SIST-TP CEN/TR 14589:2004

Characterization of waste - State of the art document - Chromium VI specification in solid matrices

This European document describes the state-of-the-art extraction and determination methods for the total content of hexavalent chromium in raw waste and other solid materials.

Charakterisierung von Abfällen - Bestimmung von Chrom in AbfallStatusbericht

Caractérisation des déchets - Etat de l'art - Spécification du Chrome (VI) dans les matrices solides

Le présent document décrit l'état de l'art des méthodes d'extraction et de détermination de la teneur totale en chrome hexavalent dans les déchets bruts et autres matières solides.

Karakterizacija odpadkov - Dokument o stanju tehnike - Določevanje Cr (VI) v trdnem

General Information

• Status: Published
• Publication Date: 30-Apr-2004
• ICS: 13.030.10 - Solid wastes
• Technical Committee: KAT - Soil quality
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 01-May-2004
• Due Date: 01-May-2004
• Completion Date: 01-May-2004
• Ref Project: CEN/TR 14589:2003 - Characterization of waste - State of the art document - Chromium VI specification in solid matrices

Overview

CEN/TR 14589:2003 is a CEN Technical Report that reviews the state of the art for extraction and determination of hexavalent chromium (Cr(VI)) in solid matrices. Its scope covers methods for quantifying the total content of Cr(VI) in raw waste, soils and other solid materials, and discusses chemistry, speciation challenges and practical analytical approaches for reliable Cr(VI) measurement.

Key topics

• Speciation and chemistry: importance of distinguishing Cr(III) (nutrient form) from toxic Cr(VI); redox instability and mobility of chromium in environmental matrices.
• Extraction criteria: three essential requirements for any Cr(VI) extraction procedure:
  • solubilize all forms of native Cr(VI);
  • avoid reduction of native Cr(VI) to Cr(III) during extraction;
  • avoid oxidation of native Cr(III) to Cr(VI) during extraction.
• Alkaline extraction approach: the report highlights alkaline leaching as preferred (rather than acidic) to limit reduction of Cr(VI). It documents the commonly used USEPA SW-846 Method 3060A (Na2CO3 0.28 M + NaOH 0.5 M, heated at 90–95 °C for 60 min) and explains its advantages and limitations.
• Quality control and interpretation: use of matrix spikes, ancillary redox-indicating parameters (pH, ORP, sulfides, TOC) to interpret poor recoveries and distinguish method artefacts from true matrix effects.
• Analytical detection techniques: discussion of common determination methods referenced in the report (e.g., diphenylcarbazide spectrophotometry, ion chromatography, IC-ICP-MS, and electrochemical methods) and associated abbreviations and symbols.

Applications and users

CEN/TR 14589:2003 is aimed at professionals involved in environmental and waste characterization, including:

• environmental and commercial testing laboratories validating Cr(VI) methods;
• regulatory agencies and compliance officers assessing contaminated sites and landfill waste;
• remediation contractors and consultants performing risk assessment and cleanup planning;
• researchers and soil chemists studying chromium speciation and mobility.

Practical applications include site contamination assessment, landfill waste characterization, monitoring of remedial effectiveness (Cr(VI) reduction), and supporting regulatory compliance for hazardous substances.

Related standards

Relevant normative references and complementary methods cited in the report include:

• USEPA SW-846 Method 3060A (alkaline extraction for Cr(VI));
• ISO 11083 (photometric determination with diphenylcarbazide);
• ASTM methods for Cr(VI) in water/atmosphere (e.g., ASTM D5257/D5281);
• DIN methods and other national standards for Cr(VI) extraction and photometric determination.

Keywords: CEN/TR 14589:2003, Chromium VI, Cr(VI), hexavalent chromium, waste characterization, solid matrices, extraction methods, USEPA 3060A, speciation, environmental analysis.

Standards Content (Sample)

SLOVENSKI STANDARD
01-maj-2004
.DUDNWHUL]DFLMDRGSDGNRY'RNXPHQWRVWDQMXWHKQLNH'RORþHYDQMH&U9,Y
WUGQHP
Characterization of waste - State of the art document - Chromium VI specification in solid
matrices
Charakterisierung von Abfällen - Bestimmung von Chrom in AbfallStatusbericht
Caractérisation des déchets - Etat de l'art - Spécification du Chrome (VI) dans les
matrices solides
Ta slovenski standard je istoveten z: CEN/TR 14589:2003
ICS:
13.030.10 Trdni odpadki Solid wastes
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL REPORT
CEN/TR 14589
RAPPORT TECHNIQUE
TECHNISCHER BERICHT
May 2003
ICS 13.030.10
English version
Characterization of waste – State of the art document –
Chromium VI specification in solid matrices
Caractérisation des déchets - Etat de l'art - Spécification Charakterisierung von Abfällen - Bestimmung von Chrom in
pour la détermination du Chrome VI dans les matrices AbfallStatusbericht
solides
This Technical Report was approved by CEN on 7 April 2003. It has been drawn up by the Technical Committee CEN/TC 292.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and United
Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2003 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 14589:2003 E
worldwide for CEN national Members.

