The reaction of the SO3·- radical with FeII in acidic aqueous solution—A pulse radiolysis study
Abstract
The reaction between FeII 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 FeIII were not proportional to [FeII] and also depended on [SIV]. It is concluded that an FeII-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-FeII. The oxidation of hexaaquo-FeII by SO3- involves electron transfer within an inner-sphere precursor complex. On the assumption that the rate of oxidation of the FeII-sulfito complex is negligible compared with that of the hexaaquo-FeII, 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 FeIII produced depends on the [SIV] and [FeII] and sulfito bridged FeIII–FeII complexes are thought to exist at the higher [FeII].