persulfate. The potential feasibility of such a digestion
medium was predicated on the possibility tff maintaining a highly oxidizing
medium with potassium persulfate In the digestion solution to keep Cr(YI) In an
oxidized state even under extremely acidic conditions.
ENVIRONMENTAL SAMPLE ANALYSES
The eight solid samples and one liquid sample were analyzed for total
chromium concentrations by ICP-OES. Independent sample digestion methods were
used: nitric add - perchloric acid digestions for the eight solid samples and
nitric add digestions for the river water sample. These analyses were
performed prior to hexavalent chromium analyses to determine total chromium
concentrations 1n the environmental samples using rigorous digestion methods;
the total chromium concentrations were used to estimate Cr(VI) and Cr(III)
spike concentration levels 1n the hexavalent chromium experiments.
The nine environmental samples were analyzed for hexavalent chromium
using an alkaline digestion and quantification by DPC spectrophotometry; three
of the solid samples were also analyzed for hexavalent chromium using a mild
nitric acid digestion. All sample digest solutions were also analyzed for
total chromium concentrations by ICP-OES to provide Information on the relative
solubilities of chromium species In alkaline and nitric add digestion media.
Except for the two tannery sludge samples, the environmental samples
were digested and analyzed as received. Preliminary sample preparation of the
tannery sludges included an attempt to partially homogenize the moist bulk
samples with respect to moisture content. Approximately 250 g of each sludge
sample was transferred to an acid-cleaned 500-mL plastic bottle and stirred for
5 minutes with a ceramic spoon. The "homogenized" sub-samples were stored in a
refrigerator until sampling for analyses was required. After a homogenized
sub-sample was removed from the refrigerator and wanned to room temperature for
sampling, the tannery sludge sub-sample was stirred again with a ceramic spoon
prior to taking test portions for analyses.
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Nitric Acid - Perchloric Acid Digestions
Prior to hexavalent chromium analyses, total chromium concentrations
were determined 1n the eight solid environmental samples using an Independent
digestion method and quantification by ICP-OES. One-gram test portions of the
solid samples were Initially digested In 20 mL of 50 percent nitric add.
After the addition of 10 mL of concentrated perchloric add, each- sample
solution was further digested until dense perchloric add fumes appeared.
Heating of each sample was continued until slightly less than 10 ml of digest
solution remained. The digest solutions were gravity-filtered, with delonlzed
water rinsing, Into 100-mL volumetric flasks and diluted to calibrated volume
for ICP-OES analyses.
Calibration for ICP-OES analyses of the solid environmental samples
was performed using a standard blank and a 10 mg/L chromium standard solution.
The standard blank and calibration standard solutions were prepared In 10
percent (v/v) perchloric add to approximate the matrix acid and' concentration
of the sample digest solutions.
A rigorous nltrfc acid digestion without perchloric acid was used as
an alternate digestion method for total chromium measurements of river water
samples. Duplicate 30-mL allquots of .river water were digested 1n 30 mL of
add medium (50 percent nitric add) 1n a 100-mL beaker on a hot plate with
stirring for 2 hours. The river water samples were digested at a temperature
of 80°C +/- 10°C. The sample digest solutions were vacuum-filtered through
0.45-um filters, collected 1n 100-mL volumetric flasks and diluted to
calibrated volume with delonlzed water. A reagent blank was also carried
through the same digestion procedure and analyzed with the samples.
Calibration for ICP-OES analyses of river water samples was performed using a
standard blank and a chromium standard solution within the linear range of the
Instrument; the standard blank and standard solutions contained the same amount
of nitric add as the digested sample solutions. Control check standards were
also analyzed to verify stability of the calibration curve during sample
analyses.
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Alkaline Digestions (Method 3060)
Hexavalent and total chromium concentrations were determined In
alkaline digest solutions of the nine environmental samples by OPC
spectrophotometry and ICP-OES, respectively* One-gram test portions of the
eight environmental samples were digested In 50 ml of the alkaline medium (an
aqueous mixture of 2 percent sodium hydroxide and 3 percent sodium carbonate),
except where stated otherwise, in a 100-mL beaker on a hot plate with stirring
for approximately 45 minutes. The sample solutions were heated to a
temperature of 80°C V- 10°C. After a cooling period, the sample digest
solutions were then vacuum-filtered through a 47-mm filter membrane (0.45-um
pore size) of a glass MilUpore filtering apparatus, transferred Into 100-mL
volumetric flasks and diluted to near calibrated volume with deionized water.
After addition of concentrated nitric add to adjust the pH to approximately 7,
each sample solution was diluted to calibrated volume with delonized water.
Fifteen-mi aliquots of the river water sample were digested in 15 ml
of alkaline digestion medium as described above for the solid samples. Because
of the low endogenous chromium concentration in the river sample, each filtrate
was diluted to a final volume of 50 mL to maintain a minimal dilution factor.
Two sets of the alkaline digestions were performed for quantification by OPC
spectrophotometry and ICP-OES. For the DPC spectrophotometrie measurements,
color development was performed in the 50-mL volumetric flasks for collection
of the sample filtrates to alleviate further dilution. The additional set of
sample filtrates diluted to 50 ml were analyzed directly for total chromium by
ICP-OES. On this basis, the ICP-OES measurements would be directly comparable
to the OPC spectrophotometrfc measurements for the samples within the same
sample sets.
Calibration for ICP-OES analyses was performed using a standard blank
and a chromium standard solution within the linear calibration region of the
instrument. The standard blank and calibration standard solutions were
prepared in an alkaline digest matrix equivalent to the final matrix of the
diluted samples; the pH of the alkaline matrix solution was neutralized with
concentrated nitric acia followed by further acidification to pH 2 with
sulfurlc acid.
21
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Nitric Add Digestions
Hexavalent and total chromium concentrations were determined In
nitric add digest solutions of three solid samples (River Sediment, Municipal
Digested Sludge, and Contaminated Soil "A") by OPC spectrophotometry and ICP-
OES, respectively. One-gram test portions of the three environmental samples
were digested 1n 50 ml of add medium (50 percent nitric acid) in a 100-mL
beaker on a hot plate with stirring for 2 hours. The samples were digested at
a temperature of 80°C +/- 10°C. The sample digest solutions were vacuum-
filtered through 0.45-um filter membranes of a glass MllUpore filtering
apparatus, transferred Into 100-mL volumetric flasks and diluted to calibrated
volume with delonlzed water.
DILUTION SCHEMES FOR ENVIRONMENTAL
SAMPLE ANALYSES
Various dilution schemes required for DPC spectrophotometrlc and ICP-
OES analyses of the nine environmental samples due to a wide range of
endogenous chromium concentrations 1n the samples. The dilution schemes for
analyses of the environmental samples following alkaline and nitric acid
digestions are summarized in TABLES 2 and 3, respectively.
CHROMIUM SPIKING SCHEMES FOR
ENVIRONMENTAL SAMPLE ANALYSES
Pre-01gestion Spikes
The redox behavior of Cr(VI) and Cr(III) during digestions of
environmental samples was evaluated by a series of pre-digestion chromium
spiking experiments. Six L-gram test portions of solid samples and 15-mL test
allquots of river water were used to form three sets for testing of each
environmental sample: (1) duplicate unspiked samples, (2) duplicate samples
spiked with Cr(III), and (3) duplicate samples spiked with Cr(YI). The
concentrations of the Cr(III) and Cr(YI) spikes were similar to the total
endogenous chromium concentrations measured by ICP-OES following the
independent digestion methods. Each chromium spike was added to the
environmental sample prior to addition of the digestion media.
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TABLE 2. SUMMARY OF DILUTION SCHEMES FOR ENVIRONMENTAL SAMPLE ANALYSES BY
DPC SPECTROPHOTOMETRY AND ICP-OES FOLLOWING ALKALINE DIGESTIONS
Sample
Digestion Final Filtrate Filtrate Dilution^)
(Sample/Medium) Volume (mL) DPCIcF
River Sediment
Municipal Digested
Sludge
Contaminated Soil "A"
Contaminated Soil "B"
Electroplating Sludge
Estuarine Sediment
Tannery Sludge "A"
Tannery Sludge "B"
River Water
1 g/400 mL
1 g/50 mL
1 g/50 mL
1 g/50 mL
1 g/50 mL
1 g/50 mL
1 g/50 mL
' 1 g/50 mL
30 ml/30 mL
15 mL/15 mL
1000
100
100
100
100
100
100
100
100
50
20X
10X
SOX
100X
100X
1.43X
100X
100X00
None
None
10X
None
2.5X
10X00
10X00
None
2X
10X
None
(a) Dilution of digest filtrate of unsplked, Cr(III)-sp1ked, and Cr(VI)-sp1ked
samples except where noted otherwise.
(b) Dilution of digest filtrate of Cr(YI)-sp1ked sample.
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TA8LE 3> ^MMARY OF OILUTI0N SCHEMES FOR ENVIRONMENTAL SAMPLE ANALYSES BY
DPC SPECTROPHOTOMETRY AND ICP-OES FOLLOWING NITRIC! ACIDI OIGESTIONS
Sample
f* M i Rnal F1ltrate Filtrate Dilution^)
(Sample/Medium) Volume (mL) 0PC TCP"
River Sediment
Municipal Digested
Sludge
Contaminated Soil "A"
1 g/50 mL
1 g/50 mL
1 g/50 mL
10QQ
100
100
1QX
10X
SOX
None
None
None
slmpfls"
Cr(III)-splked. and Cr(VI}-spiked
24
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Hexavalent chromium spikes were added to the environmental samples In
the form of allquots of a chromium standard solution prepared by dissolving
solid K2<>207 in delonlzed water. Trivalent chromium spikes were added to the
environmental samples In the form of allquots of a chromium standard solution
prepared by dissolving solid Cr(N03)3-9H20 In delonlzed water. The pre-
dlgestlon chromium spiking manipulations applicable o the environmental sample
analyses are summarized 1n TABLE 4.
Post-Digestion Spikes
The presence of multiplicative Interferences In the quantification
steps by DPC spectrophotometry or ICP-OES was checked by post-digestion spikes.
Post-digestion spikes would differentiate between multiplicative Interferences
which altered the slope of the calibration curve during the quantification step
and Incomplete recoveries of pre-digestlon chromium spikes. The post-digestion
chromium spiking manipulations applicable to the environmental sample analyses
are summarized in TABLE 5.
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TABLE 4. SUMMARY OF PRE-DIGESTION CHROMIUM SPIKING SCHEMES
FOR ENVIRONMENTAL SAMPLE ANALYSES
Sample^)
River Sediment
Municipal Digested Sludge
Contaminated Soil "A"
Contaminated Soil "B"
Electroplating Sludge
Estuarlne Sediment
Tannery Sludge "A"
Tannery Sludge "B"
River Water (30 mL)
River Water (15 ml)
Spike
Addition
30 mg
0.2 mg
1.0 mg
8.5 mg
8.5 mg
76 ug
5.7 mg
7.8 mg
50 ug
25 ug
Standard
Concentration^1'0)
3 mg/mL
0.2 mg/mL
1.0 mg/mL
8.5 mg/mL
8.5 mg/mL
76 ug/mL
5.7 mg/mL
7.8 mg/mL
50 ug/mL
50 ug/mL
Aliquot
Added
10 mL
1 mL
1 mL
1 mL
1 mL
1 mL
1 mL
1 mL
1 mL
0.5 mL
(a) 1-g test portions of solid"samples; 30-mL or 15-mL test altquots of water
sample.
(b) Cr(III) aqueous spiking solution prepared from solid
dissolved in deionized water.
(c) Cr(VI) aqueous spiking solution prepared from solid KgC^O/ dissolved 1n
deionized water.
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TABLE 5. SUMMARY OF POST-DIGESTION CHROMIUM SPIKING SCHEMES FOR
ENVIRONMENTAL SAMPLE ANALYSES AFTER ALKALINE DIGESTIONS
Cr(VI) Spike Concentration (mg/mL)(a)
Sample DPCICP
River Sediment
Municipal Digested Sludge
Contaminated Soil "A" —W
Contaminated Soil "B" 0.5 1.0
Electroplating Sludge 0.5 1.0
Estuarlne Sediment 0.76 0.76
Tannery Sludge "A" 0.5 1.0
Tannery Sludge "B" 0.5 2.0
ttter — 0.5
(4) Cr(VI) spike concentration In final dilution of alkaline digest solution
presented to the instrument for analysis.
(b) Cr(Vl) spike concentration of 0.5 rag/L 1n final dilution of nitric acid
digest presented to the Instrument for analysis.
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SECTION 6
RESULTS AND DISCUSSION
PHASE I - CHARACTERIZATION AND RUG6EDNESS EVALUATION
OF DIPHENYLCARBAZIDE SPECTROPHOTOMETRY
Parameter and ruggedness evaluations of the EPA protocol for SW-846
Method 7196 were Initially performed to test the feasibility of using Method
7196 as a probe for monitoring chromium redox phenomena during digestions and
analyses of solid chromate materials and environmental samples. Concentration
f1gures-of-mer1t for hexavalent chromium were determined and selected
ruggedness parameters were tested using Method 7196.
Detection Limit
The detection limit (CL) for Cr(VI) using Method 7196 was estimated
by extrapolation from measured analyte concentrations to an analyte
concentration with a signal-to-noise ratio of 3 using the equation recommended
by the International Union of Pure and Applied Chemistry (IUPAC)9:
(Wi) * K*»
where k Is an arbitrary confidence factor, Npns represents the root-mean-square
(standard deviation) noise, and m is the slope of the calibration curve. A
confidence factor of 3 for Ic is used to comply with the IUPAC criterion. The
root-mean-square noise is estimated by the standard deviation of the absorbance
measured for seven replicate 0.01 mg/L Cr(VI) standard solutions prepared and
analyzed on a single day; 0.0005 absorbance unit is a representative value for
the root-mean-square noise. A slope of 0.82 absorbance per mg/L is a
representative value for m. Substitution of these values Into equation (1)
gives a detection limit of approximately 0.002 mg/L Cr(VI). This detection
limit also corresponds to the concentration value calculated for the method
detection limit (MDL)10. However, practical detectability of the given
spectrophotometers with analog meter readouts, limited by readout error of the
meter needle (uncertainty of approximately 0.002 absorbance unit), is
approximately 0.01 mg/L Cr(YI). Detectability may be increased, if necessary,
by using larger path-lenth absorption cells.
28
-------
Linear Dynamic Range
The optimal range of calibration standards for Cr(VI) as dlchromate
and chromate was established using Method 7196. The experiments were conducted
on different days to estimate day-to-day variability 1n the calibration curve.
The absorbances, corrected for DPC blanks, for a series of Cr(VI)
standard solutions, ranging from 0.01 mg/L to 1.5 mg/L, are presented 1n TABLE
6. Absorbance measurements were performed on the Beckman Model DU-2
spectrophotometer except for the absorbance data 1n parentheses which were
obtained on the Gary Model 14M spectrophotometer. No detectable blank
contribution due to Cr(YI) contamination 1n the reagents or glassware was
observed. Linearity was observed over a 100-fold concentration range from the
detection limit, 0.01 mg/L, to approximately 1 mg/L.
The absorbance data 1n TABLE 6 were statistically examined with a
linear regression algorithm using a hand calculator. The sensitivity (slope),
Intercept, and correlation coefficient were calculated for Cr(YI), as
dlchromate and chromate, on different days. An average slope of 0.82
absorbance per mg/L was obtained. Excellent day-to-day reproduc1bH1ty of the
calibration curve was achieved. No systematic uncertainty 1s Introduced when
either dlchromate or chromate 1s used as the primary stock solution for Cr(VI).
Typical absorbance data for Cr(YI), as dlchromate, are graphically
presented In Figures 2 and 3. The lower concentration or absorbance range Is
presented 1n Figure 2 and the higher concentration or absorbance range Is
presented in Figure 3. The constructed lines 1n Figures 2 and 3 represent the
linear regression fit (r - 0.9999) for 12 data points between 0.01 mg/L and 1.0
mg/L, Inclusively.
A second set of typical calibration absorbance data for Cr(YI) as
dlchromate and chromate, measured over approximately a 100-fold concentration
range on different days, were compiled. An average slope of 0.83 +/- 0.008
absorbance per mg/L was obtained for Cr(YI) as dlchromate on six different days
with a day-to-day variability of 1.0 percent. The average slope of the
calibration curves for Cr(YI) as chromate on two different days was 0.84 *•/-
0.006 absorbance per mg/L with a relative reproduc1bH1ty of approximately 0.8
percent.