Contents
Foreword. 3
Introduction . 3
1 Scope . 3
2 Normative references . 4
3 Symbols and abbreviations. 4
4 Chromium VI speciation in solid matrices . 6
5 Final conclusions. 19
Bibliography . 38
Foreword
This document (CEN/TR 14589) has been prepared by Technical Committee CEN/TC 292 "Characterization of
waste", the secretariat of which is held by NEN.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to announce this European Standard: Austria, Belgium, Czech Republic, Denmark, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal,
Slovakia, Spain, Sweden, Switzerland and the United Kingdom.
Introduction
Speciation is one of the growing features of analytical chemistry of the last years. It is now recognized that the
determination of total trace element contents is no longer sufficient, because the biological and environmental
impact of an element is dictated by the physico-chemical form in which it is present in the sample.
Chromium belongs to the category of problematic elements in analytical chemistry, because it behaves as a
valence chameleon. The chemistry of chromium compounds is rather complicated, inorganic chromium compounds
may occur in oxidation states ranging from -II to +VI [1,2]. However, in natural systems, Cr(III) and Cr(VI) are the
most stable forms. Besides Cr(III) which is an essential trace element for mammals, including man, Cr(VI)
compounds are genotoxic and potentially carcinogenic in humans. Evidence exists for the carcinogenity of calcium,
strontium and zinc chromate [2,3]. The inoffensive nature of Cr(III) ions results from the fact that in biotic
n-3
environment, it usually appears in aqua-hydroxo complexes of the form []Cr()H O (OH ) and their size
2 6-n
n
excludes them almost entirely from penetrating cell membranes [4].
From chemical point of view, Cr(III) shows similarities with that of Al O : Cr O is amfoteric, albeit more basic than
2 3 2 3
acidic. In contrast, Cr(VI) is strongly acidic; all Cr(VI) compounds, except for CrF are oxocompounds:
-2-2-
HCrO CrO Cr O
(hydrochromate), (chromate) and (dichromate) species which are powerful oxidants.
4 4 2 7
Under environmental conditions, dichromates are not formed at a total chromium concentration less than
0,01 mol/l. Certain forms of Cr(III) may oxidize to Cr(VI) in soils and that Cr(VI) may be reduced to Cr(III) in the
same soil. Since under alkaline to slightly acidic conditions, Cr(VI) compounds are not strongly absorbed by many
soils, they can be very mobile in surface environments. On the other hand, under these conditions, Cr(III) readily
precipitates as Cr(OH) . Cr(VI) can be reduced to Cr(III) in soils by redox reactions with aqueous inorganic species,
electron transfer at mineral surfaces, reactions with non-humic organic substances such as carbohydrates and
proteins or reduction by soil humic substances [5]. The latter, which constitutes the majority of the organic fraction
in most soils, represents a significant reservoir of electron donors for Cr(VI) reduction. As a result, the opposing
solubility and toxicity characteristic of Cr(III) and Cr(VI) and the potential for Cr(III) oxidation in soil represent a
unique regulatory challenge for the establishment of protective, health-based clean-up standards for Cr-
contaminated soils [6]. Remediation of Cr(VI) containing soils through reduction to Cr(III) will lower the health and
ecological hazard of such soils.
As a consequence of previous considerations, most attention is paid to Cr(VI) determination in environmental
matter. Unfortunately, just this task is difficult to handle. Intricacies are primarily because of instability of the
oxidation states of chromium and the complex character of environmental samples.
1 Scope
This European document describes the state-of-the-art extraction and determination methods for the total content
of hexavalent chromium in raw waste and other solid materials.
2 Normative references
This document incorporates by dated or undated reference, provisions from other publications. These normative
references are cited at the appropriate places in the text and the publications are listed hereafter. For dated
references, subsequent amendments to or revisions of any of these publications apply to this document only when
incorporated in it by amendment or revision. For undated references the latest edition of the publication referred to
applies (including amendments).;
AS 2882 : 1986 Waters-Determination of chromium (VI)(diphenylcarbazide spectrophotometric
(Australia): method);
Test Method for Dissolved Hexavalent Chromium in Water by Ion Chromatography;
ASTM D 5257 1997
ASTM D 5281: 1998 Standard Test Method for Collection and Analysis of Hexavalent Chromium in
Ambient Atmospheres;
DSF 38929: 1999 Packaging-Requirement for measuring and verifying four heavy metals and other
dangerous substances present in packaging and their release into the environment-
Part 1: Requirements for measuring and verifying four metals present in packaging
(lead, cadmium, chromium VI and mercury);
IRSA (Italy): 1986 Analytical Methods for Waste-Physico-Chemical Parameters, Method No. 16,
Hexavalent Chromium
(colorimetric reaction with diphenylcarbazide);
ISO 11083: 1994 Water quality-Photometric determination of Chromium VI with 1.5 diphenylcarbazide;
ISO 3856-5: 1984 Determination of hexavalent chromium content of pigment portion of liquid paint or
paint in powder form-spectrophotometric method with diphenylcarbazide;
Soil quality-Determination of Chromium (VI) in phosphate extract;
DIN 19734: 1999
DIN 38405-24: 1987 German standard methods for the examination of water, waste water and sludge;
photometric determination of Chromium (VI) using 1.5 DPC
;
DIN 53780: 1999 Pigments and extenders-Determination of matter soluble in water-hexavalent
chromium content
3 Symbols and abbreviations
For the purposes of this documentError! No text of specified style in document., the following symbols and
abbreviations apply:
50 52
III
is the mass bias corrected measured isotope ratio of Cr to Cr of Cr(III) in the spiked sample;
R
50/52
is the atomic fraction of Cr in the sample;
A
x
III
is the concentration of Cr(III) in the sample (μmol/g, unknown);
C
x
50 50
50 III
is the atomic fraction of Cr in the Cr(III) spike;
A
s
III
is the concentration of Cr(III) in the Cr(III) spike (μmol/g);
C
s
VI
is the concentration of Cr(VI) in the Cr(VI) spike (μmol/g);
C
s
III
is the weight of the Cr(III) spike (g);
W
s
VI
is the concentration of Cr(VI) in the sample (μmol/g, unknown);
C
x
B is the bias per mass unit;
c , c is the concentration of analysed sample and spike, respectively (μg/g);
x s
I is the true number of counts, that means the number of counts that would have been detected if
there had been not dead time;
I is the number of counts measured on a channel;
m
K is the mass discrimination factor;
M , M is the atomic mass of isotope "1" and "2", respectively;
1 2
M , M is the atomic mass of analysed sample and spike, respectively (μg/mol);
x s
N is the number of atoms;
i
N , N is the total number of atoms in unknown sample and spike, respectively;
x s
R is the real isotope ratio;
R' is the measured isotope ratio;
R , R is the corrected-isotope ratio and the measured dead-time-corrected isotope ratios of the sample,
c m
respectively;
R , R is the certified isotope ratio and the measured isotope ratios of the standard material (dead time
t m
corrected);
W , W is the weigh of unknown sample and spike, respectively (g);
x s

