Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 145

Table 2.11 Chromium speciation with the suggested configuration and using the
MECROM program

Found (µg L −1 )

Added (µg
mL −1 )

Species of Cr(III) Species of Cr(VI)

Cr
(III)

Cr
(VI)

pH Cr

(H 2 O) 6
3+

Cr
(OH)
2+

Cr
(OH)
2+

P *

Cr
(OH) +

H 2 CrO 4

HCrO − Cr 2 O 7

2−

CrO 4
2−

3.000 0.200 4.51 0.573 2.032 0.203 - - - 0.200 - -

2.500 0.400 4.58 0.497 1.763 0.176 - - - 0.411 - 0.004

2.000 0.600 4.65 0.287 1.440 0.203 - - - 0.600 - 0.008

1.500 0.800 4.78 0.158 1.072 0.204 - - - 0.792 0.002 0.015

1.000 1.000 4.95 0.069 0.696 0.196 - - - 0.960 0.002 0.027

0.500 1.200 5.24 0.015 0.290 0.160 - - - 1.138 0.003 0.064

0.500 0.500 5.26 0.015 0.309 0.178 - - - 0.486 0.001 0.028

2.000 1.200 4.65 0.280 1.405 0.198 - - - 1.178 0.004 0.017

1.500 0.300 4.75 0.180 1.115 0.193 - - - 0.314 - 0.005

P * =fraction precipitated as hydroxide
Source: Reproduced with permission fromTaylor and Francis, UK [221]

analysed by atomic absorption using the chromium 357.9 line to give the concentrations
of cationic, anionic and non-ionic chromium. The concentration of chromium(III) is
probably closely related to the sum of the cationic and non-ionic fractions and the
concentration of chromium(VI) corresponds to the anion portion.
The concentrations of total, anionic, cationic and non-ionic chromium in the original
3L sample are obtained by difference. The precisions of the analyses for total, anionic,
cationic and non-ionic chromium were respectively ±14, ±20, ±20, and ±25% relative.
Subramanian [226] studied the factors affecting the determination of trivalent and
hexavalent chromium (chromate) by direct complexation with ammonium
pyrrolidinedithiocarbamate, extraction of the complex into methyl isobutyl ketone, and
determination by graphite furnace atomic absorption spectrometry. Factors studied
included the pH of the solution, concentration of reagents, period required for complete
extraction, and the solubility of the chelate in the organic phase. Based on the results,
procedures were developed for selective determination of trivalent and hexavalent
chromium without the need to convert the trivalent to the hexavalent state. For both states
the detection limit was 0.2µg L −1 .