keywords: Binuclear complex, buffer, kinetics, mechanism, metabisulphite, oxo-bridged, redox reaction
Redox reaction between S2O and [Fe2O(phen)4(H2O)2]4+ in aqueous phenanthrolinium buffer medium, pH 4.50, has been studied. This was carried out under pseudo first order condition such that [S2O ] >> [Fe2O(phen)4(H2O) ] by monitoring the formation of [Fe(phen)3]2+, the reduced product of the binuclear complex, on a UNICO UV–2102PC Spectrophotometer at 510 nm, its wavelength of maximum absorption (λmax). The initial reaction rates were determined with variation in the initial concentrations of S2O , pH and ionic strength of the solution. First order dependence on both the oxidant and reductant concentrations were observed. The reaction rate was observed to increase with increase in pH in the range 3.50–4.50 but decreases with increase in ionic strength from 0.10 to 0.60 mol dm-3. Based on the kinetic data and other experimental observations, a plausible reaction mechanism was suggested utilizing the redox characteristics of the binuclear complex and S2O . This mechanism involves the oxidation of sulphur from the oxidation state of +4 in SO to +6 in SO and the reaction appears to proceed through the formation of an adduct intermediate complex, with HSO considered to be sulphur bonded to [Fe2O(phen)4(H2O) ]+. Evidence for the formation of sulphite ion radical (SO .) via a one–electron transfer step is presented.