is the percentage of Cr(III) oxidized to Cr(VI) after spiking (unknown);
 is the percentage of Cr(VI) reduced to Cr(III) after spiking (unknown);

is the dead time;
Aliquat-336: methyltricaprylammonium;
APDC: ammonium pyrolidine dithiocarbamate;
DDTC: diethyldithiocarbamate;
DIN
: direct injection nebulization;
DPC: Diphenylcarbazide;
DPP: differential pulse polarography;
DPTA: Diethylenetriaminepentaacetic;
HHPN
: hydraulic high-pressure nebulization;
HMDE: hanging mercury drop electrode;
HPLC:
high pressure liquid-chromatography;
IC: ion-chromatography;
IC-DPC: ion chromatography diphenylcarbazide;
IC-ICP-MS: ion chromatography inductively coupled plasma mass spectrometry;
ICP-MS:
inductively coupled plasma-mass spectrometry;
ID: Isotope Dilution method;
LC: liquid-chromatography;
LiFDDC: lithium bis(trifluoroethyl) dithiocarbamate;
LL
: Liquid-liquid extraction;
MIBK: methyl isobuthyl ketone;
NPP: normal pulse polarography;
ORP: Oxidation Reduction Potential;
SFE
: Supercritical fluid extraction;
SIDMS: Speciation Isotope Dilution-Mass Spectrometry;
TBDTC: dibuthyl-dithiocarbamate;
TOC: Total Organic Carbon;
TSN
: thermospray nebulization;
4 Chromium VI speciation in solid matrices
4.1 Chromium VI extraction from solid matrices
4.1.1 Sample pre-treatment
To quantify total Cr(VI) in solid matrices, three criteria must be satisfied:
a) the extraction solution must solubilize all forms of Cr(VI);
b) the conditions of the extraction must not induce reduction of native Cr(VI) to Cr(III);
c) the method must not cause oxidation of native Cr(III) contained in the sample.
Thus, it has been recognized that Cr(VI) must be leached from samples in an alkaline medium rather than in acidic
medium in order to inhibit Cr(VI) to Cr(III) reduction by possible reductants present in sample [2]. An alkaline
extraction procedure, USEPA SW-846 Method 3060 for the preparation of soil samples in view of analysis of total
Cr(VI) was used for a number of years. But an USEPA funded research study did not achieved consistent results
among samples using this method [7]. The researches concluded that the Cr oxidation state is matrix specific and
may be unstable and unpredictable (in environmental samples) once it is solubilize in either an acidic or basic
aqueous extraction medium [8]. Based on these considerations, in June 1997 the USEPA promulgated SW-846
rd
Method 3060A for inclusion in the Third Update to the Test Method for Evaluating Solid Waste, SW-846, 3 ed. [5].
Although the basic chemistry has remained the same, the modifications to USEPA SW-846 Method 3060 have
enhanced the efficiency of the extraction process, principally by reducing the soil sample weight and decreasing the
ratio of sample weight to alkaline digest volume.
4.1.2 USEPA SW-846 Method 3060A [9]
4.1.2.1 Summary of USEPA SW-846 Method 3060A
The solid sample is digested using a mixed solution (pH>11,5) consisting of Na CO (0,28 M) and NaOH (0,5 M)
2 3
and heating at 90 °C – 95 °C for 60 minutes, in order to dissolve the Cr(VI) and stabilize it against reduction to
Cr(III)
For waste materials or soils containing soluble Cr(III) greater than four times the laboratory Cr(VI) detection limit,
Cr(VI) results obtained using this method may be high biased because of method-induced oxidation. Thus the
method recommends the addition of Mg(II) in a phosphate buffer to the alkaline extraction solution to suppress this
oxidation. When analysing a sample digest for total Cr(VI) it is appropriate to determine the reducing/oxidizing
tendency of each sample matrix. This can be accomplished by characterization of each sample by means of four
major redox-indicating ancillary parameters:
— pH (USEPA SW-846 Method 160);
— Oxidation Reduction Potential (ORP) (ASTM D1498-76);
— sulfides (USEPA SW-846, Method 9030);
— Total Organic Carbon (TOC) (USEPA SW-846 Method 9060, ASTM-1976).
Based on the research performed on a wide variety of samples using USEPA SW-846 Method 3060A and
departing from the conventional interpretative approach for QC data for total metals, data associated with low or
0 % Cr(VI) matrix spike recoveries must be evaluated in accordance with established redox chemistry of Cr in soils
or sediments. With pH and ORP having such significance with regards to the redox status of a soil or sediment
-
HCrO
sample the method refers to an E-pH diagram for /Cr(OH) (see Annex A), which can be used to asses the
redox characteristics of a sample.
4.1.2.2 Advantages of Method 3060A
The proposed method meets the three previous criteria for a wide spectrum of solid matrices. Under the alkaline
conditions of the extraction, minimal reduction of Cr(VI) or oxidation of Cr(III) occurs.
A quite comprehensive study concerning the efficiency of different methods of Cr(VI) extraction from soils was
carried out [6]. For this task, the USEPA SW-846 Method 3060A (Na CO , 0,28 M and NaOH, 0,5 M and heating)
2 3
was compared with four other digestion methods, using different extractants, namely:
— Distilled water (pH=5,7);
— Phosphate buffer (pH=7,0);
— NaOH 0,1 M (pH=13,0) with sonication;
— Mixture Na CO (0,28 M) + NaOH (0,5 M), without heating.
2 3
Distilled water and phosphate buffer extractions can be used just to quantify soluble and exchangeable forms of
Cr(VI). The fraction of Cr(VI) which can not be solubilized in water or phosphate buffer solution is the non-
exchangeable form. The soluble and exchangeable fractions of Cr(VI) are useful parameters for estimating soils
