Tennessee
Valley
Authority
Office of Natural
Resources
Chattanooga TN 37401
TVA/ONR79/09
United States
Environmental Protection
Agency
Office of Energy, Minerals, and
Industry
Washington DC 20460
EPA 600 7 79 086
March 1979
Research and Development
Critical Evaluation of
Differential Pulse
Polarography for
Determining
Chromium(lll) and
Chromium (VI) in
Water Samples
Interagency
Energy/Environment
R&D Program
Report
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
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essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
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mental issues.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/7-79-086
TVA/ONR-79/09
March 1979
CRITICAL EVALUATION OF DIFFERENTIAL PULSE POLAROGRAPHY
FOR DETERMINING CHROMIUM(III) AND CHROMIUM(VI) IN
WATER SAMPLES
by
Lyman H. Howe, Isaac E. Jones
and Norman K. Stanley
Office of Natural Resources
Tennessee Valley Authority
Chattanooga, Tennessee 37401
Interagency Agreement No. D5-E721
Project No. E-AP 80BDH
Program Element No. INE 625C
Project Officer
James Stemmle
Office of Energy, Minerals, and Industry
U.S. Environmental Protection Agency
Washington, D.C. 20460
Prepared for
OFFICE OF ENERGY, MINERALS, AND INDUSTRY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
-------
DISCLAIMER
This report was prepared by the Tennessee Valley Authority and
has been reviewed by the Office of Energy, Minerals, and Industry,
U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the
views and policies of the Tennessee Valley Authority or the U.S.
Environmental Protection Agency, nor does mention of trade names or
commerical products constitute endorsement or recommendation for use.
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ABSTRACT
The Tennessee Valley Authority critically evaluated differential pulse
polarography for determining chromiura(VI) and chromium(III) in water samples
from coal-fired steam-electric generating stations. After reagents are
added to overcome interference, the peak currents for chromium(III) and
chromium(VI) are measured separately in different electrolytes and quantified
by standard addition. Total chromium is the algebraic sum of chromium(VI)
and chromium(III). The effective range for quantification is 0.1 to at
least 10 mg/1 of chromium(VI) and 0.6 to at least 10 mg/1 of chromium(III).
Interferences by lead(II) and chromium(VI) on chromium(III) are discussed.
Neither lead(II) nor chromium(III) interferes with measurement of chromium(VI)
Copper(II), zinc(II), and iron(III) do not interfere with measurement of
either chromium(III) or chromium(VI).
This report was submitted by the Tennessee Valley Authority, Office of
Natural Resources, in partial fulfillment of Energy Accomplishment Plan
79BDH under terms of Interagency Energy Agreement D5-E721 with the
Environmental Protection Agency. Work was completed in October 1978.
111
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CONTENTS
Page
Abstract iii
Figures vi
Tables vii
Acknowledgments viii
Abbreviations and Symbols ix
1. Introduction 1
2. Conclusions 3
3. Recommendations 4
4. Experimental 5
Sample Preparation 5
Equipment 5
Preparation of Equipment and Solutions 6
Determination of Chromium(VI) by Differential
Pulse Polarography 7
Determination of Chromium(III) by Differential
Pulse Polarography 8
5. Results and Discussion 9
References 27
List of Presentations 30
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FIGURES
Number Page
1 Typical differential pulse polarograms for standard
chromium(VI) concentrations in 0.1 M sodium hydroxide
at pH 12.7 10
2 Calibration curve for 0 to 1.0 mg/1 of chromium(VI) in
0.1 M sodium hydroxide at pH 12.7 by differential pulse
polarography 12
3 Calibration curve for 0 to 10 mg/1 of chromium(VI) in
0.1 M sodium hydroxide at pH 12.7 by differential
pulse polarography 13
4 Peak shift caused by pH in differential pulse polarograms
for chromium(VI) in 0.1 M sodium hydroxide 14
5 Typical differential pulse polarograms for standard
chromium(III) concentrations in 0.2 M ammonium citrate
with 0.01 M EDTA at pH 5.1 16
6 Calibration curve for 0 to 1.0 mg/1 of chromium(III) in
0.2 M ammonium citrate with 0.01 M EDTA at pH 5.1 by
differential pulse polarography ... 18
7 Calibration curve for 0 to 10 mg/1 of chromium(III) in
0.2 M ammonium citrate with 0.01 M EDTA at pH 5.1
by differential pulse polarography 19
8 Interference of chromium(VI) in differential pulse
polarograms for chromium(III) in 0.2 M ammonium citrate
with 0.01 M EDTA at pH 5.1 21
9 Use of EDTA to eliminate interference of zinc(II) in
differential pulse polarograms for chromium(III) in
0.2 M ammonium citrate with 0.01 M EDTA at pH 5.0 23
10 Interference of lead in differential pulse polarograms
for chromium(III) in 0.2 M ammonium citrate with 0.01 M
EDTA at pH 5.0 24
VI
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TABLES
Number
Effect of chromium(VI) in analysis of chromium(III) by
differential pulse polarography in 0.2 M ammonium
citrate with 0.01 M EDTA at pH 5.0 22
Determinations of chromium(VI) and chromium(III) by
differential pulse polarography for triplicate solutions
of spiked surface water 26
VI1
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ACKNOWLEDGMENTS
This work was conducted as part of the Federal Interagency Energy/
Environment Research and Development Program with funds administered
through the Environmental Protection Agency (EPA Contract No.
EPA-IAG-D5-E721, TVA Contract No. TV-41967A).
