DIFFERENTIAL DETERMINATION OF CHROMIUM(VI) AND TOTAL CHROMIUM IN NATURAL-WATERS USING FLOW-INJECTION ONLINE SEPARATION AND PRECONCENTRATION ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY

A rapid, sensitive and selective method for the differential determination of Cr(III) and Cr(VI) in natural waters is described. Chromium(VI) can be determined directly by flow injection on-line sorbent extraction preconcentration coupled with electrothermal atomic absorption spectrometry using sodium diethyldithiocarbamate as the complexing agent and C18 bonded silica reversed-phase sorbent as the column material. Total Cr can be determined after oxidation of Cr(III) to Cr(VI) by potassium peroxydisulfate. Chromium(III) can be calculated by difference. The optimum conditions for sorbent extraction of Cr(VI) and oxidation of Cr(III) to Cr(VI) are evaluated. A 12-fold enhancement in sensitivity compared with direct introduction of 40-mu-l samples was achieved after preconcentration for 60 s, giving detection limits of 16 ng l-1 for Cr(VI) and 18 ng l-1 for total Cr (based on 3-sigma). Results obtained for sea-water and river water reference materials were all within the certified range for total Cr with a precision of better than 10% relative standard deviation in the range 100-200 ng l-1. The selectivity of the determination of Cr(VI) was evaluated by analysing spiked reference materials in the presence of Cr(III), resulting in quantitative recovery of Cr(VI).