levels of Cr(VI) that may leach to groundwater, form a soluble "blush" on soil surface or be absorbed by plants and
micro organisms. However, the quantification of total Cr(VI) in soil samples is necessary to assess the Cr hazard in
the environment. The study demonstrated that the heated carbonate-hydroxide solution (USEPA SW-846 3060A)
was the most effective extractant for total Cr(VI) in soils that contain native Cr(VI) or in soils that had a sufficiently
high redox status to maintain chromium as Cr(VI). To asses the redox status of solid matrix, ancillary chemical
2-
parameters, including ORP, pH, S and TOC should be quantified and interpreted to explain poor Cr(VI)
recoveries. It was demonstrated [6] that the strongly reducing samples cannot maintain Cr(VI) laboratory matrix
spikes. Thus, if reducing conditions are shown for Cr(VI), poor spike recovery is probably due to soil reduction and
not attributable to method-induced reduction. But if oxic conditions are indicated by the ancillary parameters (above
-
HCrO Cr OH
/ ( )
4 3
the line, see Annex A), poor spike recovery is probably the result of technical error, since
method-induced reduction is improbable.
Concluding, the high frequency of acceptable matrix spike recoveries attained using even the sparingly soluble
chromate compounds, BaCrO and PbCrO has demonstrated the reliability and robustness of USEPA SW-846
4 4
Method 3060A [5]. The collective research that established the basis for SW-846 Method 3060A demonstrated that
method-induced reduction of Cr(VI) to Cr(III) did not contribute to low or 0 % matrix spike recoveries. The method
contains detailed decision to assist the user in the interpretation of quality control (QC) data that are needed to
substantiate the quantification of the Cr(VI) results. In situation where low or zero percent matrix spike recoveries
were observed and a reducing sample is suspected, USEPA SW-846 Method 3060A stipulates the measurement
of a number of previous ancillary redox-indicating parameters.
4.1.2.3 Limitations of USEPA SW-846 Method 3060A
With respect to a limitation of USEPA SW-846 Method 3060A, method induced oxidation of Cr(III) to Cr(VI) has
been observed in samples demonstrated to contain soluble forms of Cr(III) and high levels of MnO . However, in
most cases, the percentage of Cr(VI) formed will not exceed 15 % [2]. When subjected to aerated alkaline
-
Cr OH
( )
conditions, soluble forms of Cr(III) can form a fresh Cr(OH) precipitate and at pH=12-13. This fresh
precipitate is available to be partially oxidized to Cr(VI) under the aerated conditions. However, with the exception
of a fresh spill of soluble Cr(III), the soil-born forms of Cr(III) found in environmental samples are aged, crystalline,
Cr(OH) and Cr O , both of which have not been observed to oxidize under the aerated alkaline conditions of the
3 2 3
method. Performing a water extraction and analysing the resultant leached for both Cr(VI) and total Cr ; the
presence of soluble Cr(III) in samples can be approximated. If soluble Cr(III) or freshly precipitated Cr(OH) is
suspected of being present in a sample, the method specifies the addition of Mg(II), which is capable to reduce or
eliminate the occurrence of oxidation of Cr(III) to Cr(VI). Moreover, for the fixation of Cr(III), a solution of EDTA is
recommended to use. In this way, losses of Cr(VI) over 7 days have been reported to be reduced from 80 % to less
than 20 % [2].
4.2 Chromium (VI) speciation methods
With regards to chromium speciation in solids, the accuracy of the methods remains a field of additional effort and
improvement. A review on analytical methodologies for chromium speciation in solid matrices by Marques et al [10]
emphasis the lack of reported recovery by most authors. However, the variety of methods for Cr(VI) speciation may
be classified into two fundamental categories:
a) valence-specific-direct measurements (4.2.1)- which include:
— spectrophotometric methods (4.2.1.1);
— electrochemical methods (4.2.1.2).
b) valence-specific-separation measurements (4.2.2)- based on selectively removing of one chromium species
from the sample and subsequent unspecific measurement, by means of straightforward methods, such as: AES,
AAS and MS.
4.2.1 Valence-specific-direct measurements
4.2.1.1 Spectrophotometric methods
Spectrophotometric methods are often used for the determination of the speciation forms of some elements without
preliminary separation. The existing spectrophotometric methods for chromium speciation have a series of
limitations and they are not always suitable for the analytical practice [11]. Thus, the disadvantages of these
methods are the following:
— the molar absorptivities of the ions associated used in these methods are rather low (0,14 x 10 l/mol cm -
8,0 x 10 l/mol cm);
— the color of the used dyes is not stable and the value of blank tests is high.
Therefore, the development of the new analytical procedures with improved sensitivity and selectivity is a very
important question. However, it should be emphasized that nowadays in a quite large number of laboratories the
spectrophotometric methods for Cr(VI) speciation are still used (see Annex B). The most widely used is the method
with diphenylcarbazide (DPC) and this due to the fact it doesn't require organic extractants and more, it is easily
associated with USEPA SW-846 Method 3060A for chromium extraction. The reaction of Cr(VI) with DPC is the