The EPA Project Officer for this research is James Stemmle, Office of
Energy, Minerals, and Industry, U.S. Environmental Protection
Agency, Washington, D.C., and the TVA Project Director is C. Wayne
Holley, TVA Laboratory Branch, Chattanooga, Tennessee. Their contri-
butions to the direction of the research and constructive review of the
reported results are gratefully acknowledged.
viii
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
A — ampere .
c — centi-, xlO- (as a prefix, e.g., cm)
°C — degrees Celsius (centigrade)
DPP — differential pulse polarography
EDTA — ethylenediaminetetraacetic acid
EPA — U.S. Environmental Protection Agency
g — grams
hr — hour
in. — inch
1 --liter
m --meter _«
ffl- — milli-, xlO (as a prefix, e.g., mm)
min — minute
M — molar, mole per liter
NBS --National Bureau of Standards
psi — pound per square inch
PAR — Princeton Applied Research Corporation
s --second
t — student t statistic
TVA — Tennessee Valley Authority
V —volt
V vs. nee — volts vs. a calomel electrode filled with 1 M potassium
chloride
SYMBOLS
— micro-, xlO (as a prefix, e.g., |Jg)
— percent
IX
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SECTION 1
INTRODUCTION
Chromate, the highest valence state for chromium, has potential for
use as a corrosion inhibitor in process waters for heat exchangers and
in other service water systems at steam-electric generating stations.1
Chromate may also be deposited in aqueous wastes that are formed when
heat exchangers are cleaned with acid solvents or that are extracted
when coal ash is sluiced with water into settling ponds. Even though
hexavalent chromium, including the lower trivalent species, can be
removed by adsorption,2 3 precipitation,4 5 or recycling,6 some may
eventually reach surface or ground waters.
Tandon7 concluded recently that, although damage to kidneys in
rabbits by hexavalent chromium far exceeds that by trivalent chromium,
care must also be taken to avoid trivalent chromium compounds, particu-
larly those that are water-soluble. The purpose of this work, funded by
an interagency agreement by TVA under sponsorship of EPA, is to examine
polarography as a possible means for determining chromium(VI) and
chromium(III) concentrations individually.
As required by the Federal Water Pollution Control Act, test pro-
cedures have been published for determining concentrations of total and
hexavalent chromium by atomic absorption or colorimetry to demonstrate
that effluent discharges meet applicable pollutant discharge limita-
tions.8 Although not a compliance method, the determination of chromium
by differential pulse polarography is attractive because it offers a
potential means for quantifying chromium(VI) and chromium(III) indi-
vidually, with a sensitivity of about 1/20 of that for direct-current
polarography.9'12 Valence-specific methods for determining concentra-
tions of hexavalent and trivalent chromium, including polarography, and
the significance of these measurements in biological systems have been
reviewed by Purdy13 and Mertz.14
Sodium hydroxide has been used to determine chromium(VI) concentra-
tions by differential pulse polarography15'16 and by direct-current
polarography.17'18 The usefulness of this electrolyte in a concentration
of 1 M is limited by interference from lead(II) and an increase in peak
height thought to be caused by the peroxide formed when the dropping
mercury electrode is placed in solution during deaeration.15 Crosmun
and Mueller19 resolved these limitations by determining chromium(VI) in
a different electrolyte, 0.1 M acetate buffer with 5 x 103 M ethylene-
diamine at pH 7. For this report, sodium hydroxide in a concentration
of 0.1 M was selected instead of the 1 M concentration for evaluation as
an electrolyte for determining chromium(VI). It was hoped that the
0.1 M concentration might eliminate peroxide formation and interference
by lead without lowering the pH sufficiently to allow interference by
copper.15
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-2-
Several electrolytes have been recommended for determining
chromium(III) concentrations by polarography.20"24 Information for
interferences in the determination of chromium(III) is available only
for two electrolytes: (1) 0.2 M acetic acid with 0.2 M sodium
thiocyanate at pH 3.2;21'22 and (2) 0.1 M potassium chloride with 0.01 M
ethylenediaminetetraacetic acid (EDTA) at pH 6.24 Although not specifi-
cally mentioned in the literature,21'22*24 thiocyanate21'22 or EDTA24 is
probably required to maintain chromium(III) in solution by complexing.
Both zinc(II) and chromium(VI) cause some indirect interference by
shifting the baseline when chromium(III) concentrations are determined
in 0.2 M acetic acid with 0.2 M sodium thiocyanate at pH 3.2.21'22
Preliminary data in the TVA laboratory showed that copper(II) also
produces similar results. Because further preliminary results in the
TVA laboratory indicated that the baseline was not shifted by copper(II)
and zinc(II) when chromium(III) concentrations were determined in 0.2 M
ammonium citrate with 0.01 M EDTA at pH 5, this electrolyte was chosen
for detailed evaluations for quantifying chromium(III). Some inter-
ference by chromium(VI) was observed, but it was less than that for
0.1 M potassium chloride with 0.01 M EDTA at pH 6.0, because in this
electrolyte, the peaks for chromium(III) and chromium(VI) coincide.24
This report evalutes the effect of
1. pH, zinc(II), iron(III), copper(II), lead(II), and chromium(III) in
determining concentrations of chromium(VI) in 0.1 M sodium hydroxide
by differential pulse polarography; and
2. pH, zinc(II), iron(III), copper(II), lead(II), and chromium(VI) in
determining concentrations of chromium(III) in 0.2 M ammonium citrate
with 0.01 M EDTA.
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-3-
SECTION 2
CONCLUSIONS
A differential pulse polarographic method has been critically
evaluated for determining chromium(VI) and chromium(III) concentrations
in natural water samples. Differential pulse polarography is effective
for determining 0.1 to at least 10 mg/1 of chromium(VI) and 0.6 to at
least 10 mg/1 of chromium(III).
Chromium(VI) is measured at about -1.14 V vs. nee at pH 12.7 in 0.1
M sodium hydroxide. The height and potential of the chromium(VI) reduction
peak are nearly the same at pH 11.7 and 12.7. At pH 13.2, the peak
height is the same as at pH 11.7 and 12.7, but the potential is -0.95 V
vs. nee rather than -1.15 V. Chromium(III) is measured at about -1.26 V
vs. nee at pH 5.0 in 0.2 M ammonium citrate buffer solution with 0.01 M
EDTA. The potential of the chromium(III) peak is the same at pH values
between 4.7 and 5.2. The height of the peak is the same at pH 4.7 and
5.0, but at pH 5.2 it is 43 percent of the value at pH 5.0.
A concentration of 5.0 mg/1 of copper(II), zinc(II), and iron(III)
does not interfere with measurement of either chromium(III) or chromium(VI),
and a concentration of 5.0 mg/1 of lead(II) and chromium(III) does not
interfere with measurement of chromium(VI). However, a concentration of
5 mg/1 of lead(II) and chromium(VI) does interfere with measurement of
chromium(III).
