The reaction of the SO₃^·- radical with Fe^II in acidic aqueous solution—A pulse radiolysis study

Abstract

The reaction between Fe^II and the SO₃^- radical has been studied in aqueous solution at pH 4.0 using the technique of pulse radiolysis. The first-order rate constants for the formation of Fe^III were not proportional to [Fe^II] and also depended on [S^IV]. It is concluded that an Fe^II-sulfito complex is formed with a stability constant, K₁₃, of (69.7±18.8) d mol^-1 and that this complex reacts more slowly with SO₃^- than does hexaaquo-Fe^II. The oxidation of hexaaquo-Fe^II by SO₃^- involves electron transfer within an inner-sphere precursor complex. On the assumption that the rate of oxidation of the Fe^II-sulfito complex is negligible compared with that of the hexaaquo-Fe^II, we conclude that the precursor complex, (Fe_(aq)²⁺···SO₃^-), has a stability constant, K₁₂, =(278±39) d mol^-1 and that electron transfer within the complex proceeds with a rate constant, k₁₂, =(3.05±0.32)×10⁴ s^-1. The speciation of the Fe^III produced depends on the [S^IV] and [Fe^II] and sulfito bridged Fe^III–Fe^II complexes are thought to exist at the higher [Fe^II].

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