most common and reliable spectrophotometric method for Cr(VI) solubilized in alkaline digestate. The use of DPC
has been well established in a large number of standardized methods, such as:
DIN 19734: 1999 Soil quality-Determination of Chromium(VI) in phosphate extract;
DIN 38405-24: 1987 German standard methods for the examination of water, waste water and sludge;
photometric determination of Chromium(VI) using 1.5 DPC;
ISO 11083: 1994 Water quality-Photometric determination of Chromium VI with 1.5
diphenylcarbazide;
ISO 3856-5: 1984 Determination of hexavalent chromium content of pigment portion of liquid paint or
paint in powder form-spectrophotometric method with diphenylcarbazide;
DIN 53780: 1999 Pigments and extenders-Determination of matter soluble in water-hexavalent
chromium content;
AS 2882 (Australia): 1986 Waters-Determination of chromium (VI) (diphenylcarbazide spectrophotometric
method);
ASTM D 5257: 1997 Test Method for Dissolved Hexavalent Chromium in Water by Ion Chromatography;
ASTM D 5281 1998 Standard Test Method for Collection and Analysis of Hexavalent Chromium in
Ambient Atmospheres
DSF 38929: 1999 Packaging-Requirement for measuring and verifying four heavy metals and other
dangerous substances present in packaging and their release into the environment-
Part 1: Requirements for measuring and verifying four metals present in packaging
(lead, cadmium, chromium VI and mercury);
IRSA (Italy): 1986 Analytical Methods for Waste-Physico-Chemical Parameters, Method No. 16,
Hexavalent Chromium (colorimetric reaction with diphenylcarbazide);
USEPA SW-846 1992 Chromium, Hexavalent (Colorimetric)-method using diphenylcarbazide
Method 7196A:
4.2.1.1.1 Summary of DPC method [12]
Dissolved hexavalent chromium, in the absence of interfering amounts of substances such as Mo, V and Hg is
determined colorimetrically by reaction with DPC, in acidic solution (pH=2). The reaction is sensitive
(
=4,17 x 10 l/mol cm); addition of an excess of DPC yields the red-violet product and its absorbance is
540 nm
measured photometrically at 540 nm. The Cr(VI) reaction with DPC is usually free from interferences; however,
certain substances may interfere if the chromium concentration is relatively low. Hexavalent molybdenum and
mercury salts also react with DPC forming color with the reagent; however, the red violet intensities produced are
much lower than those for chromium at the specified pH. Concentrations of up to 200 mg/l of molybdenum and
mercury can be tolerated. Vanadium interferes strongly, but concentrations up to 10 times that of chromium will not
cause trouble. Iron in concentration greater than 1 mg/l may produce a yellow color, but it is not strong and difficulty
is not normally encountered if the absorbance is measured at the appropriate wavelength.
Even the well-established DPC reaction with Cr(VI) is in fact valence-specific, however it is subject to be interfered
by metal ions and by Cr(VI) reduction in acidic solution. Thus, to avoid the limitations, an extensive sample pre-
treatment is required comprising the following steps [1]:
a) precipitation of polyvalent cations including Cr(III) by phosphate buffer/aluminum sulfate(floculant agent);
b) oxidation of strong reductants by hypochlorite addition;
c) destruction of hypochlorite excess;
d) finally, color development with DPC.
However, DPC method is still one of the most used spectrophotometric method, but with inherent limitations.
A recent paper was published with regards to spectrophotometric determination of Cr(VI) by means of formation
and extraction(in toluene) of Cr(VI) ion associates with symmetric cyanine dyes. The study was carried out with a
number a five cyanine dyes and the molar absorptivity of ion associates is ranging from 2,501 05 l/mol cm -
3,621 05 l/mol cm, depending on the dye used. This method is suitable for speciation measurements without
separation of Cr(VI) and by comparison with known spectrophotometric methods is more sensitive and avoids the
use of hazardous chemicals [11].
4.2.1.1.2 Limitations of the spectrophotometric methods [13]
The distribution of Cr(VI) and Cr(III) species strongly depends on pH and potential. According to the E-pH diagram
of Cr species as given in Annex A, Cr(VI) is thermodynamically stable at relatively high pH and E values, while
Cr(III) at relatively lower pH and E. Corresponding to the change of pH, the formal reduction potential of
Cr(VI)/Cr(III) changes from –0,04 (pH=13) to 0,52 (pH=7,4) then to 1,07 (pH=2). Based on these thermodynamic
data, the reduction of Cr(VI) may occur during neutralization step, because Cr(VI) may react with coexisting
reducing matrix rather than DPC and consequently cause negative errors. In addition, some chromate compounds
have a much lower solubility in the neutral solution than in the strong basic solution.
4.2.1.2 Electrochemical methods
4.2.1.2.1 Summary of electrochemical methods
Up to now there are not many standardized electrochemical methods for Cr(VI) speciation. In literature several
publications using this techniques have been described [1,14,15].
Among the electrochemical methods, mainly the polarographic methods have been used for Cr(VI) speciation. The
classic polarography is a voltammetric method (measuring the electrolysis cell current as a function of electrode
potential) at controlled potential in which the working electrode consists of a dropping mercury electrode and the