In all cases, correlation coefficients of the linear regressions for
the individual calibration curves were between 0.9994 and 0.9999. The data
29
-------
TABLE 6. CALIBRATION DATA FOR Cr(VI) AS OICHROMATE AND CHROMATE
f rfVH
Ur\ Y i ; _
Concentration
(mg/L)
0.010
0.020
0.025
0.050
0.075
0.10
0.15
0.20
0«5C
.& J
0.50
.75
1.0
1.5
Linear Regression
No. Data Points
Slope (Abs/mg/L)
Intercept (Abs)
Corr. Coeff.
Absorbance(a)
#1
0.015
• •_
0.040
.__
0.082
0.125
0.165
0.406
(0.814)
1.14
7
0.8L
0.001
0.9999
*2
(0.007)
0.021
(0.040)
(0.062)
(0.082)
—
—
0.209
(0.404)
0.616
0.815
—
9
0.82
0.000
0.9999
K2Cr
0.011
___
(0.022)
(0.044)
0.064
0.085
~~ —
-_ —
0.207
•(0.403)
0.608
0.805
___
9
0.80
0.004
0.9999
•04
0.010
• • •
(0.022)
(0.043)
0.064
0.085
"""
• • •
0.214
(0.423)
0.626
0.813
1.15
9
0.82
0.004
0.9998
Absorbances measured on Beckman Model DU-Z; absorbances in
nuaui uan\.sj •!•».«.«-••.- — i jiii
parentheses measured on Gary Model 14M.
30
-------
0.14
0.12
0.10
c
o
0
u
o
i
cxoa
0.06
0.04
0.02
XOO
O Beckman DU-2
Q Gary 14M
I I L
OOO 0.02
0.04 0.06 OJJ8 aiO 0.12
Cr (VI) Concentration (mg/L)
0.14
Rguw 2. Calibration Curve for Cr (VI) as Bichromate
in Low Abaorbanc* Range
31
-------
1.4
O Beckman DU-2
n Gary 14M
Cr (VI) Concentration (mg/L)
Figure 3. Calibration Curve tor Or (VI) a* Dlchromate
In High Abaorbanca Range
32
-------
Indicated that the linear dynamic range of the method 1s sufficient to permit
measurements of Cr(YI) concentrations at the 0.05 mg/L and 0.5 mg/L levels
using the same Instrument parameters and conditions.
Precision
The repeatability of Method 7196 was assessed by consecutive
measurements of the absorbances of seven solutions of Cr(VI) as dlchromate at
the 0.01 mg/L, 0.05 mg/L, and 0.5 mg/L concentrations. The data from each set
of measurements, repeated on separate days, are presented 1n TABLE 7. The
average repeatability (reported as percent relative standard deviation) for the
two days was 5.7 percent at the 0.01 mg/L level, 1.6 percent at the 0.05 mg/L
level, and 0.41 percent at the 0.5 mg/L level.
The day-to-day variability of measurements for Cr(VI) as dlchromate
at three concentrations was examined. Each Cr(VI) concentration was measured
on a minimum of 11 different days. The data for the three Cr(VI)
concentrations are presented In TABLE 8. The day-to-day variability was 2.5
percent for 0.05 mg/L, 1.2 percent for 0.5 mg/L, and 0.95 percent for 1 mg/L.
These precision data Indicate that the repeatability and day-to-day variability
of DPC spectrophotometHc measurements 1n delonized water using Method 7196 are
excellent over the relevant concentration range for Cr(YI).
Time Stability of the Cr(Iin-DPCO Complex
The absorbances of a reagent blank and eight standard solutions,
extending from 0.01 mg/L to 1 mg/L, were monitored for 90 to 120 minutes after
color development. No color degradations were observed for any of the
solutions for the first 90 minutes. Standards with less than 0.50 mg/L Cr(YI)
were monitored for an additional 30 minutes; the measured absorbances remained
constant throughout the test period. The data is summarized 1n TABLE 9.
This experiment verified that sample dilutions could be made to
contain as much as 1 mg/L Cr(YI) using the adopted test procedure. The red-
violet color developed 1n the solutions is stable for at least 90 minutes in a
delonized water matrix. However, the present data do not preclude
complications which may occur 1n the presence of redox agents and other metals
1n environmental samples.
33
-------
TABLE 7. REPEATABILITY OF MEASUREMENTS FOR Cr(VI)
AS OICHROMATE AT SELECTED CONCENTRATIONS
Absorbance
Average
Std. Dev.
Rel . Std. Dev.
0.01 mq/L
n
0.009
0.008
0.009
0.009
0.009
0.009
0.009
0.009
0.0004
4.45
Cr(VI)
#2
0.007
0.007
0.008
0.007
0.008
0.008
0.007
0.007
0.0005
7.15
0.05 mq/L
11
0.040
0.040
0.040
0.041
0.040
0.041
0.041
0.040
0.0005
1.31
Cr(VI)
#2
0.042
0.042
0.042
0.042
0.044
0.042
0.042
0.042
0.0008
1.91
0.5 mq/L
n
0.413
0.411
0.416
0.411
0.416
0.413
0.412
0.413
0.0021
O.S1S
Cr(VI)
n
0.412
0.413
0.415
0.415
0.412
0.413
0.412
0.413
0.0013
0.311
34
-------
TABLE 8. DAY-TO-DAY VARIABILITY OF MEASUREMENTS FOR Cr(VI) AS
DICHROMATE AT SELECTED CONCENTRATIONS
Cr(YI) Concentration Number of Absorbance RSD
mg/L Data Points Average Percent
0.05 11 °-043 2'5
0.5 15 0.428 1.2
11 0.844 0.95
35
-------
TABLE 9. TIME STABILITY OF THE Cr(III)-OPCO COMPLEX
••^••^••B
Cone.
mg/L
0.00
0.01
0.05
0.10
0.50
0.70
0.80
0.90
1.00
•uHmmm^n
Time
Hrs
0.167
0.183
0.167
0.200
0.217
0.183
0.333
0.333
0.367
•KH^KKVH
ABS
-0.003
0.006
0.040
0.087
0.430
0.597
0.685
0.764
0.845
^^HHBK^
Time
Hrs
••^•^^^••^^^^^
0.530
0.530
0.520
0.560
0.550
0.783
0.767
0.750
0.600
•l^BMMH^BBB
ABS
-0.004
0.006
0.041
0.085
0.428
0.598
0.685
0.767
0.845
I^BHBB^Bi
Time
Hrs
1.18
1.20
1.35
1.40
1.38
1.45
1.42
1.38
1.42
HBMMM^^B
ABS
-0.003
0.006
0.042
0.064
0.429
0.597
0.684
0.761
0.835
Time
Hrs ABS
1.95 -0.003
1.93 O.OOS
1.93 0.041
1.98 6.086
2.05 0.425
._
—
•• — —
-_ —
36
-------
Specificity
Experiments were performed to verify the specificity of DPC
spectrophotometry for Cr(VI) 1n the presence of high concentrations of Cr(III).
These specificity tests required' prellmjnary determinations of: (1) the
spectral background absorbance at 540 nm due to the chromophorlc property of
aqueous Cr(III) and (2) the quantification of Cr(VI) contamination 1n the
chromium nitrate salt used for Cr(III) spike solutions for this and subsequent
experiments.
An absorption spectrum (310 nm to 730 nm) for a 500 mg/L Cr(III) test
solution 1s presented 1n Figure 4. This test solution contains DPC reagent and
has been adjusted to pH 2 with sulfuric add. As shown 1n Figure 4, the 540-nm
wavelength for DPC spectrophotometry Hes on the shoulder of an absorption band
for Cr(III) centered at approximately 580 nm.
Cr(VI) contamination 1n the chromium nitrate salt was determined by
measuring the absorbances for two different 500 mg/L Cr(III) solutions at 540
nm. The first solution was pH-adjusted to I without the addition of DPC. Any
measured absorbance from this solution would be due solely to background
absorbance resulting from the chromophorlc property of aqueous Cr(III). The
second solution was prepared with the addition of DPC and pH adjustment to 2.
The measured absorbance from this solution represented the sum of Cr(III)
background absorbance and absorbance due to reaction of Cr(VI) contaminant with
DPC reagent.
The results of the absorbance measurements are presented In TABLE 10.
The net absorbance difference, 0.002 "absorbance unit, for measurement of the
two solutions corresponds to a Cr(YI) concentration below the detection limit
of 0.01 mg/L. These data Indicate that Cr(VI) contamination In the
Cr(NG3)3«9H20 salt represents less than 0.002 percent of the total chromium
present.
The specificity of Method 7196 for Cr(VI) as dichromate and chromate
was assessed by relative absorbance measurements of Cr(YI) solutions at the
0.05 mg/L and 0.5 mg/L concentration levels 1n the presence of 100X, 200X,
500X, and 1000X concentrations of Cr(IIl). Individual blank solutions were
also prepared without Cr(VI) for each Cr(III) concentration and analyzed by DPC
spectrophotometry. The absorbance data, corrected for DPC blanks, for the
studies at the 0.05 mg/L and 0.5 mg/L concentrations of Cr(VI) are presented in
37
-------
0.0
310
370
430
490 550
Wvmtength (nm)
610
730
Figure 4. Absorption Spectrum for 500 mg/L Cr (III)
by Diphenylcarbazide Spectrophotometry
38
-------
TABLE 10. DETERMINATION OF RESIDUAL Cr(YI) IN TRIYALENT
CHROMIUM NITRATE
Calculated Cr(YI)
Solution Absorbance Concentration (mg/L)la;
Standard Blank
0.05 rag/L Cr(YI)
0.50 mg/L Cr(YI)
0.75 mg/L Cr(YI)
500 mg/L Cr(III) (No DPC)
500 mg/L Cr(III) (+ DPC)
500 mg/L Cr(III) + 0.05 mg/L Cr(YI)
•••• TaawTi»^rwTM
0.000
0.043
0.440
0.661
0.094
0.096
0.141
—
0.049
0.500
0.749
0.002(b)
•0.052(c)
(a) Calculated from linear regression of 3 Cr(YI) standards.
(b) Corresponds to < 0.002 percent Cr(YI) contamfnatloir In Cr(N03)3-9H20.
(c) Recovery of 0.05 mg/L Cr(YI) spike Is 100 percent.
39
-------
TABLES 11 and 12, respectively. The average percent relative absorbances for
0.05 mg/L Cr(VI) as dlchromate and chromate were 107 percent and 93 percent,
respectively. The average percent relative absorbances for 0.5 mg/L Cr(VI) as
dlchromate and chromate were 99 percent and 98 percent, respectively. These
data Indicate excellent specificity of 'Method 7196 for Cr(VI) as either
dlchromate or chromate 1n the presence of 100- to 1000-fold excess
concentrations of Cr(III) provided that DPC blank corrections are taken Into
account.
Order of Dlphenylcarbazlde Reagent
and Sulfuric Add Additions
Although Method 7196 specifies the addition of dlphenylcarbazlde.
reagent before acidification with sulfurlc add 1n the color development state,
other DPC spectrophotometrlc methods for Cr(VI) 1n the literature specify
acidification with sulfuHc add before addition of dlphenylcarbazlde reagent.
The ruggedness of this procedural step for color development was examined for
two concentrations of Cr(YI) In solutions of similar pH.
The results of the study for Cr(YI), as dlchromate and chromate, are
presented In TABLE 13. The data Indicate that absorption measurements for 0.05
mg/L and 0.5 mg/L concentrations of Cr(YI), with and without 100-fold ratios of
Cr(III), were not sensitive to order of additions of OPC or sulfuHc add. No
differences were found for Cr(YI) added 1n the form of dlchromate or chromate.
However, addition of OPC before acidification may be preferred for analyses of
alkaline sample digests to minimize the possibility of reduction of Cr(YI) by
reducing species 1n the sample matrix prior to color development.
PHASE II - ANALYSES OF SYNTHETIC AQUEOUS SOLUTIONS
The effects of Cr(III) and sulfide matrix constituents,'collectively
and Individually, on the DPC spectrophotometrlc measurements of Cr(VI)
solutions were conducted. The studies focused on 'the examination of selected
fundamental factors including concentration. pH, redox potential, and holding
time of the simulated sample solution.
40
-------
TABLE 11. MEASUREMENTS OF 0.05 mg/L Cr(YI) IN THE PRESENCE OF Cr(III)
01 chroma te
mg/L
0
5
10
25
50
Absorbance
0.042
0.042
0.048
0.044
0.047
Percent
Relative
Absorbance
~
100
114
105
112
Chroma te
Absorbance
0.045
0.043
0.041
0.042
0.042
Percent
Relative
Absorbance^)
~
96
91
93.
93
(a) Cr(III) added as a solution of CKNOsJa-SHgO dissolved 1n delonlzed water.
(b) Relative to 0.05 mg/L Cr(VI) with no Cr(III) addition.
41
-------
TABLE 12. MEASUREMENTS OF 0.5 mg/L Cr(YI) IN THE PRESENCE OF Cr(III)
Bichromate
mg/L
0
50
100
250
500
Absorbance .
0.415
0.409
0.411
0.412
0.412
Percent
Relative
AbsorbanceloJ
—
99
99
99
99
Chroma te
Absorbance
0.427
0.419
0.419
0.418
0.417
• ' •••
Percent
Relative
Absorbancelb)
—
98
98
98
98
(a) Cr(III) added as a solution of Cr(N03)3-9H20 dissolved 1n delonlzed water.
(b) Relative to 0.5 mg/L Cr(VI) with no Cr(III) addition.
42
-------
TABLE 13. EFFECT OF ORDER OF OIPHENYLCARBAZIDE REAGENT AND SULFURIC
ACID ADDITIONS ON Cr(YI) ABSORBANCE MEASUREMENTS
Cr(VI)
mg/L
0.05
0.05
0.5
0.5
Cr(III)U)
mg/L
0
5
0
50
.
Cr(VI)
01 chromate
01*07^2504
0.042
0.041
0.412
0.417
H2S04/DPC
0.042
. 0.041
0.409
0.417
Absorbance
Chromate
OPcVhgSO^
0.043
0.041
0.409
0.425
H2S04/DPC
0.043
0.041
0.419
0.427
(a) Cr(III) added as a solution of Cr(N03)3-9H20 dissolved 1n delonlzed water.
43
-------
oH Measurements
Selected pH measurements of Cr(YI) as dlchromate and chromate at the
0.05 n,g/L and 0.5 mg/L concentration levels In the presence of varying
concentrations of Cr(III) and sulfldt are presented In TABLE 14. The pH of
each test solution was measured Immediately after the addition of OPC reagent.
Identical trends In solution PH were observed for Cr(VI) as either dlchromate
or chromate although the pH for each chromate solution was marginally higher
than for the corresponding dlchromate solution.
Addition of Increasing Cr(III) concentrations, prepared from
Cr(N03)3-9H20 as the stock source, to Cr(YI) solutions decreased the solution
pH. In the presence of 500 mg/L Cr(III). the solution pH of 0.5 mg/L Cr(VI)
was 2.9; such a low pH may be critical In maintaining Cr(YI) In an oxidized
state If in the presence of a reducing agent that does not have alkaline
properties. Addition of 5 mg/L sulflde to Cr(YI) solutions containing 5 mg/L
Cr(III) Increased the solution pH to where It was almost neutral. In the
presence of 50 mg/L sulflde, the pH of 0.5 mg/L Cr(Vl) solutions, with and
without 50 mg/L Cr(III). was 4.9 and 10.6. respectively. Thus, for the given
concentrations, sulflde may not reduce Cr(YI) In the absence of any acidic
matrix components because of the inherent alkaline properties of sulflde.
Effect of Holding Time on Acidic Cr(vr. Solutions
Synthetic aqueous solutions containing 0.5 mg/L Cr(YI), 50 mg/L
Cr(III) and 50 mg/L sulflde were prepared. Hexavalent chromium was studied by
using both dichromate and chromate as stock solutions for Cr(YI). The pH of
the solutions was 4.9. The buffering capacity of 50 mg/L Cr(III) prevented
sulflde from raising the solution pH to approximately 11. The test solutions
were analyzed for Cr(VI) by DPC spectrophotometry after holding times of 0, 12.
and 60 minutes.
The results of the OPC spectrophotometrlc measurements of the test
solutions are presented in TABLE 15. Mthln the 12 minute holding period,
sulflde had sufficient reaction time to reduce Cr(YI) to a concentration level
approximately 50 percent of its original concentration. Within the 60 minute
holding time, the Cr(VI) concentration had been reduced to approximately 10
percent of its original concentration.
