Speciation of Cr(VI) in soil extracts by polarographic methods

Polarography is very suitable to determine Cr(VI) in soil extracts. The best supporting electrolyte is NaOH 0.1 M. The detection limit (DL) of differential pulse polarography (DPP) for Cr(VI) was 1.59 x 10(-5) M, and for Cr(III) was 2.85 x 10(-5) M. The extraction of Cr(VI) was carried out with tris-NaOH at pH 13. Recovery of this element added to an anthrophic soil of a lacustrine zone was 98.6%. Two signals were observed by DPP in basic extracts from soils contaminated with Cr(VI) and equilibrated at pH 2 and 4. One corresponded to the Cr(VI)/Cr(III) system. The other was asymmetric and disappeared with the addition of CrO2-2. This signal can be related to a chromium-natural organic matter interaction (Cr NOM). The soluble humic acids have the same suppressing effect of Triton-X-100, inhibiting the appearance of a polarographic maxima. The formation of a Chromium (III) oxidized organic matter chelate (Cr(III)-NOMox) is discussed.