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-4-
SECTION 3
RECOMMENDATIONS
The differential pulse polarographic method described in this
report lacks the sensitivity for directly determining the low concen-
trations of total chromium, chromium(VI), and chromium(III) that are
normally expected in ash ponds that receive effluents from steam-electric
generating plants.25 This method may be applicable to the determination
of chromium(VI) and chromium(III) in industrial effluents or process
waters containing higher concentrations of chromium. This, of course,
remains to be proven because of the possible presence of various
interferences in such samples.
Further studies should be conducted to determine the applicability
of the differential pulse polarographic method described in this report
for low concentrations of chromium(VI) and chromium(III) after precon-
centration of chromium(VI) by ion exchange26 and of chromium(III) by
coprecipitation with iron(III) hydroxide.27
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-5-
SECTION 4
EXPERIMENTAL
SAMPLE PREPARATION
Standard Reference Sample
A standard reference sample, consisting of known amounts of concen-
trated trace metals in very pure water, was obtained from EPA. The reference
sample was diluted to the desired concentration according to accompanying
instructions.28
Synthetic Samples
Test solutions containing 0, 0.05, 0.1, 0.3, 0.5, and 1.0 of
chromium(III) or chromium(VI) were prepared by spiking exactly 5 or 9 ml
of very pure water in a series of 50-ml Pyrex beakers with 0, 5, 10, 30,
50, and 100 pi of a 100-mg/l standard of chromium(III) or chromium(VI).
After the chromium solutions were mixed, exactly 5 ml of 0.4 M ammonium
citrate buffer with 0.02 M EDTA (pH 5.0) was added to the chromium(III),
and exactly 1 ml of 1 M sodium hydroxide was added to the chromium(VI).
Test solutions containing 2.0, 5.0, 7.5, and 10 mg/1 chromium(III) and
chromium(VI) were prepared similarly by spiking with 20, 50, 75, and
100 pi of a 1000-mg/l standard of chromium(III) or chromium(VI). The
5-mg/l concentrations of zinc(II), copper(II), lead(II), chromiura(III),
chromium(VI), and iron(III) were prepared similarly, too, by spiking
with 50 (Jl of a 1000-mg/l standard solution of the respective element.
After being mixed, the solutions were transferred to the cell for the
polarographic measurements.
All elements used to prepare synthetic samples, except iron(III),
chromium(III), and chromium(VI), were drawn from 1000-mg/l certified
atomic absorption standards (Fisher Scientific Company, Fairlawn, New
Jersey). The 1000-mg/l concentration of iron(III) was prepared gravi-
metrically from ferric ammonium sulfate-12-water (8.634 g in 1000 ml of
reagent water) with weights checked against reference weights traceable
to the National Bureau of Standards (NBS). The 1000-mg/l concentration
of chromium(III) was prepared by dissolving exactly 0.7696 g of chromium(III)
nitrate-9-water in reagent water and diluting to 100 ml in a volumetric
flask. The 1000-mg/l concentration of chromium(VI) was prepared by
dissolving exactly 0.2828 g of anhydrous potassium dichromate with
dilution to 100 ml in a volumetric flask.
EQUIPMENT
All measurements were made with the Princeton Applied Research (PAR)
model 174 polarographic analyzer with mechanical drop timer and Houston
omnigraphic X-Y recorder model 2200-3-3. The dropping mercury electrode
was a 2- to 5-s capillary from Sargent-Welch Company with part no. S-29419.
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-6-
Other instruments—the platinum counter electrode; the calomel electrode;
the salt bridge with slow-leakage Vycor tip (Corning Glass Works, Corning,
New York) for isolating the calomel electrode from the test solution;
the outgassing tube; cell holder; and cell—were obtained from Princeton
Applied Research Corporation.29 An adjustable digital microliter pipette
(Analtech, Newark, Delaware, part no. P-20D or P-200D) was used to spike
solutions.
PREPARATION OF EQUIPMENT AND SOLUTIONS
Glassware that contacted the sample solution was soaked overnight
in a solution of two volumes of concentrated nitric acid in three volumes
of water. After leaching, the glassware was rinsed with reagent water
and dried in an oven at 110°C. Clean glassware and solutions were covered
with Parafilm (Fisher Scientific Company, Fairlawn, New Jersey) to prevent
contamination from trace elements in atmospheric particles.
Nitrogen gas, used to deaerate solutions for polarographic analysis,
was purged of oxygen. Zero-grade nitrogen gas was passed through a fur-
nace containing a special catalytic converter (a gas purifier, model 02-2315,
purchased from Supelco, Beliefonte, Pennsylvania) and heated to 600°C.
After the gaseous effluent from the furnace was passed successively through
a Hydro-Purge unit and a Dow gas purifier (Applied Science Laboratories,
State College, Pennsylvania), it was passed through sintered glass frits
in three scrubbing towers. One scrubbing tower contained 100 ml of reagent
water, and the other two contained 100 ml of 0.1 M chromous chloride in
2.4 M hydrochloric acid with amalgamated zinc. The amalgamated zinc particles
used were 0.8 to 3.2 mm in diameter for a Jones reductor (Fisher Scientific
Company, Fairlawn, New Jersey). Details for preparing the chromous chloride
scrubbers are given by Meites.30
Reagent-grade chemicals were used to prepare all solutions except
the 1 M potassium chloride in the salt bridge with Vycor tip that isolated
the reference electrode from the test solution. The 1 M potassium chloride
was prepared from an "Ultrex" grade salt (Baker Chemical Company, Phillipsburg,
New Jersey) by dissolving 7.45 g of the salt in reagent water and diluting
the solution to 100 ml.
The 1 M sodium hydroxide electrolyte was prepared by dissolving 4.0
g of the salt in reagent water that had been boiled to remove carbon dioxide
and diluting to 100 ml. One milliliter of this solution was diluted with
9 ml of reagent water to yield 0.1 M sodium hydroxide with a pH of 12.7.
