SIST-TP CEN/TR 14589:2004
(Main)Characterization of waste - State of the art document - Chromium VI specification in solid matrices
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 matieres solides.
Karakterizacija odpadkov - Dokument o stanju tehnike - Določevanje Cr (VI) v trdnem
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
SIST-TP CEN/TR 14589:2004
01-maj-2004
.DUDNWHUL]DFLMDRGSDGNRY'RNXPHQWRVWDQMXWHKQLNH'RORþHYDQMH&U 9, 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
SIST-TP CEN/TR 14589:2004 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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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.
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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
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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:
6
-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,
3
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.
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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
50
50
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
50
III
is the concentration of Cr(III) in the Cr(III) spike (μmol/g);
C
s
53
VI
is the concentration of Cr(VI) in the Cr(VI) spike (μmol/g);
C
s
50
III
is the weight of the Cr(III) spike (g);
W
s
4
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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;
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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.
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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
4
sample the method refers to an E-pH diagram for /Cr(OH) (see Annex A), which can be used to asses the
3
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;
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— 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
2
most cases, the percentage of Cr(VI) formed will not exceed 15 % [2]. When subjected to aerated alkaline
-
Cr OH
( )
4
conditions, soluble forms of Cr(III) can form a fresh Cr(OH) precipitate and at pH=12-13. This fresh
3
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
3
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.
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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:
4
— the molar absorptivities of the ions associated used in these methods are rather low (0,14 x 10 l/mol cm -
4
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
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4
( =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]
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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 a
...
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