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The reaction between FeII and the radical has been studied in aqueous solution at pH 4.0 using the SO 3 ~technique of pulse radiolysis. The Ðrst-order rate constants for the formation of FeIII were not proportional to [FeII] and also depended on [SIV]. It is concluded that an FeII-sulÐto complex is formed with a stability constant, of (69.7 ^18.8) dm3 mol~1 and that this complex reacts more slowly with than does K 13 , S O 3 ~hexaaquo-FeII. The oxidation of hexaaquo-FeII by involves electron transfer within an inner-sphere SO 3 ~precursor complex. On the assumption that the rate of oxidation of the FeII-sulÐto complex is negligible compared with that of the hexaaquo-FeII, we conclude that the precursor complex, has a (Fe (aq) 2`É É ÉSO 3 ~), stability constant, \(278 ^39) dm3 mol~1 and that electron transfer within the complex proceeds with a K 12 , rate constant, \(3.05 ^0.32) ] 104 s~1. The speciation of the FeIII produced depends on the [SIV] and k 12 , [FeII] and sulÐto bridged FeIIIÈFeII complexes are thought to exist at the higher [FeII].

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