The pH was determined potentiometrically with a meter calibrated against a
standard buffer with pH of 12.0 (Micro Essential Laboratory, Brooklyn, New
York). The 0.1 M sodium hydroxide solutions with pH values of 13.2 and
11.7 were prepared by pipetting 10 ml of 1 M sodium hydroxide into two
vessels to which about 50 ml of reagent water has been added, adjusting
the pH of the solution in one vessel to 13.2 with 5 M sodium hydroxide and
the pH of the solution in the other vessel to 11.7 with 6 M hydrochloric
acid, and diluting to a final volume of 100 ml.
•The 0.1 M EDTA solution was prepared by dissolving 7.44 g of
disodium EDTA-2-water in reagent water and diluting to 200 ml. The
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-7-
solution was heated as necessary to complete dissolution of the salt.
The 1 M citric acid solution was prepared by dissolving 210 g of citric
acid-1-water in reagent water with dilution to 1000 ml.
The 0.4 M citrate buffer with 0.02 M EDTA was prepared by mixing
400 ml of 1 M citric acid with 200 ml of 0.1 M EDTA, adding enough
concentrated ammonium hydroxide (about 63 ml) to bring the pH to 5.0
while the solution was still hot, and after cooling, diluting to 1000
ml. Buffer solutions with pH values of 4.7 and 5.2 were prepared
similarly by adjusting the pH with ammonium hydroxide before diluting.
This solution was not sterilized by autoclaving for 15 min at 121°C
and 1.03 x 105 Pascals (15 psi) because this treatment produced a
polarographically active interfering species between -1.0 and -1.4 V
vs. nee. New buffer solutions were prepared every two weeks.
DETERMINATION OF CHROMIUM(VI) BY DIFFERENTIAL PULSE POLAROGRAPHY
Exactly 9 ml of test solution and 1 ml of 1 M sodium hydroxide were
pipetted into a 50-ml Pyrex beaker, mixed, and transferred to the polarographic
cell. After deaerating the solution for 10 min with nitrogen gas treated
to remove the oxygen, a differential pulse polarographic scan was made
between -0.650 and -1.80 V vs. nee under suitable conditions. The peak
for chromium(VI) appeared at about -1.14 V vs. nee. Typically, the height
of the mercury column above the capillary from Sargent-Welch Company
(part no. S-29419) was adjusted to about 45 cm to provide a natural drop
time of about 2.8 s in 0.1 M sodium hydroxide (pH 12.7) at open circuit.
The typical settings used for the PAR model 174 polarographic analyzer
with mechanical drop timer were
drop time 1 s
scan rate 5 mV/s
display direction positive
scan direction negative
initial potential -0.650 V
range 1.5 V
sensitivity 2 pA for 0 to 1 mg/1
chromium(VI)
5 |JA for 2 mg/1
10 \iA for 5 mg/1
20 MA for 7.5 and
10 mg/1
modulation amplitude 50 mV
operation mode differential pulse
output offset negative settings as
required
With the Houston omnigraphic 2200-3-3 recorder, the recorder Y-axis
was adjusted to 1 V/in. (0.039 V/mm), and the X-axis was adjusted to 100
mV/in. (3.94 mV/mm). The Vycor tip of the 1 M potassium chloride salt
bridge was stored in reagent water between experiments because dilute
sodium hydroxide slowly dissolves Vycor.
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1200 PennsylvaniaAvenue
Washington DC 2Q4bU
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-8-
DETERMINATION OF CHROMIUM(III) BY DIFFERENTIAL PULSE POLAROGRAPHY
Exactly 5 ml of test solution and 5 ml of 0.4 M ammonium citrate with
0.02 M EDTA were pipetted into a 50-ml Pyrex beaker, mixed, and transferred
to the polarographic cell. After deaerating the solution for 10 min with
nitrogen gas treated to remove the oxygen, a differential pulse polaro-
graphic scan was made between -1.00 and -1.40 V vs. nee under suitable
conditions. The peak for chromium(III) appeared at about -1.26 V vs. nee.
Typically, the height of the mercury column above the capillary from
Sargent-Welch Company (part no. S-29419) was adjusted to about 45 cm to
provide a natural drop time of about 2.8 s in 0.2 M ammonium citrate
0.01 M EDTA (pH 5.1) at open circuit. The typical settings used for the
PAR model 174 polarographic analyzer with mechanical drop timer were
drop time 2 s
scan rate 2 mV/s
display direction positive
scan direction negative
initial potential -1.00 V
range 1.5 V
sensitivity 1 |jA for 0 to 1 mg/1
chromium(III)
2 |JA for 2 mg/1
5 |JA for 5 mg/1
10 |JA for 7.5 and 10 mg/1
modulation amplitude 50 mV
operation mode differential pulse
output offset negative settings as required
With the Houston omnigraphic 2200-3-3 recorder, the recorder Y-axis was
adjusted to 1 V/in. (0.039 V/mm), and the X-axis was adjusted to 100 mV/in.
(3.94 mV/mm). The Vycor tip of the 1 M potassium chloride salt bridge
was stored in reagent water between experiments to prevent possible plugging
of the tip.
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-9-
SECTION 5
RESULTS AND DISCUSSION
Typical polarograms are given in Figure 1 for standard chromium(VI)
concentrations under the conditions described in Section 4. With the
mercury electrode adjusted to provide a natural drop time of 2.8 s in
0.1 M sodium hydroxide at pH 12.7, standard solutions of 0.05, 0.1, 0.3,
0.5, 1.0, 2.0, 5.0, 7.5, and 10.0 rag/1 of chromium(VI) give peak currents
of 0.04, 0.07, 0.30, 0.53, 1.2, 2.3, 5.8, 9.0, and 12 \iA. No measurable
current is observed for a reagent blank. The peak currents have been
determined at -1.14 V vs. nee by measuring height above extrapolations
of the current just before and just after the wave. Sensitivity is not
improved by changing the scan rate from 5 to 2 mV/s, drop time from 1 to
2 s, and modulation amplitude from 50 to 100 mV. Improved sensitivity
is expected according to a paper on instrumental artifacts in differential
pulse polarography.31
Figure 2 gives the calibration curve for chromium(VI) for concen-
trations between 0 and 1 mg/1. The calibration curve for concentrations
between 0 and 10 mg/1 is given in Figure 3.
