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.
                                      16

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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.
                                   17

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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.
                                      18

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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.
                                       19

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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.
                                      20

-------
 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.
                                      22

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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.
                                     23

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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

-------
           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.
                                      25

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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.
                                    26

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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.
                                        27

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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

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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

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   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

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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

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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

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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

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        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

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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

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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

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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

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           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

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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

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     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

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     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

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     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

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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

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           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

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         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

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         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

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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.


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              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.

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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

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  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

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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

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  '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

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 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

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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

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   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

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     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

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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

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           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

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          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

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         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

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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.

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 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

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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

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        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

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 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

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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.

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    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

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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

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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

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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

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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 
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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

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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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    Materials.  M1kroch1m. Acta, II: 231-246, 1985.

 2.  Stumm, W. and J.  J. Morgan.  Aquatic Chemistry. An Introduction Emphasizing
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 3.  Test Methods for  Evaluating Solid Waste,  Physical/Chemical Methods, SW-846,
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 4.  Photometric Determination of Traces of Metals, E. B. Sandell and H. On1sh1,
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 5.  Sano, H.  Anal. Ch1m. Acta, 27: 398. 1962.

 6.  .Kovalenko, E.  V., and V. I.  Petrashen.  J. Anal. Chem., USSR, 18: 645,
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 7.  Najdeker, E.  Proc. Soc. Anal.  Chem., 8:  194, 1971.
    <
B.  "Mar-chart. H.   Anal. CMm. Acta. 30: 1. 1964.

9.  'international Union of Pure and Applied  Chemistry,  Analytical  Chemistry
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    Analysis - II.  Data Interpretation.  Anal. Chem., 48(14): 2294-2296, 1976.

 10  Glaser, J. A.,  0. L.  Foerst,  G.  0. McKee, S. A.  Quave, and W.  A.  Budde.
    Trace Analyses for Wastewaters.  Environ. Sc1. Techno!., 15(12):  1426-1435,
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 11. Blomqulst, G.. C.  A.  Nllsson,  and  0.  Nygren.  Scand.  J. Work Environ.
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                                          112

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