44
-------
TABLE 14. pH MEASUREMENTS OF SIMULATED AQUEOUS SAMPLES
CONTAINING Cr(YI), Cr(III) AND SULFIDE
Cr(VI)
mg/L
0
0.05
0.05
0.05
0.05
0.05
0.05
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
mg/L
0
0
5
10
25
50
5
0
50
100
250
500
5
0
50
SulfldeM
mg/L
0
0
0
0
0
• 0
5
0
0
0
0
0
5
50
50
Dlchromate
pH
5.8
5.6
4.0
3.9
3.6
3.4
6.9
5.9
3.5
3.3
3.1
2.9
6.7
10.6
4.9
Chromate
pH
5.9
6.0
4.2
4.0
3.8-
3.6
7.0
6.6
3.6
3.4
3.2
3.0
6.7
10.6
4.9
(a) Cr(III) added as a solution of Cr(N03)3«9H20 dissolved 1n deionlzed water.
(b) Sulflde added as a solution of Na£S dissolved In deionlzed water.
45
-------
TABLE 15. EFFECT OF HOLDING TIME ON ACIDIC Cr(VI) SOLUTIONS
CONTAINING Cr(III) AND SULFIDE
aConKKMBHKI
Cr(YI)
mg/L
0.5
0.5
0.5
KtKDKnUOHW*—
Cr(III)(a)
mg/L
50
50
50
Sulf1de(b) ' Holding Time Cr(VH Absorbance
mg/L PH Minutes 01 chroma te Chromate
50
50
50
4.9
4.9
4.9
0
12
60
0.32
0.15
0.042
0.34
0.17
0.038
_^_^__^_^—^^^^^^^^•^•••^••^••^^•^•^^""^"'
(a) Cr(III) added as a solution of Cr(N03)3-9H20 dissolved In delonlzed water.
(b) Sulflde added as a solution of Na2S-9H20 dissolved In delonlzed water.
46
-------
The reduction of Cr(YI) by sulflde was exacerbated because the
MM1QM wnUtnrt CTllllli «" •*"««« of Cr(XII) as trlvalent
where Cr(VI) 1s a more powerful oxldant and Is, thus, more easily reduced by
sulflde. These data verify that Cr(VI) would be reduced by sulflde In acidic
media; the kinetics of the reduction may be controlled by the degree of
acidity. Similar results were obtained for Cr(VI) as dlchromate and chromate.
Effect of Holding Time on Alkaline Cr(VI) Solutions
The effect of sulflde on the stability of 0.5 mg/L Cr(YI) as
dlchromate In alkaline solution was studied as a function of holding time over
a one-hour period. The pH of the Cr(YX) solutions containing SO mg/L sulflde
is 10.6 without the buffering capacity of 50 mg/L Cr(lII) as the nitrate.. The
data, presented In TABLE 16, reveals that there 1s no significant reduction of
0.5 mg/L Cr(VI) In the presence of 50 mg/L sulflde over at least a 1 hour
period. This confirms that Cr(YI) 1s stable 1n the presence of sulflde In an
alkaline (pH 10.6) medium.
Stability of Cr(YI) Solutions
The stability of simulated aqueous solutions containing 0.05 mg/L
Cr(VI) as chromate and various concentration ratios of sulflde were studied
over a minimal two-day period. The pH of each solution was allowed to reach
Its equilibrium value without any additions to Intentionally drive the pH to
either the addle or alkaline side. Measurements of solution pH and redox
potentials were performed on each simulated sample at different stages In the
procedure in an attempt to correlate observed decreases in Cr(YI) absorbance
with possible changes 1n solution chemistry.
The results of the stability study are summarized 1n TABLE 17. No
significant changes in absorbance for Cr(YI) solutions 1n the absence of
sulflde were observed over the too-day period. A decrease (approximately 25
percent) In absorbance was observed for the Cr(YI) solutions In the presence of
10-fold sulflde concentration (0.5 mg/L) during the first 18 hours after which
the absorbances remained relatively constant. No significant decreases In
absorbance were observed for the Cr(YI) solutions containing 100-fold sulflde
47
-------
TABLE 16. EFFECT OF HOLDING TIME ON 0.5 oig/L• Cr(YI) AS
OICHROMATE IN THE PRESENCE OF SULFIOE IN
ALKALINE SOLUTION
Sul
•^HMI^^M
(a)
(b)
Holding
fideU) Time(b)
mg/L Minutes
0 0
50 0
50 10
50 20
50 30
50 60
Sul fide prepared from
Time elapsed between
solution and addition
Cr(YI)
Absorbance
0.411
0.390
0.379
0.383
0.386
0.384
^==^====
Na2S-9H20 dissolved in deionized
preparation of simulated aqueous
of dlphenylcarbazide reagent and
Relative
Percent
—
95
92
93
94
93
=====
water.
sample
sulfuric
acid in color development stage.
(c)
Relative to 0.5 mg/L
Cr(VI) with no sulflde addition.
-------
TABLE 17. STABILITY OF 0.05 mg/L Cr(Vl) AS CHROMATE IN AQUEOUS SULFIDE SOLUTIONS
After Cr(VI)/H20
Addition^3 )
pH Redox Pot.^>)
6.0
6.0
5.8
6.1
6.1
5.8
6.0
6.1
5.9
5.9
5.9
5.7
5.6
5.7
5.8
6.0
5.7
5.9
5.9
5.8
+160
+ 111
+236
+209
+204
+82
+105
+81
+99
+ 115
+119
till
+102
+130
+148
+149
+78
+118
+98
+96
+90
Sulfide
Addition(c)
mg/L
0
0
0
0.5
0.5
0.5
0.5
0.5
5
5
5
5
5
50
50
50
50
50
50
After Sulfide
Addition
pH Redox PotJb)
--
--
7.5
7.2
7.6
7.2
7.2
9»7
9.7
9.7
9.6
9.6
10.6
10.8
10.7
10.7
10.7
10.7
--
__
-231
-220
-219
-216
-214
-297
-302
-302
-303
-319
-368
-349
-364
-339
-346
-344
Elapsed
Tlme(d)
Hours
6
28
47
0
18
24
42
47
0
.6
21
24
9Q
0
7
17
30
48
93
After Elapsed
Time
pH Redox Pot.(°)
5.8
5 ft
5.9
5.8
6.6
6.4
6.4
6.2
9.1
8.7
8.8
7.7
10.5
10.5
10.2
10.3
+110
+49
+171
-186
-124
+78
+144
-279
-295
-271
-243
-330
-337
-316
-306
After DPC
Addition
7TT
pH Redox Pot. 1°)
5.8
6.1
6.1
5.8
5.7
6.7
6.7
6.3
6.3
6.2
9.6
9.0
8.7
8.5
7.6
10.6
10.4
10.0
10.1
10.3
+259
+ '••8
+261
+124
+211
+268
+90
+73
+83
+123
+5
+10
+14
.+24
+38
-28
-8
-19
0
+12
+26
Cr(VI)
Absorbance
0.045
0.043
0.040
0.043
0.040
0.045
0.034
0.032
0.034
0.034
0.045
0.0*2
0.040
0.042
0.025
0.040
0.035
0.028
0.022
0.025
0.021
(a) Cr(Vl) aliquot a,dded to 100-mL volumetric flask and diluted to approximately 80 ml with delonlzed water of
natural pH for measurements.
Redox potential measurements 1n units of millivolts.
Sulfide prepared from Na?S dissolved In delonlzed water.
(dj Time elapsed between preparation and analysis of simulated sample.
-------
concentration (5 mg/L) over the first 24-hour period; a significant decrease
was observed after 90 hours. A small decrease 1n absorbance was observed for
Cr(YI) solutions 1n the presence of 1000-fold sulflde concentration ISO mg/L)
within the first eight hours; the absorbances were relatively constant after 24
hours for the remaining 3-day period.
A trend between the relationship of sulflde concentration with that
of solution pH, redox potential and Cr(YI) absorbance was evident. For the
0.05 mg/L Cr(YI) solutions containing Increasing concentrations of sulflde, the
following observations were evident 1n TABLE 17: (1) the solution pH Increased
with Increasing sulflde concentation, (2) the redox potential became
Increasingly more negative as a function of holding time with Increasing
sulflde concentration prior to OPC addition, or Increasingly less positive
after OPC addition, (3) Cr(YI) absorbances generally decreased with Increasing
sulflde concentration although an apparent equilibrium concentration
approximately 50 percent of the original Cr(YI) concentration was reached for
the higher sulflde concentrations.
The stability of simulated aqueous solutions containing 0.05 mg/L
Cr(YI) as chromate and various concentration ratios of sulflde after add
preservation with nitric add (pH adjustment to 2) was studied. The stability
of Cr(YI) 1n these solutions were monitored for at least 24 hours.
The Cr(YI) absorbance data are presented 1n TABLE 18. No significant
decrease 1n absorbance was observed for 0.05 mg/L Cr(YI) solutions 1n the
absence of sulflde at pH 2 for up to 96 hours. However, dramatic decreases In
absorbances for 0.05 mg/L Cr(YI) solutions containing as little as 0.5 mg/L
sulflde at pH 2 were observed due to the reduction of Cr(YI) by sulfide under
these pH conditions.
The experimental data in TABLE 18 Indicate that acid preservation by
acidification to pH 2 with nitric acid should not be recommended for 0.05 mg/L
Cr(YI) solutions containing sulfide and other similar sample solutions. Cr(YI)
solutions arc not stable under these pH conditions. Moreover, this emphasizes
the importance of adding DPC first before acidification with sulfuric add, as
specified in Method 7196. for DPC spectrophotometric measurements of Cr(VI) 1n
the presence of sulflde and probably other strong reducing agents.
50
-------
TABLE 18. STABILITY OF SIMULATED AQUEOUS SAMPLE SOLUTIONSU)
ACIDIFIED TO pH 2 WITH NITRIC ACID
Elapsed
Time(b)
Hours
0
6
24
32
50
96
0
6
24
50
96
0
6
24
0
g
U
24
Sulflde
Add1t1on(c)
mg/L
0
0
0
0
0
0
0.5
0.5
0.5
0.5
0.5
5
5
5
50
50\d)
50(0)
Cr(VI)
Absorbance
0.044
0.045
0.038
0.038
0.033
0.039
0.042
0.019
0.004
0.001
0.000
0.040
0.000
0.000
0.011
0.000
0.000
(a) Simulated sample solutions consist of 0.05 mg/L Cr(VI) as chromate
and varying concentrations of sulflde.
(b) Time elapsed between preparation and analysis of simulated sample.
(c) Sulfide prepared from NagS dissolved In deionized water.
(d) Simulated sample became cloudy 1n approximately 4 hours after
preparation and was filtered through a 0.22 urn disposable filter
prior to colorimetric measurement.
51
-------
PHASE III - ANALYSES OF INSOLUBLE STANDARD CHROMATES
Selected sample preparation procedures (e.g., alkaline, nitric add
and nitric acid/persulfate digestions) were. Investigated for the analyses of
Insoluble chromates. Preliminary experiments Investigating the effects of
organic filter media during filiations of sample digest solutions and heating
on the solubility characteristics of Cr(III) were first conducted.
Stability of Cr(VI) In Alkaline and Nitric Add Digest Solutions
During Vacuum-Filtration Through Organic Filter Membranes
Researchers have reported significant reduction of Cr(YI) to Cr(III)
in a recent article pertaining to add leaching of hexavalent chromium from
cellulose ester filters." Since both add and alkaline digest solutions will
be filtered through Mllllpore HA type (cellulose ester) filter membranes, It
MS essential to determine whether or not the filter membranes would affect the
redox states of chromium.
The diameter of each filter Is 47 mm and the thickness 1s no more
than 150 urn +/- 10 w. The average pore size Is 0.45 urn +/- 0.02 urn. The
filter Is composed of mixed esters of cellulose. According to the
manufacturer's literature, these filters are not attacked by dilute adds and
alkalies and are recommended for temperatures under 75°C.
This study was divided Into two parts: (1) filtration of add
solutions and (2) filtration of alkaline solutions. In both parts, solutions
containing known concentrations of Cr(YI) were prepared In the appropriate
digest medlus! and vacuum-filtered through cellulose ester filter membranes.
The filtrate solutions were analyzed for Cr(VI) by DPC spoctrophotometry.
Control solutions were prepared and analyzed similarly but without the
filtration step. The concentration difference measured this way would yield
the net effect of the filter membrane on the stability of hexavalent chromium
1n the particular digestion medium.
For the vacuum-filtration of alkaline Cr(VI) solutions, duplicate
solutions containing 50 ug and 500 jig of Cr(VI) as chromate were added to 50
ml of alkaline digestion medium (2 percent sodium hydroxide and 3 percent
sodium carbonate). These solutions were vacuum-filtered, transferred to 100-
mL volumetric flasks, and then diluted to calibrated volume with deionlzed
water The Cr(VI) concentrations in 10-fold dilutions of these filtrate
52
-------
solutions were measured by DPC spectrophotometry. Corresponding control
solutions of Cr(YI), not taken through the vacuum-filtration step, were also
prepared and analyzed by DPC spectrophotometry.
For the vacuum-filtration of Cr(YI) solutions 1n nitric add, six
solutions were used to form three sample sets: (1) duplicate solutions
containing 0.2 mg Cr(YI) 1n 50 mL of 50 percent (v/v) nitric add, (2)
duplicate solutions containing 0.5 mg Cr(VI) 1n 50 ml of 10 percent (v/v)
nitric add and (3) duplicate solutions containing 0.5 mg Cr(YI) In 50 mL of
50 percent (v/v) nitric add. These solutions were filtered, transferred and
diluted to 100 mL 1n volumetric flasks and further diluted 10-fold for OPC
spectrophotometry. No pH adjustment for color development was required for
these sample solutions. Corresponding control solutions were prepared by
adding either 2-mL or 5-mL allquots of 10 mg/L Cr(YI) standard to 5 mL of the1
appropriate acid medium and diluted to 100 mL for DPC analysis. The pH of
these add solutions was less than 1. Another set of control solutions were pH
adjusted to pH 2 so the pH factor would not be compounded with the filter
membrane factor.
The calibration'curves for this study were constructed with Cr(YI)
standards prepared 1n delonlzed water. Oelonized water was used as the
reference. The results for this filter membrane study are summarized 1n TABLE
19. There was no significant loss of Cr(YI) when alkaline solutions or 10
pe-cent (v/v) nitric acid solutions were filtered through the cellulose ester
membranes. However, when the medium 1s 50 percent (v/v) nitric acid,
significant loss of Cr(YI) through reduction to Cr(III) was found. This
conclusion 1s supported by combined DPC spectrophotometric and ICP-CES results.
The extent of Cr(VI) reduction 1s approximately 90 /jg regardless of
whether 0.2 mg or 0.5 mg of Cr(VI) were available. This constant amount of
Cr(VI) reduction could be a function of surface area contact and/or duration of
contact with the filter membrane. It is uncertain whether the cellulose esters
are being oxidized by the Cr(YI) and as a result, a portion of the Cr(VI) 1s
reduced or whether the filters are contaminated with trace amounts of metals
such as Fe(II).
53
-------
'ABLE 19. EFFECT OF ORGANIC FILTER MEMBRANE ON THE STABILITY OF
HEXAVALENT CHROMIUM IN ACID AND ALKALINE MEDIA
Test Solutions
Percent Recovery
of Cr(YI)
OPC
(I)
(2)
Percent Recovery
of Cr(YI)
ICP
T?7
Alkali Digestion Medium
0.05 mg Cr(YI)
0.05 mg Cr(VI) Control
0.5 mg C.-(YI)
0.5 mg Cr(YI) Control
96
96
99
100
98
98
Add Digestion Medium
0.5 mg Cr(VI)/10X (v/v) HN03,
pH = 1.2
0.5 mg Cr(VI)/10S (v/v) HN03 Control,
pH - 1.2
0.5 mg Cr(VI)/10S (v/v) HN03 Control,
pH - 2
0.2 mg Cr(VI)/50S. (v/v) HN03
pH - 0.6
0.2 mg Cr(YI)/505 (v/v) HN03 Control,
pH = 0.6
0.2 mg Cr(YI)/50S (v/v) HN03 Control, '
pH =» 2
Net reduction = 89 ug Cr(YI) to Cr(III)
0.5 mg Cr(YI)/50X (v/v) HN03,
pH • 0.6
0.5 mg Cr(VI)/50S (v/v) HN03 Control,
pH - 0.6
0.5 mg Cr(VI)/50S (v/v) HN03 Control,
pH - 2
Net reduction - 91 ug Cr(YI) to Cr(III)
96
99
102
47
92
99
71
90
102
96
47
73
99
98
97
100
54
-------
Effect of Heating on the Solubility Characteristics
of Cr(III) 1n Alkaline Digestion Medium
Chromium (III) 1s only sparingly soluble 1n alkaline medium since the
solubility product constant, KSp- °f Cr(OH)'3 Is approximately 10'31. In the
presence of excess hydroxide. Cr(OH)3 can be resolublllzed through the
formation of Cr(OH)4~ species. The formation constant, Kf, of Cr(OH)4' from
Cr(OH)3 and OH" Ions 1s 10'0-4.