The detection limit from the pooled standard deviations of the
current residuals and slope of the calibration curve for 0 to 10 mg/1 of
chromium(VI) given by Figure 3 is 0.13 mg/1 at the 95 percent confidence
level. This has been calculated by using a definition of detection
limit given in the literature,32»3^
st/k,
where
k = 1.20193, the slope of the least squares equation for Figure 3,
t = 2.365, the value for tQ Q25 for n - 2 degrees of freedom
equal to 7,
s = standard deviation for the current residuals from the least
squares straight line (i = 1.20193C - 0.061236, where i =
the least squares peak for each concentration, C).
Tests have been conducted with 0.50 mg/1 of chromium(VI) solutions
at pH 11.7, 12.7, and 13.2 in 0.1 M sodium hydroxide. Differential
pulse polarograms for the test results are shown in Figure 4. The
height and potential of the chromium(VI) reduction peak are nearly the
same at pH 11.7 and 12.7. At pH 13.2 the peak height is the same as at
pH 11.7 and 12.7, but the potential is -0.95 V vs. nee rather than -1.15
V. The peak potentials for chromium(VI) in a 0.1 M sodium hydroxide,
illustrated by the bottom two differential pulse polarograms in Figure
4, are about 0.3 V more negative than those reported in the literature
for 1.0 M sodium hydroxide.15'17 The shift in peak potential at pH 13.2
-------
0.05mg/l Cr (3ZL)
(Cr(3ZD
o
I
-0.65
-0.85
-1.05 -1.25 -1.45
VOLTS vs. N.C.E.
-1.65
-1.85
Figure 1. Typical differential pulse polarograms for standard chromium(VI) concentrations in 0.1 M
sodium hydroxide at pH 12.7.
-------
i i i i r
lOmg/ICrGZT)
7.5mg/ICr(H)
j I I I I I I I I I | I
-0.65 -0.85 -1.05 -1.25 -1.45 -1.65 -1.85
VOLTS vs. N.C.E.
Figure 1 (continued)
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-12-
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
CHROMIUM (VI) (mg/l)
1.0
Figure 2. Calibration curve for 0 to 1.0 mg/l of chromium(VI) in
0.1 M sodium hydroxide at pH 12.7 by differential pulse
polarography.
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-13-
14
12
10
- 8
01 fi
a: D
o
0
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
CHROMIUM (VI) (mg/l)
Figure 3. Calibration curve for 0 to 10 mg/l of chromium(VI) in
0.1 M sodium hydroxide at pH 12.7 by differential pulse
polarography.
-------
T 1 1 1 1 I I T
T T
pH 13.2
0.5mg/l CHROMIUM (2L)
-0.65 -0.85
-1.05 -1.25 -1.45
VOLTS vs. N.C.E.
B5
Figure 4. Peak shift caused by pH in differential pulse polarograms for chromium(VI)
in 0.1 M sodium hydroxide.
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-15-
could be caused by the higher ionic strength from the additional sodium
hydroxide that is added to adjust the pH to 13.2.
Concentrations of 5.0 mg/1 of zinc(II), copper(II), lead(II),
chromium(III), and iron(III) do not interfere in the determination of
chromium(VI) in 0.1 M sodium hydroxide at pH 12.7. Zinc(II) gives a
peak nearby, but it does not affect the baseline above which extra-
polations are made to measure the peak height for chromium(VI).
Iron(III) causes only a slight elevation of the baseline, with a peak at
-0.86 V vs. nee and twin peaks between -1.4 and -1.8 V vs. nee. Many
natural waters contain zinc(II), copper(II), lead(II), chromium(III),
and iron(III), but most of them contain these metals in concentrations
of less than 5.0 mg/1.
Differential pulse polarograms of standard concentrations of
chromium(III) under the conditions described in Section 4 are shown in
Figure 5. With the mercury electrode adjusted to yield a natural drop
time of 2.8 s in 0.2 M ammonium citrate with 0.01 M EDTA at pH 5.1,
standard solutions of 0.05, 0.1, 0.3, 0.5, 1.0, 2.0, 5.0, 7.5, and 10
mg/1 of chromium(III) produce peak currents of 0, 0.01, 0.09, 0.26,
0.63, 1.4, 3.7, 5.8, and 6.9 pA. No measurable current is observed for
a reagent blank. The peak currents have been determined at -1.26 V vs.
nee by measuring height above extrapolations of the current just before
and just after the wave. The differential pulse polarograms have been
recorded at a scan rate of 2 mV/s, a modulation amplitude of 50 mV, and
a drop time of 2 s. Sensitivity is greater than at a scan rate of 5
mV/s and a drop time of 1 s. This agrees with theoretical consi-
derations reported in the literature.31
Figure 6 gives the calibration curve for chromium(III) for concentra-
tions between 0 and 1 mg/1. The absence of an appreciable response
below 0.3 mg/1 of chromium(III) may be caused by adsorption. The cali-
bration curve for concentrations between 0 and 10 mg/1 is shown in
Figure 7.
Using the previously discussed criteria for chromium(VI), the
detection limit from the pooled standard deviations of the current
residuals and slope of the calibration curve for 0 to 10 mg/1 of
chromium(III) given by Figure 7 is "0.61 mg/1 at the 95 percent confi-
dence level. This has been calculated by using the same definition of
detection limit that was used for chromium(VI).32»33 The detection
limit is
st/k,
where
k = 0.729001, the slope of the least squares equation for
Figure 7,
t = 2.365, the value for tQ Q25 for n - 2 degrees of freedom equal to 7,
s = standard deviation for the current residuals from the least
squares straight line (i = 0.729001C - 0.0546954, where
i = the least squares piak for each concentration, C).
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-16-
I.Omg/ICr(D
0.5mg/ICr(I)
0.3mg/ICr(I)
O.lmg/ICrfl)
0.05mg/l Cr(I)
-1.0 -1.1 -1.2 -1.3 -1.4 -1.5
VOLTS vs. N.C.E.
Figure 5. Typical differential pulse polarograms for standard
chromium(lll) concentrations in 0.2 M ammonium citrate
with 0.01 M EDTA at pH 5.1.
