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

A simple and rapid potentiometric FIA method far the sequential determinati on of Cr(VI) and Cr(III) is described, using both a redox electrode detecto r 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 (H2 SO4 solution or Ce(IV) solution containing H2SO4) and the Fe(III)-Fe(II) po tential buffer solution, was constructed by equipping a six-way valve betwe en 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) solut ion containing H2SO4 in the reagent solution. For the selective determinati on of Cr(VI) in a mixed sample of Cr (VI) and Cr(III), the valve was snitch ed to flow a reagent stream of the H2SO4 solution into the potential buffer solution. Then, Cr(Vl). reacted with the potential buffer, and a peak sign al was obtained by the detector. For determining the total chromium, the su m of Cr(VI) and Cr(III), the valve was switched to flow the reagent stream of Ce(TV) 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 s ignal was obtained by the detector. Cr(III) in the mixed sample was determi ned by the difference of the peak heights obtained by the two reagent strea ms. The detection of Cr(VI) was based on the measurement of a large transie nt potential change due to bromine generated by an oxidation reaction betwe en Cr(VI) and bromide in the potential buffer solution. The linear relation ships between the peak heights and the sample concentration were observed o ver the range of 1 x 10(-6) to 10 x 10(-6) M for Cr(VI) and from 4 x 10(-6) to 4 x 10(-5) M for Cr(III) by using a 0.01 M Fe(III)-Fe(II) potential buf fer containing 0.6 NI NaBr and 3.6 M H2SO4.