To provide an estimate of chromium (III) precipitation without sample
matrix Interference, duplicate solutions were prepared by adding 50 ml of
alkaline digestion medium to 7.8 mg Cr(III). One solution was heated for 45
minutes at 80°C +/- 10°C with mechanical stirring. The other solution was
maintained at room temperature for 24 hours. When the alkaline digestion
medium was added to each Cr(III) mass, a light-blue precipitate formed which
quickly redlssolved to form a light-green solution as more alkaline solution
was added. The heated solution formed a fine light-blue precipitate whereas
the unheated solution remained light green. No precipitate was observed In the
unheated solution. Both.solutions were filtered. The light-blue precipitate
1n the heated solution was retained on a 0.45-um pore size filter membrane; the
filtrate was clear.
Analyses of the filtrates by ICP-OES revealed that 7.0 mg/L chromium
remained 1n solution for the unheated solution, which represents 90 percent of
the original 7.8 mg/L chromium concentration. The filtrate from the heated
solution contained only 0.13 mg/L dissolved chromium, representing only 2
par-cent of th« original 7.8 mg/L chromium concentration. Tho« data provide
qualitative confirmation that aqueous trlvalent chromium fs largely removed
from the dissolved fraction through precipitation as a hydroxide after
digestion 1n alkaline media.
Effects of MJtric Acid and pH on Dlphenvlcarbazide
The digestion methods using a nitric acid medium result 1n highly
acidic digests (generally less than pH 1) that may need to be analyzed for
Cr(Vt) by the OPC spectrophotometrlc method. Because of the highly acidic and
oxldlilno, nature of tha nitric acid w\d of tha reducing characteristics of the
55
-------
Interaction between the HNOa and OPC that may adversely effect the redox
reaction between Cr(VI) and DPC 1n the formation of the colored complex.
Synthetic aqueous test solutions of 0.5 ppm Cr(VI) were prepared in
various concentrations of HNOa as Indicated in TABLE 20. Each test solution
was prepared in duplicate for spectrophotometric measurement, one without pH
adjustment and one with pH adjustment to pH 2 using dilute NaOH prior to
addition of the DPC reagent.
The absorbance measurements for the paired test solutions under
different pH conditions are presented in Table 20. These absorbance data
indicate that at least up to 30 percent HN03 solutions do not significantly
degrade the efficacy of the DPC reagent. Test solutions resulting from nitric
acid digestions may, therefore, be analyzed directly by the DPC
spectrophotometric method without previously raising the pH of the test
solution to pH 2.
Alkaline Digestions of Insoluble Chromates
In the Presence of Cr(IIIJ
The alkaline digested method was.investigated to ascertain whether or
not 1t could solubilize chromates typically insoluble in aqueous solution and
If it would oxidize Cr(III). Dichromate and Insoluble chromates, spiked with
two different masses of solid Cr(N03)3-9H20, were analyzed by DPC
spectrophotometry following alkaline digestions. In these experiments, an
abbreviated method was used 1n which the leachate was not filtered but instead
diluted to calibrated volume without pH adjustment. Lead chromate in the test
sample did not visibly precipitate'because a small mass was used and the
alkaline leachate solution was diluted considerably before an aliquot was
neutralized with nitric add prior to the DPC spectrophotometric measurement.
As Indicated by the measured absorbances 1n TABLE 21, good recoveries
of Cr(YI) were obtained and Cr(III) was not significantly oxidized during the
alkaline digestion in the absence of other oxidizing compounds. For the
present test conditions, the alkaline digestion method proved satisfactory in
terms of solubilizing Cr(YI) from insoluble chromates and not oxidizing Cr(III)
in alkaline media.
56
-------
TABLE 20. RESULTS OF OIPHENYLCARBAZIDE SPECTROPHOTOMETRIC MEASURE-
MENTS OF 0.5 mg/L Cr(VI) SOLUTIONS IN VARYING CONCEN-
TRATIONS OF NITRIC ACIO
Percent
HNOi
Matrix
0
1
5
10
20
30
Without pH
Adjustment
0.428(b)
0.422
0.402
0.420
0.410
0.400
Absorbance
With pH
Adjustment^3)
0.420
0.422
0.420
0.420
0.418
0.408
(a) Solution pH adjusted to pH 2 with dropwise additions of one molar
or ten molar NaOH.
(b) Test solution without HN03 required 0.2 mL of 10 percent H2S04 for
adjustment of solution pH to pH 2 for color development.
57
-------
TABLE 21. RESULTS OF ANALYSES OF OICHROMATE AND INSOLUBLE
CHROMATES IN'THE PRESENCE OF TRIVALENT CHROMIUM
FOLLOWING ALKALINE DIGESTIONS
SampleU.b.c) Absorbance
Blank - nnA
4- 0.77 g Cr(N03)3.9H20 (100 mg Cr) 0.004
* 7.7 g Cr(N03)3.9H20 (1000 mg Cr) °-004
155 mg PbCr04 (25 mg Cr) .
+ 0.77 g Cr(N03)3-9H20 (100 mg Cr) }
+ 7.7 g Cr(N03)3-9H20 (1000 mg Cr) <
122 mg BaCrO^ (25 mg Cr)
+ Q.;77 g Cr(N03)3-9H20 (100 mg Cr) 0.414
+ 7.7 g Cr(N03)3-9H20 (1000 mg Cr) l
70.8 mg K2Cr207 (2S m9 Cr) % n
* 0.77 g Cr(N03)3'9H20 (100 mg Cr) 0-
* 7.7 g Cr(N03)3-9H20 (1000 mg Cr) °-
70.8 mg K2Cr207 (25 mg Cr) Q
(aqueous standard solution) «.
(a) All Cr(VI) salts added so that Cr(VI) concentration would
be 0.5 mg/L in final dilution for analysis.
(b) Sample leachate solutions not filtered; initially diluted to
1000 mL volume without pH adjusonent.
(c) 2-mL allquots of leachate solutions diluted to about 30 mL
volume with delonized water; PH adjusted to •PP««;«JJ1J
7 with NaOH prior to DPC/H2S04 addition and final dilution
to 100 mL volume for colorimetric measurement.
58
-------
Alkaline Digestions of Barium Chromate
Alkaline digestions were evaluated to ascertain the extent of any
reduction of Cr(VI) during the digestions of an Insoluble chromate. Barium
chromate, selected as the Insoluble chromate for testing, was studied In the
absence and presence of two different reducing compounds, sodium sulflde and
ascorbic add, each at two different concentrations.
Test portions (487 mg) of solid barium chromate (equivalent to 100 mg
of hexavalent chromium) containing additions of sodium sulflde or ascorbic add
were digested according to the alkaline digestion procedure; sulflde and
ascorbic add were added Individually 1n two different amounts equivalent to a
one- and ten-fold ratio of the Cr(VI) mass 1n the barium chromate. The
alkaline digestions and analyses for each of the test samples were repeated on
separate days to confirm the experimental observations and data.
Observations and results of the analyses of barium chromate solutions
for alkaline digestions are presented 1n TABLE 22. A fine precipitate formed
during the alkaline digestions of all barium chromate test samples. The
precipitates observed for all the barium chromate test samples except the one
with high-level sulflde were white; a yellowish-green precipitate was observed
for barium chromate with high-level sulflde. The'white precipitates were
probably barium carbonate; chromium (.III) hydroxide may have also
coprecipiated. The yellow-green precipitate was probably a mixture of barium
carbonate and elemental sulfur which was visible only for the higher sulflde
mass addition.
Serial dilutions of 10-fold and 20-fold for a combined 200-fold
dilution were performed on the filtrate of each barium chromate test sample to
provide a target concentration of 0.5 ppm Cr(VI) for spectrophotometric
analyses. In the absence of reducing compounds, an average absorbance of 0.413
(approximately 97 percent of an aqueous 0.5 ppm Cr(VI) calibration standard)
was measured for the test solutions which represents full recovery of Cr(YI).
Average recoveries of approximately 85 percent and 89 percent were obtained for
Cr(YI) in the barium chromate test samples containing equivalent amounts of
sulflde or ascorbic acid, respectively. No detectable Cr(YI) was measured in
the barium chromate samples containing sulflde or ascorbic acid at 10-fold
greater masses.
59
-------
TABLE 22. RESULTS OF ANALYSES OF BARIUM CHROMATE SOLUTIONS
FOLLOWING ALKALINE DIGESTIONS
^^^=^==^===^^=
Sample
487 mg BaCHty (100 mg Cr)
+ 0.243 g Na2$ (0.1 g sulfide)
+ 2.43 g Na£S (1 g sulfide)
+ 0.1 g Ascorbic Acid
+ 1 g Ascorbic Acid
Sample Diqest Appearance
Precipitate Filtrate
white yellow
white yellowU)
yellow-green colorless'*'
white golden brown
white brown
Percent
Reduction
of Cr(VI)
#1 #Z
4 2
15 16
100 100
8 14
100 100
(a) Alkaline sample filtrate solutions turned cloudy upon neutralization
with nitric acid.
60
-------
The data 1n TABLE 22 Indicate: (1) Cr(YI) 1s completely solublllzed
and 1s not significantly reduced during the alkaline digestions of Insoluble
barium chromate In the absence of representative reducing compounds, (2)
approximately 10 to 15 percent of the Cr(YI) Is reduced to Cr(III) during the
alkaline digestions of Insoluble barium chromate 1n the presence of sulflde or
ascorbic add at equivalent mass ratios of reducing species to Cr(VI) 1n the
barium chromate, and (3) Cr(YI) 1s completely reduced to Cr(III) during the
alkaline digestions of Insoluble barium chromate 1n the presence of sulflde or
ascorbic add at mass ratios 10-fold greater than Cr(YI) 1n the barium
chromate. Reduction of Cr(YI) occurred, even under alkaline conditions,
although the extent of such redox behavior may be more significant under
solution conditions of low pH.
The complete reduction of Cr(YI) 1n the presence of 10-fold ratios of
sulflde during alkaline digestions (pH approximately 12) conflicts with
previous data (TABLE 16) in which Cr(Y-I) 1n simulated solutions containing 100-
fold ratios of sulflde (pH 10.6) was not significantly reduced. The data
suggest that reduction of Cr(YI) 1n alkaline media may be promoted through
heating and mixing effects.
Nitric Acid Digestions of Barium Chromate
The solubilization of Cr(YI) from an Insoluble chromate and redox
behavior 1n add media were examined. Test portions (487 mg) of barium
chromate containing different additions-of sodium sulflde or ascorbic acid were
digested using a nitric acid digestion method. The test samples are the same
types as those studied for the preceding alkaline digestions. Serial dilutions
of 10-fold and 20-fold for a combined 200-fold dilution were performed on the
filtrate of each barium chromate test solution to provide a target
concentration of 0.5 mg/L Cr(YI) for spectrophptometrlc analyses. The nitric
add digestions and analyses for each of the test samples were" repeated on
separate days to confirm the experimental observations and data.
Observations and results of the analyses of barium chromate test
solutions for nitric add digestions are presented 1n TABLE 23. Whereas a fine
precipitate formed during the alkaline digestions of each of the barium
chromate test samples, no visible precipitates were observed for any of the
same test samples during the nitric add digestions. Of all the sample test
61
-------
TABLE 23. RESULTS OF ANALYSES OF BARIUM CHROMATE SOLUTIONS
FOLLOWING NITRIC ACID DIGESTIONS
Sample
Sample Digest Appearance
Precipitate Filtrate
Percent
Reduction
of Cr(VI)
487 mg BaCr04 (100 mg Cr) none
+ 0.243 g Na2S (0.1 g sulfide) none
+ 2.43 g Na2S (1 g sulfide) none
+ 0.1 g Ascorbic Acid none
+ 1 g Ascorbic Acid none
yellow 3 6
yellow-green 94 100
blue(fl) 100 100
blue-light red 100 100
blue-red 100 100
(a) Sample digest solution had cloudy appearance before filtering.
62
-------
solutions, only the barium chromate with high-level sulflde formed a cloudy
solution before filtering, presumably due to tiny particles of elemental
sulfur.
In the absence of reducing compounds, an average absorbance of 0.408
(approximately 96 percent of an aqueous 0.5 mg/L Cr(YI) calibration standard)
was measured for the test solutions representing full recovery of Cr(YI).
However, Cr(VI) was completely reduced 1n all the barium chromate test samples
containing reducing compounds. These experimental data confirm that Cr(VI) 1s
more easily reduced to Cr(III) under strongly addle conditions.
Alkaline Digestions of THvalent Chromium Nitrate
Test portions (0.77 g and 7.7 g) of solid trlvalent chromium nitrate
(equivalent to 0.1 g and 1 g of trlvalent chromium, respectively) were digested
according to the alkaline digestion procedure fc, ascertain the extent of any
oxidation of Cr(III) 1n alkaline media. Persulfate and manganese dioxide were
also added Individually in two different amounts equivalent to one- and 10-fold
ratios of the Cr(III) mass in the chromium nitrate. The alkaline digestions
and analyses for each of the test samples were repeated on separate days to
confirm the experimental observations and data.
Fluffy white precipitates were observed 1n the Initial alkaline
digestions of trlvalent chromium nitrate with and without potassium persulfate
additions. The relative amounts of the precipitates were inversely
proportional to the amounts of K2S208 added; the largest amount of precipitate
was observed with no addition of K2S208 and the smallest amount of precipitate
was observed with the largest K2S208 addition. Whereas Cr(III) Is sparingly
soluble in alkaline media, the presence of persulfate may oxidize Cr(III)
during the digestions to Cr(YI) which is soluble in alkaline media.
The relative extents of oxidation of Cr(III) to Cr(YI) during the
alkaline digestions were ascertained by analyzing appropriate dilutions of the
sample digests for Cr(YI) by DPC spectrophotometry. The results of the
analyses of trlvalent chromium nitrate solutions.for alkaline digestions are
presented 1n TABLE 24. Based on the DPC spectrophotometric measurements for
Cr(VI), less than 0.1 percent oxidation of Cr(III) was observed for both
concentrations of trivalent chromium nitrate In the absence of oxidizing
compounds. Approximately 17 percent and 80 percent oxidations of Cr(III) were
63
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TABLE 24. RESULTS OF ANALYSES OF TRIVALENT CHROMIUM NITRATE
SOLUTIONS FOLLOWING ALKALINE DIGESTIONS
Sample Dlqest Appearance .
Sample
0.77 g Cr(N03)3-9H20
(0.1 g Cr)
+ 0.141 g K2S208 (0.1 g
persul fate)
+ 1.41 g K2S2°8 (1 9
persul fate)
^ 0.1 g Mn02
+ 1 g Mn02
7.7 g Cr(N03)3-9"20
(1 g Cr)
+ 1.41 g K2S2Oa(l g
persul fate)
+ 14.1 g K2S208 (10 g
persul fate)
+ 1 g Mn02
+ 10 g Mn02
^^===^==
Precipitate
blue
• yellow-green
yellow-green
blue-green
blue-green
blue-green
yellow-green
yellow-green
blue-green
blue-green
Solution
colorless
faint yellow
yellow
colorless
faint yellow
colorless
faint yellow
yellow
faint yellow
yellow
Percent
Oxidation
of Cr(III)
tl
<0.l(«)
17(«)
7l(a)
0.6 '
3
<0.l(b)
17(b)
88(b)
0.1
0.7
<0.1
18
71
0.4
6
<0.1
16
86
0.2
1
(a) Sample digests filtered through 0.45-um filter by vacuum filtration
prior to dilution to one liter calibrated volume; all remaining
test solutions diluted to one liter calibrated volume wiJhout
vacuum filtration allowing fluffy precipitate to settle to bottom.
fb> Test samples analyzed for Cr(VI) by OPC colorimetric method after
(b' I s 2 samples diluted to one liter calibrated volume and stored
over the weekend; all remaining test solutions analyzed later in
same day of sample digestions.
-------
observed for the alkaline digestions of trlvalent chromium nltr*..- 1n the
presence of persulfate at 1- and 10-fold mass ratios of the Cr(III) mass 1n the
original test material, respectively. Approximately 0.3 percent and 3 percent
oxidations of Cr(III) were observed for the alkaline digestions of trlvalent
chromium nitrate 1n the presence of manganese dioxide at 1- and 10-fold mass
ratios of the Cr(III) mass 1n the original test material, respectively.
Based on the observed oxidations of Cr(III), although to different
extents, 1n a standard trlvalent chromium sample 1n the presence of two
different oxidizing compounds, the alkaline digestion method may be
unsatisfactory for the digestions of solid environmental samples of unknown
composition and redox properties 1n which Cr(YI) must be determined.
Significant oxidation of endogenous Cr(III) during the alkaline digestion of an
environmental sample would result In a positive bias 1n the Cr(YI) measurement.