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-17-
I I I
T T
I0.0mg/l
Cr(M)
.5mg/l
Cr(I)
5.0 mg/1
Cr(I)
2.0mg/ICr(E)
I I I
-1.0 -1.1 -1.2 -1.3 -1.4 -1.5
VOLTS vs. N.C.E.
Figure 5 (continued)
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-18-
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
CHROMIUM (ID) (mg/l)
Figure 6. Calibration curve for 0 to 1.0 mg/l of chromium(III)
in 0.2 M ammonium citrate with 0.01 M EDTA at pH 5.1
by differential pulse polarography.
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-19-
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
CHROMIUM (IE) (mg/l)
Figure 7. Calibration curve for 0 to 10 mg/l of chromium(III)
in 0.2 M ammonium citrate with 0.01 M EDTA at pH 5.1
by differential pulse polarography.
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-20-
The pH of the test solution affects the peak height of chromium(III).
Tests have been conducted with 0.50 mg/1 of chromium(III) solutions at
pH values of 4.7 to 5.2 in 0.2 M ammonium citrate with 0.01 M EDTA. The
potential of the chromium(III) peak is the same at pH values between 4.7
and 5.2. The height of the peak is the same at pH 4.7 and 5.0, but at
pH 5.2 it is 43 percent of the value at pH 5.0. The decrease in pH from
5.0 to 4.7 increases the slope of the baseline just before and just
after the peak. This obscures the peak for the determination of
chromium(III) in the 0.2 M ammonium citrate with 0.01 M EDTA. This
agrees with Zarebski's observations of pH in differential pulse
polarograms of chromium(III) in 0.1 M potassium chloride with 0.01 M
EDTA at pH 6.O.24
The interference of chromium(VI) in the analysis of chromium(III)
has been determined by measuring peak heights for differential pulse
polarograms of mixtures of 0.50 mg/1 of chromium(III) and from 0 to 5.0
mg/1 of chromium(VI) in 0.2 M ammonium citrate with 0.01 M EDTA at pH
5.0. Figure 8 gives the differential pulse polarograms for these mixtures.
Concentrations of 0.50 to 5.0 mg/1 of chromium(VI) increase the peak
heights for 0.50 mg/1 of chromium(III). Table 1 gives the raw current
readings and composition of the mixtures. These increases in peak
heights are equivalent to those for concentrations of chromium(III) of
0.06, 0.12, 0.30, and 0.88 mg/1 caused by chromium(VI) concentrations of
0.5, 1.0, 2.0, and 5.0 mg/1, respectively. A concentration of 5.0 mg/1
of chromium(VI) alone gives a peak height that is equivalent to that
produced by 0.88 ppm of-chromium(III). A graph of apparent concen-
tration of chromium(III) for each concentration of chromium(VI) can be
used, when necessary, to correct the chromium(III) concentration for the
concentration of chromium(VI) that is determined by differential pulse
polarography in 0.1 M sodium hydroxide at pH 12.7. Some of the
interference by chromium(VI) in the determination of chromium(III) in
0.2 M ammonium citrate with 0.01 M EDTA at pH 5.0 could be caused by
reduction of chromate by the metallic mercury that accumulates in the
cell during the 15 min taken to deaerate and record the polarogram.
Although this effect was not determined, Crosmun19 has measured the
amount of chromium(VI) reduced during the polarographic determination of
chromate in ammonium acetate. Although 0.2 M ammonium citrate with
0.01 M EDTA at pH 5.0 is not entirely free from interference by
chromium(VI), it is a much better electrolyte for the differential pulse
polarographic determination of chromium(III) than 0.1 M potassium chloride
with 0.01 M EDTA at pH 6.0 because, in this electrolyte, the peaks for
chromium(III) and chromium(VI) coincide.24
Figure 9 gives the interference of 5.0 mg/1 of zinc in the determina-
tion of chromium(III) when EDTA is not added to the 0.2 M ammonium
citrate electrolyte at pH 5.0. A concentration of 5.0 mg/1 of zinc
produces a large peak that interferes severely at -1.26 V, the reduction
potential for chromium(III). This figure also shows that 0.01 M EDTA
eliminates interference by 5.0 mg/1 of zinc.
Figure 10 gives interference of 5.0 mg/1 of lead in the deter-
mination of chromium(III). As shown by this figure, the peak for lead
appears at -1.19 V vs. nee with a height of 1.08 pA, which is equivalent
to about 0.8 mg/1 of chromium(III).
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-21-
SAMPLE COMPOSITION (ma/1)
CHROMIUM(M) CHROMIUM (H)
5.0
2.0
1.0
0.5
•1.0 -I.I -1.2 -1.3 -1.4 -1.5
VOLTS vs. N.C.E.
Figure 8. Interference of chromium(VI) in differential pulse
polarograms for chromium(III) in 0.2 M ammonium citrate
with 0.01 M EDTA at pH 5.1.
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-22-
TABLE 1. EFFECT OF CHROMITJM(VI) IN ANALYSIS OF CHROMIUM(III)
BY DIFFERENTIAL PULSE POLAROGRAPHY IN 0.2 M AMMONIUM CITRATE
WITH 0.01 M EDTA AT pH 5.0
Current
(MA)
0.17
0.28
0.31
0.41
0.68
0.44
Chromium (II I)
concentration
(mg/1)
0.5
0.5
0.5
0.5
0.5
0
Chromium (VI)
concentration
(mg/1)
0
0.5
1.0
2.0
5.0
5.0
-------
O
< O
Sr-o
Kits g.
CT>ns S Q.Q-
o>~ a «> -S
'
5mg/l Zn(H)
without EDTA
5mg/l Zn(I) with EDTA
Cr(I)
to
OJ
I
-1.0
-1.2 -1.4
VOLTS vs. N.C.E
-1.6
Figure 9. Use of EDTA to eliminate interference of zinc(II) in differential pulse polarograms
for chromium(III) in 0.2 M ammonium citrate with 0.01 M EDTA at pH 5.0.
-------
-24-
Cr (DO
r5mg/l Pb(fl)
-1.0
-1.1 -1.2 -1.3 -1.4
VOLTS vs. N.C.E.
