Sequential preconcentrative separation of Cr(VI) and Cr(III) species from tannery effluents and subsequent determination by flame AAS

A procedure for chromium speciation prior to flame atomic absorption spectrometry (FAAS) detection is proposed. In this procedure, chromium(III) and chromium(VI) species were sequentially separated using an off-line batch coprecipitative preconcentration onto bivalent or trivalent metal-diethylammonium diethyl-dithiocarbamate (DDTC) collectors. This separation is based on the selective coprecipitation of chromium(III) onto a nickel-DDTC collector followed by coprecipitation of chromium(VI) onto a bismuth(III)-DDTC collector. The preconcentrated metal-DDTC collectors were dissolved in 5 mL of a 1:1 mixture of nitric acid and methanol. Linear calibration plots were obtained for both chromium(III) and chromium(VI) species in the concentration range of 0-200 mu g/L in the initial solution. A 200-fold enrichment of chromium(III) and chromiurn(VI) was observed with detection limits of 0.4 and 0.6 mu g L-1, respectively. Five replicate determinations of 20 mu g of chromium(III) and chromium(VI) present in a 1.0-L sample solution gave a relative standard deviation of 2.8 and 3.2%, respectively. The developed procedure has been successfully utilized for the estimation of chromium(III) and chromium(VI) in several tannery industry effluents.