potential is changed in a linear mode.
But the most widely electrochemical methods used are normal (NPP) and differential pulse polarography (DPP). In
NPP, the potential is kept at a suitable constant base potential throughout the drop lifetime but in DPP the potential
does not return to a constant value.
An overview of mostly used electrochemical analytical methods, including the USEPA SW-846 Method 7198 [16] is
shown in Table 1 [2]
Table 1 — Electrochemical methods for Cr(VI) speciation
Electrochemical methods for Cr(VI) speciation Detection Limit
(μg/l)
Ammonia buffer solution as the supporting electrolyte; detection with differential 1,6
pulse polarography (USEPA method) ;
Addition of ammonium acetate buffer and ethylenediamine detection with 10,0
differential pulse polarography (DPP);
Differential pulse polarography in 0,2 M NaF solution; 0,8
DPP in 0,1 M dibasic ammonium citrate solution; 2
Polarographic determination in 0,1 M NH Cl+NH /KNO ; catalytic current; 2
4 3 2
pH=10,0;
Separation of interfering cations with aluminum from phosphate-buffered solution; 30
detection with DPP.
By A. C. Harzdorf [1], the polarographic method provides the best potential for Cr(VI) speciation. Hexavalent
chromium is electrochemically active over the entire pH-range, so that medium pH can be chosen throughout which
offers ideal conditions for stabilizing the oxidation states of chromium. Furthermore, a variety of supporting
electrolytes is suitable so that the operating conditions can readily be adapted to the composition of the given
sample. In order to eliminate the interferents as much as possible, polyatomic inorganic cations are removed by
precipitation with phosphate buffer solution. Removal is completed by addition of aluminum sulfate as floculant.
During this treatment, coprecipitation of Cr(VI) proved to be negligible. In the residual phosphate buffer solution,
Cr(VI) can readily measured.
Although differential pulse polarography is the most sensitivity direct polarographic technique, an even greater
sensitivity can be obtained by imploying stripping voltammetry. This technique involves a preconcentration step
before the final voltammetric determination. This step consists of the controlled electrodeposition, at a fixed
potential, of the species of interest on a stationary electrode. This is followed by the determination step, which
consists of electrolytically stripping the deposited species back into solution.
A new sensitive stripping voltammetry method for the determination of trace amounts of total chromium Cr(III) and
Cr(VI) was proposed by Golimovski [14]. The method is based on preconcentration of the Cr(III)-DPTA
(diethylenetriaminepentaacetic) complex by adsorption at HMDE (hanging mercury drop electrode) at the potential
–1,0 V. The adsorbed complex is then reduced producing a response and the peak height of the Cr(III) reduction is
measured. The determination limit is 20 ng/l and the RSD is 5 % for chromium concentrations > 200 ng/l.
4.2.1.2.2 Limitations of the electrochemical methods
Despite the promising features of polarography in the given field, it does not cover all requirements in
environmental chromium analysis because of the limited sensitivity. Moreover, the detection limit strongly depends
on the sample background and the lower limit of the methods is not always suitable for environmental studies.
Stripping voltammetry is an important, but limited technique, mainly because of the pre-concentration step, which
requires the production of an insoluble product that can be reproducibly stripped from the electrode surface in the
determination step.
4.2.2 Valence-specific-separation methods
A separate category of methods for Cr(VI) speciation is based on selective removing of one species from the
subjected sample and subsequent measurement. The separation step comprises primarily:
— chromatography (4.2.2.1);
— extraction (4.2.2.2);
— coprecipitation (4.2.2.3).
The detection usually includes non-specific methods, such as: AES, AAS, MS or spectrophotometric methods.
An interesting overview of the detection techniques for determination of Cr(VI) and/or Cr(III) is presented in
Figure 1 [17]. Figure 1 shows all the techniques employed to determine Cr(VI) and/or Cr(III). UV-VIS spectrometry
is most often used, (33 %). Other techniques such as Atomic spectrometry techniques either flame or furnace
techniques (23 %) or chromatographic techniques (11 %) have also often been used to determine chromium
species.
Figure 1 — Detection methods used for the determination of Cr(VI) and/or Cr(III)
4.2.2.1 Chromatographic separation
The application of chromatographic separation techniques is continuously increasing in the field of Cr(VI)
speciation.
Preconcentration of Cr(VI) has been carried out using different types of columns, such as columns with melamine-
formaldehyde resin, a C18 bonded silica reversed phase sorbent with diethyldithiocarbamate (DDTC) as the
complexing agent, a column of Chromabond NH , a column containing phosphate treated sawdust as adsorbent, a
column containing ZnO and microcolumns such as an alumina micro-column. Different types of resins such as
anion-exchange resins, resins with Amberlite diluted in MIBK and liquid anion exchangers such as Amberlite LA-1
or LA-2 have also been used. Different eluents are used to elute Cr(VI), such as HNO , NH OH or sodium
3 4
acetate [17].
