United States
Environmental Protection
Agency
Environmental Monitoring and
Support Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S4-86/039 March 1987
SERA Project Summary
Determination of Stable Valence
States of Chromium in Aqueous
and Solid Waste Matrices-
Experimental Verification of
Chemical Behavior
J. D. Messman, M. E. Churchwell,
D. Wong, and J. Lathouse
The objective of this research effort
was to experimentally assess the chemi-
cal behavior of the stable species of
chromium during the preparation,
chemical manipulation, and spectro-
photometric analyses of simulated and
authentic environmental samples for
hexavalent chromium. The effort for
this research was divided into four ex-
perimental phases, addressing specific
objectives: (1) characterization and
ruggedness evaluation of the diphenyl-
carbazide (DPC) spectrophotometric
method for hexavalent chromium; (2)
evaluation of the stability and reactivity
of hexavalent chromium under simu-
lated but controlled aqueous matrix
conditions; (3) evaluation of alkaline
and acidic digestions for the analysis of
insoluble chromate standards and tri-
valent chromium; and (4) evaluation of
alkaline and acidic digestions for chro-
mium analyses of environment samples.
Each research phase is individually
discussed.
This Protect Summary was developed
by EPA's Environmental Monitoring and
Support Laboratory, Cincinnati, OH, to
announce key findings ot the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
The analyses of solid waste materials
for Cr(VI) represent formidable challenges
to the analytical scientist. A metal specia-
tion scheme to differentiate between
trivalent and hexavalent species, Cr(lll)
and Cr(VI), must address the solubilization
of chromium species from solid matrices,
while maintaining the integrity of the
individual chromium species during all
sample manipulation phases of the
analytical method. Whereas much re-
search has focused on the separation
and detection of dissolved chromium
species in synthetic aqueous mixtures or
relatively clean liquid environmental
samples, the chemical solubilization and
determination of Cr(VI) in solid waste
materials have not been adequately
addressed.
The present study has focused on
evaluations of three digestion methods
for the chemical solubilization of Cr(VI) in
barium chromate test compounds and in
real environmental samples: (1) an alka-
line digestion medium, consisting of an
aqueous solution of sodium carbonate
and sodium hydroxide; (2) a nitric acid
digestion method; and (3) a nitric acid/
persulfate digestion method. The relative
merits of the digestion methods have
been based on the analytical results for
solubilization of insoluble chromates as
-------
well as stability of Cr(lll) and Cr(VI) spikes
in various test solutions and environ-
mental samples.
Results and Discussion
The DPC spectrophotometric method,
as described in EPA Method 7196, was
employed to measure concentration
changes in Cr(VI) for each test sample
solution resulting from chromium redox
phenomena occurring during the diges-
tions. The DPC spectrophotometric
method was found to be sensitive and
specific for Cr(VI) in aqueous solutions
containing up to 1000-fold ratios of Cr(lll).
Instrument response was linear over two
orders of magnitude of Cr(VI) concentra-
tion (0.01 to 1.0 mg/L). Problems of
chemical reduction were encountered
with the method for analyses of simulated
samples containing excesses of both
Cr(lll) and sulfide.
The stability of Cr(VI) in aqueous solu-
tions containing Cr(lll) and sulfide as a
function of pH was examined. In alkaline
solutions, reduction of Cr(VI) to Cr(lll) by
sulfide was slow. As predicted by standard
electrochemical potentials, the reduction
of Cr(VI) was increased in acidic solutions.
These results have two significant ramifi-
cations: (1) environmental samples for
Cr(VI) analyses should not be preserved
by acidification to pH 2 and (2) DPC
reagent should be added to an alkaline
sample before pH adjustment to 2 with
sulfuric acid to minimize Cr(VI) reduction
in the quantification step; this verifies
proper order of addition of the two
reagents as described in Method 7196.
A digestion medium consisting of 50
percent (v/v) nitric acid and 5 percent
(w/v) potassium persulfate was examined
predicated on the potential feasibility of
the potent oxidizing properties of per-
sulfate keeping the Cr(VI) in an oxidized
state even under highly acidic conditions.
However, this digestion method failed
because the results indicated that per-
sulfate exhibits reducing properties in a
50 percent nitric acid medium. At nitric
acid concentrations of 20 percent or less,
the nitric acid/persulfate medium ex-
hibited its predicted oxidizing properties.
The 50 percent nitric acid medium
(without persulfate) and the alkaline
medium (2 percent sodium hydroxide/3
percent sodium carbonate) both success-
fully solubilized Cr(VI) from the insoluble
barium chromate test compound. Further-
more, the valence states of trivalent and
hexavalent chromium were maintained
in standard solutions carried through
either digestion procedure. However, in
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.
Eight environmental solid samples were
analyzed for Cr(VI) by DPC spectro-
photometry following alkaline digestions.
Complete recoveries of Cr(VI) spikes were
obtained in 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 (organic/sulfide matrix), and
NBS-SRM 1646 estuarme sediment. Par-
tial oxidation of Cr(lll) spikes was observed
for many samples (4 out of 8) in alkaline
media; Cr(lll) spikes were stable in elec-
troplating sludge, tannery sludge (or-
ganic/sulfide matrix), NBS-SRM 1646
estuarine sediment and municipal
digested sludge. Partial oxidation of Cr(lll)
spikes in alkaline media produced mea-
surement errors in Cr(VI) concentration
by as much as 100 percent (positive
bias). Of the eight solid samples in this
study, the electroplating sludge was the
only sample analyzed in which both Cr(VI)
and Cr(lll) spikes were stable in alkaline
media.
Conclusions
Of the eight solid samples analyzed in
this study, only the electroplating sludge
sample was successfully analyzed for
Cr(VI) by DPC spectrophotometry follow-
ing an alkaline digestion. Although un-
confirmed by a collaborative method,
endogenous Cr(VI) was measured in the
electroplating sludge, as determined by
spike recovery; Cr(VI) spikes were com-
pletely recovered and no measurable
oxidation of Cr(lll) was observed.
Three solid samples (river sediment,
municipal digested sludge and contami-
nated soil) were digested in the 50
percent nitric acid medium (without per-
sulfate) and analyzed by DPC spectro-
photometry. Although no oxidation of
Cr(lll) spikes was observed in any of the
sample, Cr(VI) spikes were completely
reduced in all three samples.
DPC spectrophotometry was limited by
color interferences encountered in many
of the environmental samples. The inter-
ferences ranged from turbidity and color
formation occurring after DPC addition
Turbidity and color interference was
minimized to a certain extent by dilution
but this often resulted in high imprecision.
-------
J. D. Messman, M. E. Churchwell, D. Wong, and J. Lathouse are with Battelle
Laboratories, Columbus Division, Columbus. OH 43201 -2693.
Ted Martin is the EPA Project Officer (see below).
The complete report, entitled "Determination of Stable Valence States of
Chromium in Aqueous and Solid Waste Matrices—Experimental Verification
of Chemical Behavior," (Order No. PB 87-140 927/AS; Cost: $18.95, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring and Support Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
52
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES Pft
EPA
PERMIT No G-35
Official Business
Penalty for Private Use $300
EPA/600/S4-86/039
IL 60604
MJiiilmilJiiUiiilliiiliifil MliHiliiliiitliltliHiiiililnlil
------- |