Nitric Acid Digestions of Chromium Nitrate
Solid test portions of chromium nitrate were digested in 50 percent
nitric acid media to ascertain the extent of any oxidation of Cr(III) during
nitric acid digestions. Observations and results of these experiments,
performed using two different masses of chromium nitrate, are presented in
TABLE 25. The chromium nitrate test compounds were completely solublUzed In
the 50 percent nitric acid digestion medium. The blue color of the test sample
digest solutions provided a qualitative measure that Cr(III) was predominately
present in the solution and that little oxidation of Cr(III) to Cr(YI) had
occurred. The DPC spectrophotometric measurements for Cr(YI) in the test
solutions revealed that less than 0.1 percent oxidation of Cr(III) to Cr(IY)
had occurred during the 50 percent nitric add digestions.
Based on the analyses of standard test compounds, the data from
TABLES 23 and 25 revealed that Cr(VI) was not significantly reduced and that
Cr(III) not significantly oxidized in the 50 percent nitric acid digestion
medium. Therefore, the 50 percent nitric acid medium represents a digestion
method of potential feasibility for the analyses of environmental samples
containing an insoluble chromate and Cr(III).
65
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TABLE 25. RESULTS OF ANALYSES OF CHROMIUM (III) NITRATE
SOLUTIONS FOLLOWING NITRIC ACID DIGESTIONS
Percent
Oxidation
Sample Digest Appearance of Cr(III)
Sample Precipitate Filtrate #1 #2
0.77 g Cr(N03)3.9H20 none blue <0.1 <0.1
(0.1 g Cr)
7.7 g Cr(N03)3-9H20 none blue <0.1 <0.1
(1.0 g Cr)
66
-------
Nitric Acid/Persulfate Digestions of Barium Chromate
The nitric add/persulfate digestion procedure was evaluated to
ascertain the extent of any reduction of Cr(YI) during the digestions of solid
test portions of Insoluble hexavalent chromium 1n the form of barium chromate.
The experiments were predicated on the possibility of maintaining a highly
oxidizing medium with potassium persulfate 1n the digestion solution to keep
Cr(YI) in an oxidized state even under extremely addle conditions. Sodium
sulfide and ascorbic add also were Individually added at two different masses
to the barium chromate to form reducing environments of varying strengths. The
digestion medium consisted of a 50 (v/v) percent nitric acid mixture containing
5 percent (w/v) potassium persulfate.
Observations and results of the analyses of barium chromate test
samples for nitric add/persulfate digestions are presented 1n TABLE 26. The
digestion of each test sample resulted 1n the formation of a precipitate,
presumably due to barium sulfate. The colors of the solutions of most of the
test sample digests were blue, suggesting the presence of Cr(III) from the
reduction of Cr(VI). The OPC spectrophotometric measurements of the test
solutions did not reveal any detectable Cr(VI), thus Indicating complete
reduction of Cr(VI) to Cr(IIIJ. It was not understood why Cr(VI) 1n the barium
chromate test samples not containing reducing compounds was completely reduced
to Cr(III) even under highly acidic conditions in view of the apparent
oxidizing strength of potassium persulfate.
Nitric Acid/Persulfate Digestions of Chromium Nitrate
The nitric add/persulfate digestion procedure was evaluated to
ascertain the extent of any oxidation of Cr(III) during the digestions of solid
test portions of chromium nitrate In the absence and presence of an additional
strong oxidizing compound. The solid test samples consisted of two different
masses of chromium nitrate In the absence and presence of potassium
permanganate at 1- and 10-fold ratios to Cr(III).
Observations and results of the analyses of the chromium nitrate test
samples for nitric acid/persulfate digestions are presented 1n TABLE 27. The
digestion of each solid test sample resulted 1n the formation of a small amount
of precipitate which required filtering. The colors of the test sample digest
•67
-------
TABLE 26. RESULTS OF ANALYSES OF BARIUM CHROMATE SOLUTIONS
FOLLOWING NITRIC ACID/PERSULFATE DIGESTIONS
Percent
Reduction
Sample Digest Appearance of Cr(VI)
Sample PrecipitateSolution "71IT
487 mg BaCrfl4 (100 mg Cr) white
+ 0.243 g Na2S(0.1 g sulflde) blue-white
+ 2.43 g Na2S(l g sulfide) yellow-green
•• J,. g Ascorbic Acid yellow-blue
blue
blue
blue
blue
100
100
100
100
100
100
100
100
+ 1 g Ascorbic Acid yellow-blue grey-black 100 100
68
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TABLE 27. RESULTS OF ANALYSES OF CHROMIUM(III) NITRATE SOLUTIONS
FOLLOWING NITRIC ACIO/PERSULFATE DIGESTIONS
Sample
Sample Digest Appearance
Precipitate Solution
Percent
Oxidation
of Cr(III)
11 92
0.77 g Cr(N03)3«9H20 grey-white blue
(0.1 g Cr)
0.1 g KMnOfl. grey-white blue 0 0
t 1 g KMn04 dark-brown golden brown 90 92
7.7 g Cr(N03)3-9H20 yellow-green dark blue 2 1
(I 9 Cr)
+ 1 g KMn04 yellow-green yellow-brown 26 27
f 10 g KMn04 dark brown yellow-orange 84 84
69
-------
solutions before filtering ranged from blue to yellow; these colors provided
qualitative Indications of the extent of oxidation of Cr(III) to Cr(VI) during
the digestion procedures. The DPC spectrophotometric measurements for Cr(VI)
1n the test solutions revealed that less than 2 percent oxidation of Cr(III) to
Cr(VI) occurred for those test compounds not containing permanganate. However,
oxidation of Cr(III) occurred for those test samples containing potassium
permanganate; the extent of oxidation of Cr(III) was dependent on the
permanganate concentration.
Effects of Nitric Acid Concentrations
on Nitric Acid/Persulfate Digestions of
Mixed Valence Solutions of Chromium
The results of nitric add/persulfate digestions in TABLES 26 and 27
revealed complete reduction of Cr(YI) and no significant oxidation of Cr(IIl),
respectively In the absence of other competing redox components. Experiments
were performed to ascertain if a nitric acid concentration exists for the
nitric add/persulfate digestion medium such'that Cr(VI) 1s not-reduced or that
Cr(III) 1s not oxidized..
Absolute masses of chromium nitrate and potassium dichromate were
used to provide equivalent amounts of Cr(III) and Cr(VI) in the original solid
test sample. The mass of potassium persulfate and the volume of nitric add
added to the test chromium mixture were adjusted to simulate the digestion
conditions used 1n the previous experiments; 10-g portions of potassium
persulfate in 200 ml of diluted HN03 provided a 5 percent (w/v) <2^2Q8
concentration 1n the digestion medium. The nitric acid concentrations in the
digestion medium were varied between 50 percent and 0.5 percent.
Observations and results of the analyses of the Cr( III )-Cr(VI)
solutions for the nitric add/persulfate media are presented in TABLE 28. A
small amount of precipitate occurred only for the digestion medium containing
0.5 percent nitric add. The colors of the test sample digest solutions varied
from blue to yellow; the color of each solution provided a qualitative
Indication of the dominating redox reaction occurring under th>. given digestion
conditions. The DPC spectrophotometric measurements revealed that Cr(YI) was
completely reduced to Cr(III) 1n 50 percent nitric add. For the remaining
test sample solutions of decreasing nitric acid concentration, oxidation of
70
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TABLE 28. RESULTS OF ANALYSES OF CHROM.IUM(I 11) NITRATE/POTASSIUM DICHROMATE SOLUTIONS FOLLOWING
NITRIC ACID/PERSULFATE DIGESTIONS EMPLOYING VARIOUS NITRIC ACID CONCENTRATIONS
Sample Digest
Appearance
Sample Precipitate Solution
Percent Oxidation
of Cr(III)
fl 12
Percent Reduction
of Cr(VI)
11 12
0.77 g Cr(N03)3-9H20
+ 0.283 g K2Cr207
+ 10 g K2S20&
+ 50 percent HN03
0.77 g Cr(N03)3-9H20
+ 0.283 g K2Cr2fl7
+ 10 g K2S208
+ 20 percent HN03
0.77 g Cr(N03)3-9H20
+ 0.283 g K2Cr207
+ 10 g K2S208
+ 5 percent HN03
0.77 g Cr(N03)3.9H20
+ 0.283 g K2Cr207
+ 10 g K2$2®8
+ 2 percent HN03
0.77 g Cr(N03)v9H20
+ 0.283 g K2Cr207
i 10 g K2S208
+ 0.5 percent HN03'
none
none
none
none
brown
blue
golden brown
golden brown
yellow
yellow
60
78
80
81
81
83
100
100
-------
Cr(III) to Cr(YI) was predominate. The extent of Cr(III) oxidation Increased
with decreasing nitric add concentration.
The redox activity of the Cr(III)-Cr(VI) couple was extremely
sensitive to nitric acid concentrations for concentrations between 50 percent
and 5 percent. Below nitric add concentrations of 5 percent, the rate of
Increase of Cr(III) oxidation was less dramatic. An apparent plateau region of
approximately 80 percent oxidation was reached for nitric add concentrations
between 5 percent and 0.5 percent.
Between 50 percent and 20 percent nitric acid concentrations, the
reduction of Cr(YI) no longer occurs and oxidation of Cr(III) begins to
predominate. A nitric acid concentration between 20 percent and 50
percent may exist for the Cr(III)-Cr(YI) redox couple such that no oxidation of
Cr(III) or reduction of Cr(VI) by components of the digestion medium occurs.
Therefore, these data indicate that potassium persulfate has strong reducing
properties in 50 percent (v/v) nitric acid media which explains the anomalous
redox phenomena for Cr(YI) and Cr(III) In TABLES 26 and 27.
Nitric Acid/Persulfate Digestions of Potassium Dichromate
and Chromium Nitrate at Room Temperature
An experiment was also conducted to ascertain whether the observed
reduction of Cr(YI) or oxidation of Cr(IIl) during nitric acid/persulfate
digestions is a result of heating or whether the redox reactions will occur at
room temperature. Simulated nitric acid/persulfate digestions of Cr(VI), as
potassium dichromate, and Cr(III), as chromium nitrate, were prepared to
provide individual Cr(YI) and Cr(III) concentrations of 500 mg/L in 200 ml of
50 percent HN03/5 percent K2S208 digestion medium. The temperature varied over
a maximum 2-hour digestion period between 24°C and 310C; the small increase in
temperature over ambient temperature resulted from slight heating of the sample
digests due to mechanical stirring effects during the digestion periods. The
OPC spectrophotometric measurements were performed on test sample digests at
20-minute intervals for the 2-hour digestion period to provide insight into the
kinetic behavior of the Cr(III)-Cr(YI) redox couple at room temperature.
The results for Cr(YI) and Cr(III) are presented in TABLES 29 and 30,
respectively. .'he results of the first day's experiments for digestions of
potassium dichromate revealed that only slight reduction of Cr(YI) occurred at
room temperature; the extent of Cr(YI) reduction ranged between 2 and 6 percent
72
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TABLE 29. RESULTS OF ANALYSES OF ROOM-TEMPERATUREU) DIGESTIONS OF
POTASSIUM DICHROMATE IN NITRIC ACIO/PERSULFATE MEDIA(°)
Time from Start of
Digestion, min
20
40
60
80
100
120
#1
3
4
4
6
2
4
Percent Reduction of Cr(VI)
#2
27
38
49
57
67
75
#3
32
50
62
71
79
85
(a) For all sets of data, temperature varied between 24-31°C.
(•b) Test sample solution consists of 0.283 g KgC^Oy, 10 g ^Z^Q> and
200 mL 50 percent HN03.
-------
TABLE 30. RESULTS OF ANALYSES OF ROOM-TEMPERATURE^) DIGESTIONS OF
CHROMIUM (III) NITRATE IN NITRIC ACID/PERSULFATE
Time from Start of
Digestion, mfn
20
40
60
80
120
•
11
0
0
0
0
JJ.
*^^
0.6
Percent Oxidation of Cr(III)
tz
0
0
0
0
B ^
0
1*3
0
0
0
&
X
0
(a) For all sets of data, temperature varied between 24-31°C.
(b) Test sample solution consists of 0.77 g Cr(N03)3«9H20, 10 g
and 200 ml 50 percent HN03.
74
-------
over the 2-hour digestion period with no trend of time dependency. However,
the results of the experiments repeated on a second day differed significantly
from those of the previous day. For the second day's experiments, 27 percent
reduction of Cr(VI) occurred during the first 20 minutes of digestion. The
extent of Cr(YI) reduction continued to Increase with prolonged digestion time;
75 percent reduction of Cr(VI) had occurred at the end of the 2-hour digestion
period. Due to the discrepancy between the results of the same experiments
performed on separate days, the experiments were repeated for a third time.
The analytical data 1n TABLE 29 Indicate that the results of the
third day's experiments confirmed the results of the second day's experiments.
The combined results Indicate that approximately 30 percent reduction of Cr(VI)
occurred during the first 20 minutes of the digestion and continued to Increase
with digestion time; approximately 80 percent reduction of Cr(YI) had occurred
by the end of the 2-hour digestion period.
A summary of the results 1n TABLE 29 reveals that Cr(Y!) 1s reduced
at room temperature under these digestion conditions. Therefore, the digestion
medium consisting of 50 percent nitric add and 5 percent potassium persulfate
is not suitable for room-temperature digestions of solid materials.
Room-temperature digestions using the nitric acid/persulfate medium
were evaluated for chromium nitrate; the results are presented 1n TABLE 30.
The results of three sets of experiments, conducted on separate days, reveal
that Cr(III) oxidation does not occur over the entire 2-hour digestion period
at room temperature using this digestion medium.
Aqueous Potassium Persulfate Digestions
The feasibility of digestions of an Insoluble chromate test compound
(barium chromate) using a 5 percent (w/v) potassium persulfate solution 1n
delonized water without nitric add was Investigated. The evaluation of this
digestion medium was based on two criteria: (1) the extent to which barium
chromate test samples were solubHlzed. and (2) the extent to which Cr(III)
test samples were oxidized.
Duplicate 487-mg test portions of barium chromate, equivalent to 100
mg of Cr(VI), were weighed and transferred to Individual 500-mL beakers.
Duplicate 770-mg test portions of Cr(N03)3-9H20, equivalent to 100 mg of
Cr(III), were weighed and transferred Into Individual 500-mL beakers.
75
-------
Duplicate method blanks were also carried through the entire analytical
procedure. Each test sample was digested 1n 200-mL of persulfate medium (pH of
approximately 4.0) on a hot plate with mechanical stirring for two hours. The
sanple solutions were heated to a temperature of approximately 80°C. After
cooling In a water bath, the sample digest solutions were vacuum-filtered, with
delonlzed water rinsing, through a 47-mm filter (0.45-um pore size) of a glass
Mini pore filtering apparatus, and transferred Into 1-L volumetric flasks. The
sample filtrate solutions were then diluted to calibrated volume with delonlzed
water, providing target chromium concentrations equivalent to 100 mg/L 1n
solution.
Upon Initiation of the digestion procedure, the barium chromate test
solutions turned yellow 1n color with a cloudy appearance; the Cr(III) test
solutions turned from an Initial blue color to an Intermediate green color and
then a bright-yellow color during the first 30 minutes of the digestions. Upon
filtration of the barium chromate digest solutions, the filtrate solutions were
bright yellow-orange 1n color; solid material of light-yellow color was
retained on the filter membranes. The filtrate solutions of the Cr(III)
samples were bright yellow In color; no visible solid material was retained on
the filter membranes.
Each of the 1-L filtrate solutions was diluted 10-fold with delonlzed
water, providing target chromium concentrations equivalent to 10 mg/L in
solution. Five-mL aliquots of the 10-fold dilutions were transferred to 100-mL
volumetric flasks for the DPC spectrophotonetrlc analyses, providing target
chromium concentrations equivalent to'O.S mg/L In solution. After addition of
2 mL of the DPC reagent solution, the addition of 2 mL of 10 percent (v/v)
sulfuric acid was required to adjust the pH of the test sample solutions within
the specified pH range.
Total chromium concentrations were measured In 10-fold dilutions of
the original sample filtrates by ICP-OES. All calibration and control check
standards were prepared In an aqueous 0.1 percent (w/v) potassium persulfate
matrix solution to approximate the final matrix of the test sample solutions.