Preconcentration of both species, Cr(VI) and Cr(III) simultaneously can be carried out in different types of columns
packed with materials such as strongly basic anion exchanger with H2SO4, Dionex AG4A, DEAE-Sephadex A-25,
polyacrylonitrile sorbent modified with polyethilenepolyamine, anion exchangers Dionex CS5, Dionex IonPack A57,
Excelpak ICS-A23 etc. They can also be retained on a micro-column packed with activated alumina, on
methyltrioctylammonium chloride-loaded silica gel or on polymeric Dedata sorbent with aminocarboxylic groups.
The most commonly used solvents are HNO , HClO , HCl, H SO and methanol for Cr(III) elution and NH OH,
3 4 2 4 4
HClO , ascorbic acid, Na CO /NaHCO and hydroxylammonium chloride for Cr(VI) elution [17].
4 2 3 3
Thus, the majority of speciation separations is performed by ion exchange chromatography. The ion-exchange
separation of both chromium species in a chromatographic system can be performed based on two different
concepts. Sample species can be separated on a column based on affinity differences of the species for the
column. Using a complexing reagent in the eluant may also separate the sample species. The complexing reagent
changes the form of the sample species allowing the moving down the column to be easier. Thus, the basis of
many of the separations has been to convert Cr(III) to an anion by adding a complexing agent. Cr(VI) is already an
2-
CrO
anion (usually ), hence, anion chromatography can be used to separate the chromium species.
The most widely used complexing agents for Cr(III) are: hydroxy-ethylpiperazine-N'-3-propanesulfonic acid
buffer/methanolic-8-hydroxi quinoline, hydroxyquinoline, diethyldithiocarbamates (DDTC), SCN-,
trifluoroacetylacetone and the pH values for the complexation may vary from pH 2 to pH 8,5 [17].
Interest in element-specific detection for high performance liquid chromatography (HPLC) has increased. A large
number of reviews have been published describing the advantages associated with the use of atomic spectrometric
techniques as detectors for HPLC. Thus, recently plasma source mass spectrometry is recognized as one of the
most powerful analytical technique for trace element analysis of species when coupled to a suitable
chromatographic separation system.
With the application of ICP-MS for chromium speciation, major limitations generally arise from the occurrence of
non-spectral as well as spectral interferences. The elements that can give rise to interference at the analytical
isotope masses of Cr (i.e. 50, 52, 53, 54) are, Ar, C, Ca, Cl, K, O and S, in other words, those normally present in
environmental matrices, the plasma gas and as well as in the reagents used for extraction. Thus, a LC technique
has the great advantage that, besides the speciation of the chromium species, it gives the opportunity to separate
substances, which can otherwise interfere with the isotopes of interest.
A survey of LOD, which were found in literature [4] for different LC techniques coupled with ICP-MS, is given in
Table 2.
Table 2 — Detection limits (μg/l) for Cr(III) and Cr(VI) with different ICP-MS techniques
LC technique Detection limits for Cr(III) Detection limits for Cr(VI)
μg/l μg/l
IC-ICP-MS 0,3 0,5
HPLC-ICP-MS 0,4 1,0
HPLC-DIN-ICP-MS 0,18 0,18
HPLC-HHPN-ICP-MS 0,6 1,8
HPLC-TSN-ICP-MS 2,5 2,3
Abbreviations:
LC: liquid-chromatography;
IC: ion-chromatography; ICP-MS: inductively coupled plasma-mass spectrometry;
HPLC: high pressure liquid-chromatography; DIN: direct injection nebulization;
TSN: thermospray nebulization ; HHPN: hydraulic high-pressure nebulization.
A largely used method of Cr(VI) determinations by means of HPLC is the standardized USEPA SW-846
Method 7199 (see Table 3). This method [18] provides procedures for the determination of hexavalent chromium
using a separation on a column packed with a high capacity anion exchange resin capable of resolving
2-
CrO
from other sample constituents (Dionex IonPack AS7 or equivalent) and a post-column derivatization of
Cr(VI) with DPC in view of detecting of the colored complex at 540 nm. Also, a guard column (Dionex IonPack
2-
CrO
NG1) is used in order to remove organic constituents from the sample before Cr(VI) as is separated on an
anion exchange column. The analyzed sample is adjusted initially at pH=9 to pH=9,5 with a buffer solution and the
eluent used is a mixture of (NH ) SO (250 mM) and NH OH (49 mM).
4 2 4 4
An alternative approach by means of IC was developed by Barnovski et al [4]. In this study, an IonPack AG5
(Dionex) column for anion exchange chromatography was used. The column is designed for anion exchange with a
special selectivity for anions of higher valences like oxoanions. The column material is stable over a pH range from
pH 0 to pH 4. Even if as eluent a mixture of NaHCO -Na CO was recommended for the specified column, in order
3 2 3
to realize the desired exclusion of carbon compounds, this recommendation was not followed; thus, nitric acid was
chosen for conditioning of the column as well as for elution. This eliminated the crucial spectral interference by
40 12
Ar C at mass 52. The choice of nitric acid is possible as a result of the robustness of the column material, which
allows a wide range of pH values for the sample and eluent. It should be emphasized that both species are
retained on the column and separation during elution by one eluent under compromise conditions, on the basis of
the following considerations. The substrate material for ion exchange resins is basically the same for anion
