Simultaneous potentiometric determination of ClO₃/^--ClO₂/^- and ClO₃/^--HClO by flow injection analysis using Fe(III)-Fe(II) potential buffer

A rapid potentiometric flow injection technique for the simultaneous determination of oxychlorine species such as ClO₃/^--ClO₂/^- and ClO₃/^- HClO has been developed, using both a redox electrode detector and a Fe(III)- Fe(II) potential buffer solution containing chloride. The analytical method is based on the detection of a large transient potential change of the redox electrode due to chlorine generated via the reaction of the oxychlorine species with chloride in the potential buffer solution. The sensitivities to HClO and ClO₂/^- obtained by the transient potential change were enhanced 700-800-fold over that using an equilibrium potential. The detection limit of the present method for HClO and ClO₂/^- is as low as 5 x 10^-8 M with use of a 5 x 10^-4 M Fe(III)-1 x 10^-3 M Fe(II) buffer containing 0.3 M KCl and 0.5 M H₂SO₄. On the other hand, sensitivity to ClO₃/^- was low when a potential buffer solution containing 0.5 M H₂SO₄ was used, but could be increased largely by increasing the acidity of the potential buffer. The detection limit for ClO₃/^- was 2 x 10^-6 M with the use of a 5 x 10^-4 M Fe(III)-1 x 10^-3 M Fe(II) buffer containing 0.3 M KCl and 9 M H₂SO₄. By utilizing the difference in reactivity of oxychlorine species with chloride in the potential buffer, a simultaneous determination method for a mixed solution of ClO₃ ^--ClO₂/^- or ClO₃/^--HClO was designed to detect, in a timely manner, a transient potential change with the use of two streams of potential buffers which contain different concentrations of sulfuric acid. Analytical concentration ranges of oxychlorine species were 2 x 10^-5-2 x 10^-4 M for ClO₃/^-, and 1 x 10^-6-1 x 10^-5 M for HClO and ClO₂/^-. The reproducibility of the present method was in the range 1.5-2.3%. The reaction mechanism for the transient potential change used in the present method is also discussed, based on the results of batchwise experiments. The simultaneous determination method was applied to the determination of oxychlorine species in a tap water sample, and was found to provide an analytical result for HClO, which was in good agreement with that obtained by the o-tolidine method and to provide a good recovery for ClO₃/^- added to the sample.