Potentiometric FIA of chromium(VI) in seawater

Abstract

A sensitive and rapid potentiometric FIA method for the determination of Cr(VI) in seawater is proposed, using both a redox potential electrode and an Fe(III)-Fe(II) potential buffer containing bromide. The method is based on the detection of a large transient potential change due to bromine generated by an oxidation reaction between bromide and Cr(VI) in the presence of Fe(II). The oxidation reaction was estimated to be induced by Fe(II) from spectrophotometric studies. A sensitive determination of Cr(VI) was achieved by detecting the sharp transient potential change within 10 s after mixing the Cr(VI) sample with the potential buffer by using an appropriately designed FIA system. Analytical sensitivity obtained by the proposed method is enhanced to about 70 times that of on FIA system using a potential buffer without bromide. A detection limit of 2×10 ^-8 M Cr(VI) was achieved by using a 1×10 ^-4 M Fe(III)-Fe(II) potential buffer containing 0.4 M NaBr and 1.2 M H₂SO₄. The R.S.D. was 0.6% (n=6) for the determination of 1×10 ^-6 M Cr(VI) with the sampling rate of about 40 h ^-1. Several heavy metal ions such as Cr(III), Cu(II), Zn(II), Ni(II) and Cd(II) did not interfere with the determination of Cr(VI) even in the case where their concentration was more than 10³ times that of Cr(VI). The proposed method was applied to the determination of Cr(VI) in real seawater sample, and was found to provide a good recovery for seawater samples containing Cr(VI) at the 500 ppb level, a concentration of effluent common in Japan.