exchange as for cation exchange. If adequately conditioned, a certain exchange capacity will be preserved in anion
exchange operation for cations, also, so that a mixed bed property of the column is realized in favor of the
analytical performance. A similar approach was used by Kingston et al [20], and both Cr(VI) and Cr(III) were
separated on a anion-exchange column (CETAC ANX 4605), using HNO as eluent.
An alternative HPLC speciation of Cr(VI) is Ion Chromatography with chemiluminiscence detection [21]. The
species are separated by ion chromatography followed by post column reduction of Cr(VI) to Cr(III) before
detection. The detection is based on the measurement of the intensity of light emitted when luminol (5-amino-
2,3 dihydro-1.4 -phtalazinedione) is oxidized by hydrogen peroxide in the presence of Cr(III). Potential interference
effects caused by other metal ions that similarly catalyze the same reaction have been masked by adding EDTA to
the sample stream. The developed method is very sensitive and selective. Free Cr(III) ions were found to be only
chromium species that catalyze the reaction and in contrast to the other metals ions, the rate of formation of the
Cr(III)-EDTA complex is slow such that Cr(III) is still in the free state when the luminol reaction occurs. In a further
development, it was shown that the chemiluminescence signal, generated by the Cr(III)-luminol reaction, is
enhanced 6-fold in the presence of bromide ions. Recently, the addition of bromide ions has been used with flow
injection analysis [21]. Ion chromatography with chemiluminescence detection is a sensitive technique (0,025 μg/l,
Detection Limit), a much less costly alternative to most currently available instrumental methods for the study of the
distribution of Cr(VI) and Cr(III) in environmental samples.
Another proposed method for speciation of Cr(VI) is reversed-phase ionic-pair HPLC after formation of an ionic pair
with tetrabuthylammonium. The method does not involve preconcentration and the detection limits are 0,02 mg/l
and 0,08 mg/l for Cr(III) and Cr(VI), respectively [4].
In recent publications [4, 22] a method using a strong anionic phase to separate Cr(VI) and Cr(III) as Cr-EDTA was
proposed. For the separation of the chromium species on an anion exchange column (Excelpak ICS-A23), EDTA-
2NH and oxalic acid were used as the mobile phase (pH=7). Oxalic acid functions as an eluent of the Cr species,
while EDTA functions as an eluent of the Cr species and as a stabilizer of Cr(III) complex. For this method, the
detection limits were 0,4 μg/l for Cr(III) and 1,1 μg/l for Cr(VI).
Table 3 — The chromatographic separation methods used for Cr(VI) speciation [2].
Chromatographic method Detection Detection
method Limit
μg/l
Separation on highly anion exchange capacity column; elution with photometric 0,3
(NH ) SO +NH OH; post-column derivatization with DPC (USEPA
4 2 4 4
method);
Collection of Cr(VI) on polydithiocarbamate resin; digestion of the resin ICP-AES 36
with HNO ;
Extraction with ethylenediamine in water; treatment with Dowex anion- ICP-AES 7
exchange resin after acidification; calculation by difference with untreated
portion;
Collection of Cr(VI) on Sephadex A-25; desorption by hydroxylammonium GF-AAS 0,01
chloride solution;
Preconcentration of Cr(VI) on C-18 column after complexation with sodium GC 0,05
di(trifluorethyl)dithiocarbamate; elution with toluene;
Ion Chromatography with chemiluminiscence detection; photometric 0,025
Separation on highly anion exchange capacity column; elution by nitric MS
acid;
-
Separation of Cr(VI) and Cr(III) (as CrY ); elution with EDTA (ammonium MS 1,4
salt)+oxalic acid.
4.2.2.2 Extraction
4.2.2.2.1 Liquid-liquid extraction (LL)
Separation of Cr(III) and Cr(VI) species may be achieved by selective extraction and mostly used is the selective
separation of Cr(VI) species. The efficiency of the liquid-liquid (LL) extraction methods depends on the sample
composition, since interferences may be expected from other metal ions and should be checked beforehand.
Cr(VI) can be complexed with different dithiocarbamates, such as ammonium pyrolidine- dithiocarbamate,
dibenzyldithiocarbamate, Na-diethyldithiocarbamate, ammonium trifluoracetylacetonate, diphenylcarbazide,
tetrabuthylammonium. In the extraction these complexes, isobuthylmethylketone is most commonly used, followed
by CHCl , hexane and trybuthylphosphate [17].
An interesting approach for Cr(VI) separation by means of LL uses liquid ion exchangers. Thus, the Cr(VI) and
Cr(III) species separation was investigated with different types of liquid ion exchangers (cation and anion
exchanger resins as well as complexing resins). A good separation efficiency of Cr(VI)-Cr(III) separation, in a
-
HCO /CO
3 2
buffer solution was found by an extraction of Cr(VI) with a liquid anion exchanger [19]. A liquid
exchanger, Amberlite LA-2, which was stored in contact with a HCl solution, and MIBK was used. Under these
conditions, Cr(VI) was completely extracted into the organic phase, whereas Cr(III) remained in the aqueous phase
(99 %-100 % yield determined by labeled Cr compounds). However, a critical point using this extraction system is
the separation of the two phases. A total phase separation is only possible by using sufficiently high salt
concentration in the aqueous solution. A
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