The results for the chromium analyses of the test samples using
aqueous persulfate digestions are summarized In TABLE 31. Approximately 73
percent of the endogenous Cr(VI) was recovered from the duplicate barium
chromate test samples digested In the parsulfatt medium as determined by OPC
spectrophotometry; approximately 78 percent of endogenous Cr(YI) was recovered
76
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TABLE 31. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF
HEXAYALENT AND TRIYALENT CHROMIUM COMPOUNDS
U5ING PERSULFATE DIGESTIONS
Test Experiment
Recovery of Cr(YI),
Percent^)
Oxidation of Cr(III),
Percent(b)
Recovery of Cr(III),
Percent^)
OPC ICP
(i) (2) (i) (2;
74 72 79 78
90 83
94 87
(a) Based on analyses of BaCr04 masses equivalent to 100 mg Cr(VI).
(b) Based on analyses of Cr(N03)3-9H20 masses equivalent to 100 mg Cr(III).
77
-------
by ICP-OES. Approximately 86 percent of the endogenous Cr(III) 1n the
duplicate chromium nitrate test samples was oxidized to Cr(VI) 1n the
persulfate medium as determined by DPC spectrophotometry; approximately 90
percent of the endogenous Cr(III) was recovered from the duplicate chromium
nitrate test samples 1n the persulfate medium as determined by ICP-OES.
Separate allquots of digest solutions from each of the barium
chormate and chromium nitrate test samples were spiked with 30 mg of aqueous
Cr(VI) as potassium dlchromate at the Instrument (post-digestion spikes) to
ascertain whether the observed redox phenomena resulted 1n the persulfate
digestions or from multiplicative Interferences 1n the DPC quantification
measurements. Hexavalent chromium spike recoveries of 84 percent and 92
percent were measured for the barium chromate digest solution and the chromium
nitrate digest solution, respectively. Although the 92 percent spike recovery
is satisfactory, the 84 percent recovery of Cr(VI) 1n the barium chromate
digest solution 1s relatively low. The low recovery may be due to the
formation of barium chromate resulting from a reaction between the Cr(YI) spike
with barium ions In solution although this was not experimentally verified.
Endogenous Cr(YI) 1n test portions of barium chromate, equivalent to
100 mg Cr(YI), was recovered to the extents of approximately 73 percent by DPC
spectrophotometry and approximately 78 percent by ICP-OES. The mechanism for
the solubillzation of the barium chromate test compounds in the persulfate
digestion medium is inconclusive. A possible mechanism Involves dissolution of
Cr(Yl) from barium chromate by preferential precipitation of barium sulfate
where sulfate is produced via redox by the reduction of persulfate. Due to the
similarities in solubilities of barium chromate (Ksp = 2.4 x lO'lO) and barium
sulfate (Ksp • 1.3 x 10-1°), 1*. Is Inconclusive whether or not the sulfate
concentration 1n solution would be high enough to preferentially precipitate
barium sulfate.
Oxidation of Cr(III) 1n the chromium nitrate test samples to the
extent of approximately 86 percent may possibly be explalned'by a redox
reaction between Cr(III) and persulfate based on the following half-reactions:
S208-2 + 2e- <-> 2S04-2 E° * * 2.01 V
Cr*3 + 4H20 <--> HCr04- + 7H+ + 3e~ E° - - 1.195 V
78
-------
Even though insoluble chromates, such as barium chromate, may be solublllzed to
a large extent, the persulfate medium 1s not recommended for digestions of
solid environmental samples because of the relative ease In oxidation by
persulfate of Cr(III) that may be endogenous 1n the original samples.
PHASE IV - ANALYSES OF ENVIRONMENTAL SAMPLES
The Cr(VI) spike experiments were not designed to Investigate the
solublllzatlon properties of the alkaline digestion medium; the Cr(VI) spikes
were added as soluble potassium dlcnromate solutions 1n order to test only the
redox properties of the alkaline digestion medium.
River Sediment - Alkaline Digestions
The texture of SRM 1645 made It difficult to wet completely upon
addition of the alkaline digestion medium; sample particles had a tendency to
creep up the beaker wall during the digestion. When the digests were vacuum
filtered, a brown precipitate from each SRM test portion was retained on the
filter medium. The colors of all digest solutions were yellow of varying
Intensities; the digest solutions from the SRM test portions spiked with Cr(VI)
exhibited the brightest yellow colors. The pH of each of the 1-L digest
solutions was aporoxlmately 11.
The results for the chromium analyses of test portions of NBS-SRM
1645 using alkaline digestions are summarized In TABLE 32. Approximately 0.7
mg/g of chromium was measured In the unspiked samples by both DPC
spectrophotometry and ICP-OES. The chromium concentrations measured by ICP-OES
represent approximately 3 percent of the certified chromium concentration. The
low recovery by ICP-OES Indicates that only 3 percent of the endogenous
chromium In the river sediment was solublllzed In the alkaline medium. The
same concentration of chromium measured In the digest solutions" by the two
techniques suggests that all of the solublllzed chromium was hexavalent
chromium. However, It 1s not known whether the chromium measured by DPC
spectrophotometry was due to endogenous Cr(VI) In the sample or that
part of the solublllzed chromium was Initially Cr(III) which was oxidized to
Cr(YI) during the digestion.
79
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TABLE 32. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF N8S-SRM 1645
(RIVER SEDIMENT) USING ALKALINE DIGESTIONS
DPC •
Test Experiment (1) (2)
Cr(VI) Measured, mg/g 0.74 0.74
Total Cr Measured, mg/g
Recovery of Certified
Cna), Percent
Recovery of Cr(YI)
Spike^, Percent 96 95
Oxidation of Cr(Itl)
Spike^), Percent 2 4
Recovery of Cr(III)
Spiked, Percent
ICP
(1) (2)
—
0.75 0.77
3 33
99 100
—
3 5
(a) Certified chromium concentration 1s 29.6 mg/g (uncertainty 1s 2.8 mg/g).
(b) One gram of SRM 1645 spiked with 30 rag Cr(VI).
(c) One gram of SRM 1645 spiked with 30 mg Cr(III).
80
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Recoveries of Cr(YI) spikes were 96 percent by DPC spectrophotometry
and 100 percent by ICP-OES. The complete recoveries Indicate that the Cr(VI)
spikes are not reduced 1n the river sediment 1n the alkaline digestion medium.
Analyses of Cr(III) spike solutions by DPC spectrophotometry revealed
that approximately 2-fold higher concentrations of Cr(YI) were measured
relative to the unsplked samples. The Increase 1n measured Cr(YI)
concentrations Indicated that the Cr(III) spikes were oxidized to some extent
to Cr(VI) 1n the alkaline digestion medium; 3 percent oxidation of Cr(III) was
observed. For the present test conditions with an approximate 40-fold ratio of
Cr(III) to Cr(YI), 3 percent oxidation of the Cr(III) spikes resulted 1n a 100
percent measurement error for Cr(YI). The ICP-OES analyses revealed
approximately a 4 percent recovery of the Cr(III) spikes. The low recoveries
of Cr(III) spikes by ICP-OES were due to precipitation of Cr(III) 1n the
alkaline digestion medium.
River Sediment - Nitric Acid Digestions
The SRM samples wetted more effectively 1n the 50 percent nitric add
digestion medium than in the alkaline digestion medium. When the digest
solutions were filtered, a brown precipitate from each SRM test sample was
retained on the filter medium. The colors of the sample filtrates were varying
shades of green.
The ICP-OES analyses for total chromium measurements were performed
on the 1-L digest solutions. The DPC spectrophotometrlc measurements were
performed on 10-fold dilutions of the I-L filtrate solutions for each of the
six SRM samples. The pH of each of the 10-fold dilutions was approximately 1.5
from the residual nitric add present.
The diluted samples were colorless and clear In appearance before
addition of DPC. However, upon addition of the DPC reagent to each of the SRM
test samples, the sample solutions turned a golden or yellow-brown color
Instead of the red-violet color expected. Since the yellow color changes were
not observed when DPC was added to the reagent blank and chromium calibration
standards, the yellow-brown color resulted from an Interaction between the DPC
and an unidentified sample matrix component. If an additive background
interference Is present, there is no easy method to compensate for it. A
separate aliquot of the unsplked SRM solution, without the addition of DPC, was
81
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used as the reference solution for the DPC colorlmetHc analyses; the
absorbance produced by this solution was approximately the same absorbances
produced by the reagent blanks.
The results for the chromium analyses of test portions of NBS-SRM
1645 using nitric add digestions are summarized In TABLE 33. Approximately
0.1 mg/g of chromium was measured In the unsplked samples by DPC
spectrophotometry. Aoproxlmately 27 mg/g of total chromium was measured by
ICP-OES; the experimentally measured chromium concentration represents 93
percent of the certified chromium concentration. This compares favorably with
the total chromium determinations oy ICP-OES of 2.7 percent (27 mg/g) using the
independent nitric acid-perchloric add digestion method. The measured
concentrations of total chromium by ICP-OES using both add digestion methods
are within the uncertainty limits of the certified chromium concentration, 2.96
percent, In NBS-SRM 1645 River Sediment. The complete recovery of chromium,
within the uncertainty limits of the certified chro^um concentration, by ICP-
OES indicates that the entire amount of endogenous chromium In the SRM was
solubilized in the nitric add digestion medium. However, based only on the
results from analyses of the unsplked SRM samples, It 1s not known whether the
solubillzed chromium was due completely to endogenous Cr(III) or whether
Cr(YI), originally present In the sample, was later reduced to Cr(III) 1n the
nitric add digestion medium.
Recoveries of Cr(VI) spikes by ICP-OES were approximately 106
percent; no amount of the Cr(VI) spikes was recovered by DPC spectrophotometry.
The results Indicate that the soluble Cr(YI) spikes in the SRM Hver sediment
were completely reduced to Cr(III) In the nitric add digestion medium.
Analyses of the SRM samples spiked with Cr(III) by DPC
spectrophotometry revealed no Increase In measured chromium concentrations
relative to the unsplked samples. The DPC spectrophotowtlc results Indicate
that the Cr(III) spikes In the SRM Hver sediment were not oxidized to Cr(VI)
in the nitric acid digestion medium. Approximately 104 percent recovery of the
Cr(III) spikes by ICP-OES was determined Indicating that Cr(III) was soluble In
the nitric add digestion medium.
The potential existence of a positive additive Interference 1n the
DPC spectrophotonetMc measurements was not rigorously examined. Approximately
0 1 mg/g of chromium was measured for each of the spiked and unsplked SRM
samples. Since 1t was determined that the SRM river sediment digested In a
82
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TABLE 33. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF NBS-SRM 1645
(RIVER SEDIMENT) USING ACID DIGESTIONS
Test Experiment
Cr(YI) Measured,
Total Cr Measured
DPC
ID (*)
mg/g 0.1 0.1
, mg/g
ICP
(1) (Z)
—
27.4 27.5
Recovery of Certified g.
Cr(a), Percent
Recovery of Cr(YI) .
, Percent 0 0 107 106
Oxidation of Cr(YI)
Spike^J, Percent 00 ~
Recovery of Cr(III) 10fl
l, Percent - ~ 1Q1 108
(a) Certified chromium concentration 1s 29.6 mg/g (uncertainty is 2.8 mg/g).
(b) One gram of SRM 1645 spiked with 30 mg Cr(YI).
(c) One gram of SRM 1645 spiked with 30 mg Cr(III).
83
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nitric add medium Is a reducing matrix, It 1s probable that the small measured
absorbances were due to the constant background Interference produced by the
yellow-brown color rather than due to Cr(YI) present In the sample solutions.
However, even with the presence of this type of additive, background
interference In the present OPC spectrophotomctrlc analyses, the conclusions
regarding the solubility and redox behavior of chromium species In the nitric
add digestion medium described above remain valid.
Municipal Digested Sludge - Alkaline Digestions
The sample filtrate solutions were yellow-brown In color and clear In
appearance. The pH of each filtrate solution was approximately 12.
Concentrated nitric add was added to adjust the pH of the filtrate solutions
to slightly less than 7 prior to OPC spectrophotometrlc analyses. After
addition of the DPC reagent to the sample solutions, the pH of the filtrate
solutions was adjusted to 2 with the addition of concentrated sulfuHc acid.
Upon adjustment of the ph of the sample solutions to approximately 2.with
sulfurtc acid, the solutions turned cloudy; this appearance-was observed even
if OPC was not present in the sample solution. These sample solutions were
again vacuum-filtered in an attempt to remove the newly formed partlculate
matter.
The precipitates that were retained on the filter membranes were
brown; the filtrate solutions retained only a light-yellow tint. These
filtrate solutions were analyzed for Cr(VI) within 10 to 15 minutes after DPC
addition, acidification with sulfurlc add and secondary filtration. Test
sample filtrates, taken through the same pH-adjustment and secondary filtration
steps but without the addition of DPC, were used as reference solutions In the
OPC spectrophotometrlc analyses.
The results for the chromium analyses of test portions of Municipal
Digested Sludge (HDS) using alkaline digestions are summarized in TABLE 3*. No
detectable chromium was measured In the unspUed samples by either DPC
spectrophotometry (less than 0.01 mg/g of chromium) or ICP-OES (less than 0.05
mg/g of chromium). The ICP-OES data Indicate that, based on a detection 11.11
of approximately 0.05 mg/g, less than 25 percent of the endogenous chromium in
the MDS sample was solubilized In the alkaline digestion medium. The
additional dilution required because of the filtration difficulties decreased
84
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TABLE 34. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF
MUNICIPAL DIGESTED SLUDGE USING ALKALINE DIGESTIONS
DPC
rest Experiment (l) (2)
Cr(VI) Measured, mg/g <0.01 <0.01
Total Cr Measured, mg/g
Recovery of Reference
CrU), Percent
Recovery of Cr(VI)
Spiked, Percent <5 <5
Oxidation of Cr(III)
Spiked), Percent <5
-------
the chromium concentration 1n solution so that 1t could not be reliably
measured by ICP-OES. The DPC spectrophotometrlc data Indicate that, based on a
detection limit of approximately 0.01 mg/g, less than 5 percent of the
endogenous chromium was hexavalent chromium.
No detectable chromium was measured 1n the MDS samples spiked with
Cr(VI) by either DPC spectrophotometry or ICP-OES. Based on the same
respective detection limits for the two measurement techniques, less than 5
percent of the Cr(YI) spike was recovered by DPC spectrophotome'.ry and less
than 25 percent of the Cr(YI) spike was recovered by ICP-OES. These data
indicate that the Cr(YI) spikes are reduced 1n Municipal Digest Sludge even In
the alkaline digestion medium and the Cr(III) 1s then precipitated under such
highly alkaline conditions.
Chromium was also not detected 1n the MDS samples spiked with Cr(III)'
by either DPC spectrophotometry or ICP-OES. Based on the same respective
detection limits for the two measurement techniques, less than 5 percent
oxidation of the Cr(III) spikes was determined by DPC spectrophotometry; less
than 25 percent recovery of the Cr(III) spikes was determined by ICP-OES.
These data indicate that the Cr(YI) spikes are only slightly oxidized, 1f at
all, in Municipal Digest Sludge 1n the alkaline digestion medium. The low
recoveries of the Cr(III) spikes by ICP-OES confirm that Cr(III) 1s
precipitated in the alkaline digestion medium.
Municipal Digested Sludge - Nitric Acid Digestions
The gelatinous precipitates that formed with the MDS samples in the
alkaline digestion medium were not present 1n the nitric add digestion medium.
The solid material remaining in the sample digests was not gelatinous but
rather exhibited a silt- or sediment-like appearance. Filtration of the acid
digests proceeded rapidly. The filtrates were clear in appearance and
exhibited bright yellow-orange colors; the precipitates retained orf the filter
media appeared to have the texture of sediment and were grey in color. The PH
of each of the 100-mL filtrates was less than 1.
An unspiked MDS sample filtrate, without the addition of DPC, was
used as the reference solution In f • -VC spectrophotometrlc analyses. Upon
addition of the DPC reagent to eac,, .,,' the MDS test samples, the sample
solutions turned a darker shade of yellow-brown color. Similar to the
86
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observations of nitric add digestions of NBS-SRM 1645 River Sediment, the
yellow-brown color resulted from Interaction between the DPC and an
unidentified sample matrix component.
The results for the chromium analyses of test portions of Municipal
Digested Sludge using nitric add digestions are summarized In TABLE 35.
Approximately 0.03 mg/g of hexavalent chromium was measured 1n the unsplked
samples by OPC spectrophotometry. Approximately 0.18 mg/g of total chromium
was measured In the unsplked MOS samples by ICP-OES; the chromium
concentrations measured by ICP-OES represent approximately 91 percent of the
reference chromium concentration. This concentration value compares favorably
with the total chromium concentration of 194 ug/g measured by ICP-OES using an
Independent nitric acid-perchloric add digestion. Both total chromium
concentration values are within the 95 percent confidence limits of the 204
ug/g reference concentration value as determined from analyses by EPA reference
laboratories.