-1.5
Figure 10. Interference of lead in differential pulse polarograms for
chromium(lll) in 0.2 M ammonium citrate with 0.01 M EDTA at
pH 5.0.
-------
-25-
Concentrations of 5.0 mg/1 of iron(III), copper(II), and zinc(II)
do not interfere in the determination of chromium(III) in 0.2 M ammonium
citrate with 0.01 M EDTA at pH 5.0.
The detection limits of 0.13 mg/1 for chromium(VI) and 0.61 mg/1
for chromium(III) are not low enough to determine the expected concen-
trations of chromium species in most natural water samples. Also,
interference in the determination of chromium(III) is greater than that
for chromium(VI). But sample preparation with concentration by ion
exchange,26'34 solvent extraction,27 precipitation,27 or evaporation18
should lower the detection limit as well as isolate the chromium species
from potential interference.
Table 2 gives determinations of chromium(VI) and chromium(III) by
the differential pulse polarography procedure described in Section 4 for
triplicate solutions of spiked surface water for each individual species.
The precision and accuracy based on the analysis of these three replicates,
containing 0.55 mg/1 of chromium(VI) and 0.99 mg/1 of chromium(III),
have been determined by comparison with a standard curve prepared by
polarographing a series of standard solutions. The standard deviations
are, respectively, 0.08 and 0.3 mg/1, the relative standard deviations
are 19 and 34 percent, and the percentage accuracies are -24 and -12.35
The EPA trace metals reference concentrate 575 (No. 3)28 has been
assayed for chromium(VI) by the differential pulse polarographic technique
described in this report. Exactly 10 ml of the concentrate was diluted
to 500 ml rather than 1000 ml, and no additional acid was added. Trip-
licate results for chromium(VI) give a mean value of 0.19 with a standard
deviation of 0.01.35 The expected concentration for total chromium is
0.42, so by subtraction the remainder of the chromium is 0.23 mg/1 of
chromium(III). Attempts to verify this by differential pulse polarography
led to high results for chromium(III), which are known to be caused partly
by lead(II), as shown in Figure 10, and possibly by other elements in the
reference samples that were not tested for interference in this study.
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-26-
TABLE 2. DETERMINATIONS OF CHROMIUM(VI) AND CHROMIUM(III)
BY DIFFERENTIAL PULSE POLAROGRAPHY FOR TRIPLICATE SOLUTIONS
OF SPIKED SURFACE WATER3
Chromimn(III)
or chromium(VI)
concentration
(mg/D
Chromium(VI)
determination
Qng/1)
Chromium(III)
determination
(mg/1)
0.55, Cr(VI); 0.99, Cr(III) 0.42 ±0.08
0.41, 0.35, 0.50*
0.8710.0.3
1.16, 0.72, 0.72C
Determination of chromium(VI) and chromium(III) was made by differential
pulse polarography in 0.1 M sodium hydroxide at pH 12.7 and in 0.2 M
ammonium citrate with 0.01 M EDTA at pH 5.0, respectively. Chromium(VI)
determination was made in the filtrate from an aged surface water
containing fungi and algae, while chromium(III) determination was made
in fresh surface water.
Average value and standard deviation for triplicate determinations.
£
Individual values.
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-27-
REFERENCES
1. Onstott, E. I., W. S. Gregory, and E. F. Thode. Removal of Chromate
from Cooling Tower Slowdown by Reaction with Electrochemically
Generated Ferrous Hydroxide. Environ. Sci. Technol., 7:333-337, 1973.
2. Huang, C. P., and M. H. Wu. The Removal of Chromium(VI) from Dilute
Aqueous Solution by Activated Carbon. Water Res., 11:673-679, 1977.
3. Griffin. R. A., A. K. Au, and R. R. Frost. Effect of pH on Adsorp-
tion of Chromium from Landfill-Leachate by Clay Minerals. J.
Environ. Sci. Health, A12:431-449, 1977.
4. Hiraide, M., Y. Yoshida, and A. Mizuike. Flotation of Traces of
Heavy Metals Coprecipitated with Aluminum Hydroxide from Water
and Sea Water. Anal. Chim. Acta, 81:185-189, 1976.
5. Hannah, S. A., M. Jelus, and J. M. Cohen. Removal of Uncommon
Trace Metals by Physical and Chemical Treatment Processes. J.
Water Pollut. Control Fed., 2297-2309, November 1977.
6. Shirai, M. Non-Polluting Treatment of the Waste of Chromic Acid
Mixture. J. Chem. Ed., 54:609, 1977.
7. Tandon, S. K., D. K. Saxena, J. S. Gaur, and S. V. Chandra.
Comparative Toxicity of Trivalent and Hexavalent Chromium. Environ.
Res., 15:90-99, 1978.
8. Guidelines Establishing Test Procedures for the Analysis of
Pollutants. Fed. Regist., 41(232):52780-52786, 1976.
9. Flato, J. B. The Renaissance in Polarographic and Voltammetric
Analysis. Anal. Chem., 44-.75A-87A, 1972.
10. Bond, A. M., and D. R. Canterford. Comparative Study of a Wide
Variety of Polarographic Techniques with Multifunctional Instrumen-
tation. Anal. Chem., 44:721-731, 1972.
11. Parry, E. P., and R. A. Osteryoung. Evaluation of Analytical Pulse
Polarography. Anal. Chem., 37:1634-1637, 1965.
12. Christie, J. H., J. Osteryoung, and R. A. Osteryoung. Instrumental
Artifacts in Differential Pulse Polarography. Anal. Chem., 45:210-
215, 1973.
13. Purdy, W. C. The Role of the Electro-Analytical Chemist in Bio-
chemical Research. Anal. Chem., 36:29A-39A, 1964.
14. Mertz, W. Chromium Occurrence and Function in Biological Systems.
Physiol. Rev., 49:163-239, 1969.
U.S 6P*A Hsadquaaers Library
Mail code 340*1
1200 Pennsylvania Avenue NW
Washington, DC 20460
202-566-0556
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-28-
15. Princeton Applied Research Corporation. Differential Pulse Polarography
of Chromium. Application Note 122, P.O. Box 2565, Princeton, New
Jersey, 1974.
