Sequential determination of chromium (VI) and chromium (III) by potentiometric flow-injection method

A simple and rapid potentiometric FIA method for the sequential determination of Cr(VI) and Cr(III) is described, using both a redox electrode detector and a stream of an Fe(III)- Fe(II) potential buffer containing bromide. A three-channel flow system, a carrier stream (water), a reagent stream (H₂SO₄ solution or Ce(IV) solution containing H₂SO₄) and the Fe(III)-Fe(II) potential buffer solution, was constructed by equipping a six-way valve between the stream of the reagent solution and the carrier stream. The valve was used to switch the stream of either the H₂SO₄ solution or the Ce(IV) solution containing H₂SO₄ in the reagent solution. For the selective determination of Cr(VI) in a mixed sample of Cr(VI) and Cr(III), the valve was switched to flow a reagent stream of the H₂SO₄ solution into the potential buffer solution. Then. Cr(VI) reacted with the potential buffer, and a peak signal was obtained by the detector. For determining the total chromium, the sum of Cr(VI) and Cr(III). the valve was switched to flow the reagent stream of Ce (IV) solution into the potential buffer solution. Cr(III) in the mixed sample was oxidized to Cr(VI) by Ce(IV) in the reagent stream and a peak signal was obtained by the detector. Cr(III) in the mixed sample was determined by the difference of the peak heights obtained by the two reagent streams. The detection of Cr(VI) was based on the measurement of a large transient potential change due to bromine generated by an oxidaton reaction between Cr(VI) and bromide in the potential buffer solution. The linear relationships between the peak heights and the sample concentration were observed over the range of 1 × 10^-6 to 10 × 10^-6 M for Cr(VI) and from 4 ×10^-6 to 10 ×^-5 M for Cr(III) by using a 0.01 M Fe(III)- Fe(II) potential buffer containng 0.6 M NaBr and 3.6 M H₂SO₄.