The complete recovery of total endogenous chromium, within the
concentration uncertainty limits, as determined by ICP-OES Indicates that the
total amount of endogenous chromium In the MDS matrix was solublllzed 1n the
nitric add digestion medium and that Cr(III) does not precipitate under such
addle conditions. The much lower chromium concentrations measured by DPC
spectrophotometry Indicate that the endogenous chromium in the MDS samples was
either Cr(III) or that any endogenous Cr(VI) was reduced to Cr(III) in the
nitric acid digestion medium.
Recoveries of Cr(VI) spikes were approximately 2 percent by DPC
spectrophotometry and 95 percent by ICP-OES. The low recoveries by DPC
spectrophotometry Indicate that Cr(VI) spikes In the MDS samples are reduced In
the nitric add digestion medium. The complete recoveries of Cr(VI) spikes by
ICP-OES indicate that, although reduction apparently occurs, the Cr(III)
remains soluble in the nitric add digestion medium.
Analyses of Cr(III) spike solutions by DPC spectrophotometry revealed
that no significant oxidation occurred. The 95 percent recoveries of Cr(III)
spikes to the MDS samples by ICP-OES confirm that Cr(III) is highly soluble In
the nitric add digestion medium.
The potential existence of the positive additive interference in the
OPC spectrophotometric measurements was not rigorously examined.' Approximately
0 03 to 0 04 mg/g of chromium was measured by OPC spectrophotometry in all of
87
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TA8LE 35-
Test Experiment
Cr(VI) Measured, mg/g
Total Cr Measured, mg/g
Recovery of Reference
CrU), Percent
Recovery of Cr(VI)
Spike(b', Percent
Oxidation of Cr(III)
Spiked), Percent
0.034
0.032
3
0
2
1
0.189 0.182
93 89
96 94-
(b) One gram of Municipal Digested Sludge spiked with 0.2 mg Cr(VI).
(c) One gram of Municipal Digested Sludge spiked with 0.2 mg Cr(III).
88
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the unsplked and spiked MDS digest solutions. This suggests that the measured
abosrbances may not be due to Cr(YI) but rather due to a constant additive
background Interference from the yellow-brown color formed when DPC was added
to the MDS digest solutions. However, even 1f such an additive background
Interference 1s present, the general conclusions described above regarding the
rcdox and solubHlzatlon behavior of chromium species 1n the nitric acid
digestion medium remain valid.
Contaminated Soil "A" - Alkaline Digestions
When the digest samples were vacuum-filtered, a grey-brown
precipitate was retained on the filter membrane for each of the soil samples.
The colors of the filtrate solutions were varying shades of brown. Upon
addition of DPC and acififlcatlon to .pH 2 with sulfurlc acid, the sample
solutions turned various shades of red-violet Indicating proper color
development for Cr(VI) when an obvious color Interference 1s not present.
The results for the chromium analyses of test portions of
Contaminated Soil "A" using alkaline digestions are summarized 1n TABLE 36.
Approximately 0.1 mg/g of chromium was measured in the unsplked samples by both
DPC spectrophotometry and ICP-OES. The chromium concentrations measured by
ICP-OES represent approximately 11 percent of th pre-analyzed chromium
concentration. The low recoveries by ICP-OES indicate that approximately 11
percent of the endogenous chromium 1n Soil "A" was solubilized in the alkaline
digestion medium. The same concentrations of chromium measured in the digest
solutions by both techniques suggest that all of the solubilized chromium was
hexavalent chromium. However, It 1s not conclusively known whether the
chromium measured by DPC spectrophotometry was due to endogenous Cr(YI) 1n the
sample or whether part of the solubllized chromium was Initially Cr(III) which
was oxidized to Cr(VI) during the alkaline digestion.
Recoveries of Cr(VI) spikes were 92 percent by DPC spectrophotometry
and 96 percent by ICP-OES. The complete recoveries by both techniques indicate
that the Cr(VI) spikes were not reduced in the soil sample 1n the alkaline
digestion medium.
Analyses of Cr(III) spike solutions by DPC spectrophotometry revealed
that approximately 2-fold higher concentrations of Cr(YI) were measured
relative to the unsplked samples. The Increase in measured Cr(YI)
89
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TABLE 36 SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF CONTAMINATED
TABLE 36. SUWRTMO OIGESTIONS
Test Experiment
-^——^^-~
Cr(VI) Measured, rcg/g
Total Cr Measured, mg/g
Recovery of Pre-Analzyed
CrU), Percent
Recovery of Cr(VI)
""', Percent
Oxidation of Cr(III)
Spike^), Percent
Recovery of Cr(III)
Spiked, Percent
DPC
(1)
0.10
94
10
0.12
91
13
ICP
UT
10
(a) Pre-analyzed chromium concentration Is approximately 1 mg/g.
(b) One gram of Soil "A" spiked with 1 mg Cr(VI).
(c) One gram of Soil "A" spiked with 1 mg Cr(III).
12)
0.105 0.125
10 12
96 95
15
yo
-------
concentrations indicated that the Cr(III) spikes were oxidized to some extent
In the alkaline digestion medium; approximately 12 percent oxidation was
observed. For the present test conditions with an approximate 10-fold ratio of
Cr(IH) to Cr(VI), the 12 percent oxidation of the Cr(III) spikes resulted 1n a
100 percent measurement error for Cr(YI). The ICP-CES analyses revealed
approximately a 12 percent recovery of the Cr(III) spikes. The low recoveries
of Cr(III) spikes by ICP-OES were due to precipitation of Cr(III) 1n the
alkaline digestion medium.
Contaminated Soil "A" - Nitric Add Digestions
A precipitate was retained on the filter medium; the filtrate-was
yellow-brown in color. Upon addition of DPC to the diluted filtrates, the
clear, colorless solutions turned yellow-brown, not the red-violet color
expected for Cr(YI). This color formation may represent a similar additive
background interference observed 1n the previous add digestion experiments.
The results for the chromium analyses of test portions of
Contaminated Soil "A" using nitric add digestions are summarized in TABLE 37.
No measurable hexavalent chromium was detected by DPC spectrophotometry; this
corresponds to less than 0.05 mg/g of Cr(YI) in the soil sample. An average
chromium concentration of 1.04 mg/g was measured by ICP-OES in the unspiked
soil samples, representing 104 percent of the pre-analyzed chromium
concentration. This concentration value compares favorably with 0.10 mg/g of
total chromium measured by ICP-OES following the independent nitric acid-
perchloric acid digestion method. Both concentration values for total chromium
are also in good agreement with the pre-analyzed concentration value of
approximately 0.1 mg/g as determined by independent analysts.
The results of the ICP-OES analyses Indicate that all of the
endogenous chromium In Contaminated Soil "A" was solubllized In the add
medium. However, It is 'not known from these data whether the' endogenous
chromium was Cr(III) or whether the endogenous chromium was Cr(VI) which was
reduced to Cr(III) in the nitric add digestion.
No detectable hexavalent chromium was measured In Contaminated Soil
'•Au spiked with Cr(YI) by uPC spectrophotometry; this corresponds to less than
5 percent recovery of the Cr(YI) spikes based on our detection limit for DPC
spectrophotometry. The average recovery of the Cr(VI) spikes by ICP-OES was
91
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TABLE 37. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF
CONTAMINATED SOIL "A" USING ACID DIGESTIONS
DPC '
Test Experiment (i) (2)
Cr(VI) Measured, mg/g <0.05 <0.05
Total Cr Measured, mg/g
Recovery of Pre-Analyzed
Cr(a). Percent
Recovery of Cr(VI)
Spike(b>, Percent <5 <5
Oxidation of Cr(III)
Spike(c), Percent <5 <5
Recovery of Cr(III)
Spiked"), Percent
ICP
(1) (2)
—
1.03 1.06
103 106
99 97
__•
93 96
(a) Pre-analyzed chromium concentration is approximately 1 mg/g.
(b) One gram of Soil "A" spiked with 1 mg Cr(VI).
(c) One gram of Soil "A" spiked with 1 mg Cr(III).
92
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approxir :ely 98 percent. These spike recovery data Indicate that the Cr(VI)
spikes are reduced to Cr(III) 1n the nitric acid digestion medium and that
Cr(III) 1s soluble under such highly acidic conditions.
Recoveries of post-digestion Cr(VI) spikes were approximately 94
percent. These data Indicate that reduction of the Cr(YI) spikes 1n
Contaminated Soil "A" occur 1n the nitric acid digestion and not 1n the DPC
color1metr1c measurement.
No measurable hexavalent chromium was detected 1n Contaminate Soil
"A" spiked with Cr(III) by DPC spectrophotometry; this corresponds to less than
5 percent oxidation of the Cr(III) spikes based on our detection limit. The
average recovery of the Cr(IlI) spikes by ICP-OES was approximately 94 percent.
These spike recovery data Indicate that the Cr(III) spikes are not
significantly oxidized to Cr(VI) In the nitric acid digestion medium and that
the Cr(III) spikes do not precipitate under these acidic conditions.
Contaminated Soil "B" - Alkaline Digestions
The results for the chromium analyses of test portions of
Contaminated Soil "B" following alkaline digestions are presented in TABLE 38.
Approximately 0.2 mg/g of hexavalent chromium was measured 1n the unsplked
sample solutions by both DPC spectrophotometry and ICP-OES. The chromium
concentrations measured In the alkaline digest solutions by ICP-OES represent
approximately 2 percent of the pre-analyzed chromium concentration (9.3 mg/g)
following the nitric acid-perchloric acid digestion. The low recoveries of
endogenous chromium by ICP-OES indicate that approximately 2 percent of the
endogenous chromium 1n this contaminated soil sample was solubllized in the
alkaline digestion medium. It is not known conclusively whether the chromium
measured by DPC spectrophotometry was due to endogenous hexavalent chromium in
the soil sample or whether part of the solubflzed chromium was Initially
Cr(III) which was oxidized to Cr(VI) during the alkaline digestion.'
Recoveries of Cr(YI) spikes were approximately 95 percent by DPC
spectrophotometry and approximately 94 percent by ICP-OES. The complete
recoveries Indicate that the Cr(VI) spikes were not reduced in the contaminated
soil 1n the alkaline digestion medium.
Post-digestion spike recoveries of 101 percent and 100 percent were
obtained by DPC spectrophotomet.-y Indicating the absence of a multiplicative
93
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TABLE 38. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF CONTAMINATED
SOIL "8" USING ALKALINE DIGESTIONS
DPC
Test Experiment HH
Cr(YI) measured, mg/g 0.18 0.21
Total Cr measured^), mg/g -- ~ °-24 °-22
Recovery of Pre-iiulyzed
CrU), Percent
3 Z
Recovery of Cr(Vl)
SplkeW, Percent 93 97 93 94
Oxidap'fn of Cr(Ul)
, Percent 23 —
(a) Pre-analyzed chromium concentration is 9.3 mg/g (wet weight).
(b) One gram of Soil "B" spiked with 8.5 mg Cr(VI).
(c) One gram of Soil "B" spiked with 8.5 mg Cr(III).
94
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Interference In the quantification step. A post-digestion spike recovery of
100 percent by ICP-OES was obtained Indicating the absence of a multiplicative
interference In the ICP-OES quantification step.
Analyses of Cr(III)-spiked solutions by DPC spectrophotometry
revealed that approximately 2-fold higher concentrations of Cr(VI) were
measured relative to the unsplked samples solutions. The Increase 1n measured
Cr(YI) concentrations Indicated that the Cr(III) spikes were oxidized to some
extent to Cr(YI) 1n the alkaline digestion medium; approximately 2 percent
oxidation of Cr(III) was observed. For the present test conditions with an
approximate 40-fold ratio of Cr(III) to Cr(YI), 2 percent oxidation of the
Cr(III) spikes resulted in 100 percent errors for Cr(YI) measurements. The
ICP-OES analyses revealed approximately 2 percent recovery of the Cr(III).
spikes. The low recoveries of Cr(III) spikes by ICP-CES were due to
precipitation of Cr(III) in the alkaline digestion medium.
Investigation of the Extents of Oxidation of Cr(III)
Spikes In Contaminated Soil "B" for Different Cr(III)
to Endogenous Cr(VI) Concentration Ratios
To determine whether larger spikes would result in a proportional
increase in Cr(III) oxidation, eight 1-g samples were used to form 4 different
sample sets: (1) duplicate unsplke samples, (2) duplicate samples spiked with
9.3 mg of Cr(III), (3) duplicate samples spiked with 18.6 mg Cr(III), and (4)
duplicate samples spiked with 46.5 -ing Cr(III). Spikes were added as
appropriate aliquots of a 9.3 mg/mL Cr(III) standard prepared in deionized
water.
Digestion, vacuum-filtration and neutralization of all samples
proceeded as described previously. Gelatinous precipitates formed upon
neutralization of the filtrate solutions. The precipitates were extremely fine
which severely clogged the filter membranes. After the solids had settled, one
ml was withdrawn from each filtrate solution and diluted to 100 ml for DPC
spectrophotometrlc analysis.
The results for this study are summarized in TABLE 39. Oxidation of
Cr(III) spikes remained at the 2 percent level for 9.3 mg/g and 18.6 mg/g
Cr(III) spikes. The percent oxidation decreased to 1 percent for 46.5 mg/g
Cr(II'.) spikes. Similar results were obtained by both OPC spectrophotometry
95
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TABLE 39. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF CONTAMINATED
SOIL "B" SPIKED AT DIFFERENT CONCENTRATIONS USING ALKALINE
DIGESTIONS
DPC ICP
Test Experiment [T] T?T (T) T2J
Cr(YI) measured, mg/g 0.20 0.19
Cr
Recovery of Pre-analyzed
CrU), Percent -- —
Oxidation of 9.3 mg Cr(III)
Spike, Percent 2.2 2.2
Recovery of 9.3 mg Cr(III)
Spike, Percent — — 3.0 3.0
Oxidation of 18.6 mg Cr(III)
Spike, Percent 1.7 1.6
Recovery of 18.6 mg Cr(III)
Spike, Percent — -- 1.4 1.7
Oxidation of 46.5 mg Cr(III)
Spike, Percent 0.8 0.7
Recovery of 46.5 mg Cr(III)
Spike, Percent — — 0.8 1.0
(a) Pre-analyzed chromium concentration is 9.3 mg/g (dry weight).
96
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and ICP-OES analyses. This contaminated soil sample apparently contains
sufficient ox1dants to oxidize at least three times the endogenous chromium
concentration. Such data Indicate that the probable origin of measured
hexavalent chromium 1s actually endogenous Cr(III) which Is oxidized during the
digestion.
Investigation of Heating Time on the Extent of Solublllzatlon
of Endogenous Cr(VI) from Contaminated Soil Sample "B" and on
the Extent of Oxidation of Cr(III) Spike?
Four Contaminated Soil "B" samples were divided Into two sample sets:
(1) duplicate unspiked samples and (2) duplicate samples spiked with 9.3 mg
Cr(III). Spikes were added as 1-mL aliquots of a 9.3 mg/mL Cr(III) standard
prepared In deionized water.
Alkaline digestions of these samples were carried out at 80°C +/-
10°C, with mechanical stirring for 3 hours Instead of the usual 45 minutes.
This provides for a 4-fold increase In digestion time. The DPC
spectrophotometric analyses of filtrates were performed on 100-fold dilutions
of the filtrate. The ICP-OES analyses were performed on 2.5-fold dilutions of
the filtrates.
The results from these analyses are summarized in TABLE 40.
Solubilization of chromium was Increased by extending the digestion time.
After a 3-hour digestion period, 70 percent more hexavalent chromium was
measured compared to the concentration obtained for the usual 45-minute
digestions. Analyses by ICP-OES revealed u 125 percent increase in total
chromium concentration for unspiked samples undergoing 3-hour digestions. For
unspiked samples, about 75 percent of the Increase 1n concentration occurs in
the form of Cr(VI) as determined by DPC spectrophotometry. By difference, 50
percent of the increase 1n concentration 1s attributed to Cr(III).
Oxidation of Cr(III) spikes is 4 percent and this represents a 100
percent increase In the hexavalent chromium measurement of the unspiked
samples. Recovery of Cr(III) spikes agrees well with the percent oxidation of
Cr(III) spikes as determined by ICP-OES. Since the percent of chromium (III)
97
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TABLE 40. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF CONTAMINATED
SOIL "B" USING ALKALINE DIGESTIONS FOR EXTENDED
DIGESTION PERIODS
DPC ICP
Test Experiment (1) (2) (I) U)
Cr(YI) Measured, mg/g 0.34 0.34
Total Cr Measured^), mg/g — — 0.43 0.42
Recovery of Pre-analyzed
CrU), Percent — 55.
Oxidation of Cr(III)
dation
ked.
Spiked, Percent
Recovery of Cr(III)
, Percent
(a) Pre-analyzed chromium concentration 1s 9.3 mg/g (dry weight).