16. Williams, L.F.G. Determination of the Chromate Content of Chromate
Conversion Films on Zinc. Anal. China. Acta, 94:199-200, 1977.
17. Lingane, J. J., and I. M. Kolthoff. Polarographic Study of the
Reduction of Chromate Ion at the Dropping Mercury Electrode.
J. Am. Chem. Soc., 62:852-858, 1940.
18. Ballinger, D. G., and R. A. Taft. Polarographic Determination of
Metals in Water, Wastes and Biological Samples. Water Sewage
Works, 109:338-341, 1962.
19. Crosmun, S. T., and T. R. Mueller. The Determination of Chromium(VI)
in Natural Waters by Differential Pulse Polarography. Anal. Chim.
Acta, 75:199-205, 1975.
20. Heyrosky, J., and P. Zuman. Practical Polarography. An Introduction
for Chemistry Students. Academic Press, London and New York, 1968.
p. 191.
21. Princeton Applied Research Corporation. Chromium by Differential Pulse
Polarography. Application Brief C-2, P.O. Box 2565, Princeton,
New Jersey, 1976.
22. Taylor, L. R. Personal Communication on Polarography of Chromium.
Princeton Applied Research Corporation, Princeton, New Jersey, Jan. 9,
1978.
23. Whitnack, G. C. Single-Sweep Polarographic Techniques Useful in
Micropollution Studies of Ground and Surface Waters. Anal. Chem.
47:619-621, 1975.
24. Zarebski, J. Alternating Current, Normal, and Differential Pulse
Polarographic Studies of Chromium EDTA, CDTA, and DTPA Complexes
for Application to the Determination of Chromium in Trace Amounts.
Chem. Anal. (Warsaw) (English translation) 22:1037-1048, 1977.
25. Howe, L. H. Trace Analysis of Arsenic by Colorimetry, Atomic Absorp-
tion, and Polarography. EPA-600/7-77-036, Tennessee Valley Authority,
Chattanooga, Tennessee, 1977. p. 35.
26. Van Loon, J. C., B. Radziuk, N. Kahn, J. Lichwa, F. L. Fernandez, and
J. D. Kerber. Metal Speciation Using Atomic Absorption Spectroscopy.
At. Absorpt. Newsl., l6(4):79-83, 1977.
27. Jan, T., and D. R. Young. Chromium Speciation in Municipal Wastewaters
and Seawater. J. Water Pollut. Control Fed., 50:2327-2336, 1978.
28. U.S. Environmental Protection Agency. EPA Quality Control Samples.
Publication No. 575, Environmental Monitoring and Support Laboratory,
Quality Assurance Branch, Cincinnati, Ohio.
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-29-
29. Princeton Applied Research Corporation. Electrochemical Assessories.
T-359-10M-3/76-CP, P.O. Box 2565, Princeton, New Jersey, 1976.
pp. 7-12.
30. Meites, L. Polarographic Techniques, 2nd ed. Interscience
Publishers, New York, 1967. pp. 89-90.
31. Christie, J. H., J. Osteryoung, and R. A. Osteryoung. Instrumental
Artifacts in Differential Pulse Polarography. Anal. Chera., 45:
210-215, 1973.
32. Osteryoung, J., and D. Myers. Determination of Arsenic by Differential
Pulse Polarography. Application Note AN-117, Princeton Applied Research
Corporation, Princeton, New Jersey, October 1973.
33. Skogerboe, R. K., and C. L. Grant. Comments on the Definitions of the
Terms Sensitivity and Detection Limit. Spectrosc. Lett., 3:215-220,
1970.
34. Mohammad, J., and F. Ingman. Analysis of Chromite by Cation-Exchange
Using Ethylenediaminetetraacetic Acid. Talanta, 22:1037-1040, 1975.
35. Guide for Use in Reporting Data in Analytical Chemistry. Anal. Chem,
47:2527, 1975.
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-30-
LIST OF PRESENTATIONS
A speech, "Differential Pulse Polarographic Determination of Chromium(III)
and Chromium(VI) in Water Samples," was given by Lyman H. Howe at
the Fifth Annual Meeting, Federation of Analytical Chemistry and
Spectroscopy Societies, Boston, Mass., October 30-November 3,
1978.
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
. REPORT NO.
EPA-600/7-79-086
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
CRITICAL EVALUATION OF DIFFERENTIAL PULSE POLAROGRAPHY
FOR DETERMINING CHROMIUM(III) AND CHROMIUM(VI) IN
WATER SAMPLES
5. REPORT DATE
MARCH 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
Lyman H. Howe, Isaac E. Jones and Norman K. Stanley
TVA/ONR-79/09
3. PERFORMING ORGANIZATION NAME AND ADDRESS
Office of Natural Resources
Tennessee Valley Authority
Chattanooga, TN 37401
10. PROGRAM ELEMENT NO.
INE 625C
11. CONTRACT/GRANT NO.
80 BDH
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Research & Development
Office of Energy, Minerals & Industry
Washington. D.C. 20460
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/7
16. SUPPLEMENTARY NOTES
This project is part of the EPA-planned and coordinated Federal Interagency
Energy/Environment R&D Program.
16. ABSTRACT
The Tennessee Valley Authority critically evaluated differential pulse polarography
for determining chromium(VI) and chromium (III) in water samples from coal-fired
steam-electric generating stations. After.addition of reagents to overcome inter-
ference, the peak currents for chromium(III) and chromium(VI) are measured
separately in different electrolytes and quantified by standard addition. Total
chromium is the algebraic sum of chromium(VI) and chromium(III). The effective
range for quantification is 0.1 to 10 mg/1 of chromium(VI) and 0.6 to 10 mg/1 of
chromium(III). Interferences by lead(II) and chromium(VI) on chromium(III) are
discussed. Lead(II) does not interfere with measurement of chromium(VI), and
chromium(IIl) does not interfere with chromium(VI). Copper(II), zinc(II), and
iron(III) do not interfere with measurement of either chromium(III) or chromium(VI).
17.
KEY WORDS AND DOCUMENT ANALYSIS
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