Differential determination of chromium(VI) and total chromium in natural waters using flow injection on-line separation and preconcentration electrothermal atomic absorption spectrometry

Abstract

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 µl 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 3σ). 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.