(b) One gram of Sample "B" spiked with 9.3 mg Cr(III).
98
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oxidation had not Increased dramatically and that the spikes still produced a
100 percent Increase 1n hexavalent chromium measurement, solubH1zat1on, rather
than oxidation, 1s the reaction promoted by the prolonged digestion.
Electroplating Sludge - AUallne Digestions'
The sample filtrates were clear and golden color. Brown solid
material was retained on the filter membranes for all samples. The results for
the chromium analyses of test portions of Electroplating Sludge using alkaline
digestions are summarized 1n TABLE 41. Approximately 0.38 mg/g of hexavalent
chromium was measured 1n the unsplked samples by both DPC spectrophotometry and
ICP-OES. The total chromium concentrations measured by ICP-OES represent
approximately 5 percent of the pre-analyzed chromium concentration of 7.7 mg/g
as determined by ICP-OES following nltHc acid-perchloric add digestion. This
measured concentration value 1s within 10 percent of the reference
concentration value of 7 mg/g as determined by EPA laboratories.
The low recoveries of endogenous chromium by ICP-OES Indicate that
approximately 5 percent of the endogenous chromium 1n the electroplating sludge
sample was solubillzed 1n the alkaline digestion medium. The same
concentrations of chromium measured 1n the sample digest solutions by both
techniques suggest that all of the solubillzed chromium was hexavalent
chromium. Moreover, it 1s probable that the chromium measured by DPC
colorimetry was due to endogenous Cr(YI) 1n the electroplating sludge sample.
Recoveries of Cr(VI) spikes were 91 percent by DPC spectrophotometry
and 93 percent by ICP-OES. The complete recoveries by both techniques Indicate
that the Cr(YI) spikes were stable 1n -the electroplating sludge sample 1n the
alkaline digestion medium.
Post-digestion spike recoveries of 101 percent and 108 percent were
obtained by DPC spectrophotometry and ICP-OES, respectively. Complete
recoveries of Cr(YI) post-digestion splices verified the a6sence of a
multiplicative Interference during the quantification steps of both methods.
Analyses of Cr(III)-sp1ked sample solutions by DPC spectrophotometry
revealed no apparent Increase 1n measured Cr(YI) concentrations relative to the
unsplked electroplating sludge sample solutions. The DPC spectrophotometrlc
results indicate that the Cr(III) spikes 1n the electroplating sludge sample
were not oxidized to Cr(VI) In the alkaline digestion medium. The ICP-OES
99
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TABLE 41. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF ELECTROPLATING
SLUDGE USING ALKALINE DIGESTIONS
DPC • ICP
Test Experiment (l)T^T Ul
Cr(VI) measured, mg/g 0.41 0.32
Total Cr measured^), mg/g — — Q.43 0.35
Recovery of Pre-analyzed
CrU), Percent — 65
Recovery of Cr(VI)
, Percent 94 88 96 90
Oxidation of Cr(III)
Spike(c), Percent
Recovers of Cr(III)
Spike(O, Percent
(a) Pre-analyzed chromium concentration 1s 7.7 mg/g.
(b) One gram of Electroplating Sludge spiked with 8.5 mg Cr(VI).
(c) One gram of Electroplating Sludge spiked with 8.5 mg Cr(III).
100
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an?lyses revealed no measurable recovery of the Cr(III) spikes confirming the
precipitation of Cr(III) 1n the alkaline digestion medium and no oxidation of
Cr(III) to soluble Cr(VI).
Combined Precision of Methods 3060 and 7196
for the Determination of Endogenous Cr(VI)
In Electroplating Sludge
Nine 1-g unsplked portions of this sample were carried through the
alkaline digestion procedure and the hexavalent chromium concentration
determined by DPC spectrophotometry. After vacuum-filtration, a fine
gelatinous precipitate formed within 5 minutes after adjusting the solution pH
to 7 with nitric acid. Absorbance measurements were performed on filtered and
unfUtered solutions; no significant differences were observed.
The results of the 9 determinations are presented in TABLE 42. An
average Cr(YI) concentration value of approximately 0.33 mg/g was obtained with
a relative precision of 15 percent. If one estimates the 95 percent confidence
limit by using the probability parameter t, we obtain:
•
0 329 +/- 2'31 (°-°6) = 0.329 mg/g +/- 0.05 mg/g
This means that 5 out of 100 times, an experimental value can be expected to
deviate by V- 0.05 mg/g or more. Such relatively high Imprecision may be
related to the presence of sparingly soluble chromates and variable degrees of
solubilization in the alkaline digestion procedure.
Estuarine Sediment - Alkaline Digestions
Very dark brown solids resembling fine silt were retained on the
filter membranes. Filtrate solutions were golden 1n color. Since the
concentration of solubilized chromium was anticipated to be low In these sample
digest solutions, minimum dilutions were used to conduct DPC spectrophotometHc
and ICP-OES analysis. To compensate for the brown color of the digest
filtrates, a color blank for each sample was prepared using the same dilution
ratios. No DPC color reagent was added for these blanks. jne concentration of
101
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TABLE 42. PRECISION OF HEXAVALENT CHROMIUM CONCENTRATIONS DETERMINED IN
ELECTROPLATING SLUDGE USING ALKALINE DIGESTIONS AND DPC
SPECTROPHOTOMETRY
Trial 1 0.363 mg/g
2 0.352 mg/g
3 0.244 mg/g
4- 0.330 mg/g
5 0.385 mg/g
ii 0.318 mg/g
7 0.427 mg/g
8 0.255 mg/g
9 0.290 mq/g
Average » 0.329 mg/g
Standard deviation = 0.06 mg/g
Relative Standard Deviation - 15 percent
102
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hexavalent chromium could be calculated by the net absorbance obtained by the
absorbance differences between the samples and the color blanks.
The presence of multiplicative errors was checked by post-digestion
Cr(YI) spiking of duplicate Cr(III)-spiked filtrates. Only 36 percent and 62
percent recoveries of 0.76 uc/mL Cr(VI) spikes were found by DPC
spectrophotometry. An average recovery of 89 percent for the same spike
concentration was found by ICP-OES.
For both pre-dlgestlon and post-digestion spiked solutions, the color
developed was a different shade of pink rather than the the usual red-violet
color. This color may have been due to the golden brown background color of
the filtrates or It may have been due to formation of a DPC complex with
another metal 1n the sample.
The results for the chromium analyses of NBS-SRM 1646 Estuarlne
Sediment following alkaline digestions are presented 1n TABLE 43. Neither DPC
spectrophotometry nor ICP-OES provided reliable measurements of solubilized
chromium due to extremely low chromium concentrations. Chromium was not
detected in the alkaline digest solutions by ICP-OES. The total chromium
concentration measured by ICP-OES following the nitric acid-perchloric acid
digestion procedure was 58 ug/g. This measured concentration value is
approximately 76 percent of the certified value. The low concentration value
for chromium may have been due to loss of chromium as chromyl chloride during
the digestion or to suppression of the chromium emission signal from matrix
effects in the ICP-OES measurement.
The DPC spectrophotometrlc measurements were plagued by matrix
Interferences. No oxidation of Cr(III) spikes was found by DPC
spectrophotometry. Confirmation of this result by ICP analyses Indicated that
Cr(III) spikes precipitated during the alkaline digestions. Only 34 percent of
the Cr(VI) spikes were recovered Indicating that Estuarlne Sediment represents
a reducing matrix. This reducing matrix may be a result, of the high organic
content (about 50 percent) and a significant level of sulfur (1 percent).
Although no conclusive statement can be made about the chemical states of
endogenous chromium 1n Estuarlne Sediment, It 1s plausible that most of the
hexavalent chromium measured was the residual portion of partially soluble or
completely soluble chromates within the sample.
103
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TABLE 4J. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF
NBS-SRM 1646 (ESTUARINE SEDIMENT) USING
ALKALINE DIGESTIONS
DPC
Test hxperiment (1) (2)
Cr(VI) Measured, mg/g 0.0019 0.0013
Total Cr Measured^), mg/g
Recovery of Certified
CrU), Percent
Recovery of Cr(VI)
Spike(o), Percent 34 34
Oxidation of Cr(IIl)
Spike(c), Percent 0 0
Recovery of Cr(III)
Spike(<0, Percent
ICP
U) (2)
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Tannery Sludge "A" (Low-Su1f1de) - Alkaline Digestions
Upon addition of the alkaline digestion medium, the following
observations were noted for each of the three sample sets: (1) the duplicate
unspiked sample solutions remained clear and colorless, (2) the duplicate
sample solutions spiked with Cr(III) turned light-blue in color as expected,
and (3) the duplicate sample solutions spiked with Cr(YI) turned light-green In
color Instead of the characteristic yellow color of dlchromate solutions.
After the digestions had proceeded for 10 minutes, all six sample solutions had
turned a milky appearance; the duplicate unspiked sample solutions and one of
the Cr(VI)-spiked sample solutions had a visible orange tint and che other
three sample solutons had a visible grey-brown shade.
After the 45-m1nute alkaline digestions, the sample digest solutions
were allowed to cool and then vacuum-filtered. All sample filtrate solutions
were orange In color. Solid material remaining 1n the sample digest solutions
was retained on the filter membranes for all samples. The solid material
appeared to have the texture of very fine-grained silt.
The results for the chromium analyses of test portions of Tannery
Sludge "A" (low-sulfide) using alkaline digestions are summarized in TABLE 44i
Approximately 0.16 mg/g of hexavalent chromium was measured in the unspiked
sample solutions by both OPC spectrophotometry and ICP-OES. The total chromium
concentrations measured by ICP-OES following the alkaline digestions represent
approximately 3 percent of the pre-analyzed chromium concentrations
(approximately 5.8 mg/g) following nitric acid-perchloric acid digestions.
The concentrations of total chromium measured in triplicate sub-
samples by ICP-OES following nitric acid-perchloric acid digestions were 5.9
mg/g, 5.7 mg/g, and 5.8 mg/g on a wet-weight basis. The average measured
chromium concentration of 5.8 mg/g is low relative to the pre-analyzed chromium
concentration information (25 mg/g) submitted with this tannery sludge sample.
However, if the chromium Information value of 25 mg/g is based on a dry-weight,
the 30-40 percent solids content will account for much of this discrepancy.
The low recoveries of endogenous chromium by ICP-OES following
alkaline digestions indicate that approximately 3 percent of the endogenous
chromium In this tannery sludge sample was solublllzed in the alkaline
digestion medium. The same concentrations of chromium measu-ed in the digest
solutions by both techniques suggest that all of the solubHized chromium was
105
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TABLE 44.
Cr(VI) Measured, mg/g
Total Cr Measured(a), mg/g
Recovery of Pre-analyzed
Cria;, Percent
Recovery of Cr(VI)
Spike (10, Percent
Oxidation of Cr(III)
Spike(c), Percent
Recov
Spil
of Cr(III)
Percent
95
1
95
1
0.14 0.17
2 3
95 96
Pre-analyzed chromium concentration 1s 5.7 mg/g (wet weight).
---j» j % "^ w n\, i \
One gram of Tannery Sludge "A" spiked with 5.7 mg Cr(VI)!
One gram of Tannery Sludge "A" spiked with 5.7 mg Cr(IIl).
106
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hexavalent chromium. However, it Is not known conclusively whether the
chromium measured by DPC spectrophotometr was due to endogenous hexavalent
chromium In the tannery sludge sample or whether part of the solublUzed
chromium was Initially endogenous Cr(III) which was oxidized to Cr(YI) during
the alkaline digestion.
Recoveries of Cr(VI) spikes were approximately 95 percent by both DPC
spectrophotometry and ICP-OES. The complete recoveries by both techniques
Indicate that the Cr(YI) spikes were stable in this tannery sludge matrix in
the alkaline digestion medium.
Post-digestion Cr(YI) spikes were added to the two unspiked sample
digest solutions for each technique to check for a multiplicative Interference
during the quantification step. An average recovery of 102 percent was
measured for 0.5 mg/L Cr(YI) spikes for DPC spectrophotometry; an average
recovery of 102 percent was also measured for 1 mg/L Cr(YI) spikes for ICP-CES.
The complete recoveries of post-digestion Cr(YI) spikes for both techniques
confirmed the absence of a multiplicative interference 1n the quantification
steps of the methods.
Analyses of Cr(LII)-spiked solutions by DPC spectrophotometry
revealed that approximately 1.5-fold higher concentrations of Cr(VI) were
measured relative to the unspiked sample solutions. The increase in measured
Cr(VI) concentrations Indicated that the Cr(ITI) spikes were partially oxidized
in the alkaline digestion medium; approximately 1 percint oxidation was
observed. For the present test conditions with an approximate 35-fold ratio of
Cr(III) to Cr(YI), the 1 percent oxidation of Cr(III) spikes resulted in a 50
percent error for Cr(YI) measurements. However, implications of oxidation of
Cr(III) spikes in this tannery sludge sample should be viewed in light of the
relatively high Imprecision associated with the DPC spectrophotometric
measurements In the low absorbance region (approximately twice the Instrument
detection limit) where the analyses were performed.
The ICP-OES analyses revealed approximately a 2 percent" recovery of
the Cr(III) spikes. The low recoveries of Cr(III) spikes by ICP-CES were due
to precipitation of Cr(III) in the alkaline digestion medium.
107
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Tannery Sludge "8* - Alkaline Digestions
Upon addition of the alkaline digestion medium, the duplicate sample
solutions spiked with Cr(III) Initially formed light-blue precipitates which
redissolved as more alkaline digestion medium was added. This observation was
later determined 1n a separate experiment to be a reaction between Cr(III) and
the alkaline digestion medium. After digestion, all hairs in the samples had
disappeared, but the solutions contained gelatinous grey-brown solids. These
solutions were very slow to filter, and the resultant filtrates were golden
brown in color. When neutralized, the filtrates became cloudy and emitted a
strong odor resembling that of hydrogen sulflde
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TABLE 45. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF TANNERY SLUDGE
"8" (HIGH SULFIDE) USING ALKALINE DIGESTIONS
Test Experiment
^^^^^™^^^"^^^™^"^™™™^«™^«*™™
Cr(YI) Measured, mg/g
Total Cr Measured^), mg/g
Recovery of Pre-Analyzed
CrU), Percent
Recovery of Cr(VI)
Sp1ke(o), Percent
Oxidation of Cr(III)
Spiked), Percent
Recovery of Cr(III)
SpikelO, Percent
OPC
0.01
84
0
(2)
ICP
0.008
77
0
0.11 0.11
i i
93 82
(a)
(b)
(c)
====================3========E===ao
Pre-analyzed chromium concentration 1s 7.8 mg/g (wet weight),
One gram of Tannery Sludge "B" spiked with 7.8 mg Cr(YI).
One gram of Tannery Sludge "B" spiked with 7.8 mg Cr(III).
109
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resulted in poor precision. Moreover, It 1s not certain whether or not the
sample color blanks provided an accurate compensation for the light scattering
effect caused by the gelatinous particles.
Recoveries of Cr(YI) spikes averaged 81 percent by DPC
spectrophotometry and 88 percent by ICP-OES. Post-digestion Cr(VI) spiking
measured by DPC spectrophotometry and ICP-OES gave 101 percent and 104 percent
recoveries, respectively. Complete recovery of these post-digestion spikes
Indicated the absence of multiplicative Interferences 1n the quantification
step 1n either technique. A comparison of the percent Cr(VI) recoveries for
pre- and post-digestion spikes Indicates that reduction of Cr(YI) spikes was
probably due to the high-sulflde reducing environment.
Analysis of Cr(III)-sp1ked solutions by DPC spectrophotometry and
ICP-OES revealed that no oxidation of Cr(III) spikes occurred. Furthermore, It
can be Inferred 'rom JCP-OES results thjt Cr/JJJ.) .soifc** wrr* .P«HP»** ^.v
precipitation
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TABLE 46. SUMMARY OF RESULTS FOR CHROMIUM ANALYSES OF
RIVER WATER USING ALKALINE DIGESTIONS
OPC
Test Experiment (1) (2)
Cr(VI) Measured, yg/g 0.027 0.023
Total Cr Measured^), ug/g
Recovery of Pre-Analyzed
CrU), Percent
Recovery of Cr(VH
Spiked, Percent 99 100
Oxidation of Cr(III)
Spike(c), Percent 1 1
Recovery of Cr(IIl)
SpikelO, Percent
ICP
(1) (2)
—
<0.2 <0.2
-_ -_
104 103
.- _
<10 <10
(a) Pre-analyzed chromium concentration 1s <0.2 ug/g (wet weight).
(b) Thirty grams of River Water spiked with 0.05 mg Cr(VI).
(c) Thirty grams of River Water spiked with 0.05 mg Cr(III).
Ill
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<
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