The reaction of the SO3 center dot- radical with Fe-II in acidic aqueous solution - A pulse radiolysis study

The reaction between Fe-II and the SO3.- radical has been studied in aqueou s solution at pH 4.0 using the technique of pulse radiolysis. The first-ord er 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 comp lex is formed with a stability constant, K-13, 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 wit hin an inner-sphere precursor complex. On the assumption that the rate of o xidation of the Fe-II-sulfito complex is negligible compared with that of t he hexaaquo-Fe-II, we conclude that the precursor complex, (Fe-(aq)(2+)... SO3-), has a stability constant, K-12, =(278 +/- 39) d mol(-1) and that ele ctron transfer within the complex proceeds with a rate constant, k(12), =(3 .05 +/- 0.32) x 10(4) 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 th ought to exist at the higher [Fe-II].