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

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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.

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     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
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EPA/600/S4-86/039
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