Kinetics of reduction of hexacyanoferrate(III) by thiosulfate ions mediated by ruthenium dioxide hydrate

Abstract

The kinetics of reduction of hexacyanoferrate(III) by excess thiosulfate, mediated by RuO₂·xH₂O, are investigated. At high concentrations of S₂O₃^2–(0.1 mol dm^–3) the kinetics of Fe(CN)₆^3– reduction are first order with respect to [Fe(CN)₆^3–] and [RuO₂·xH₂O] and independent of [Fe(CN)₆^4–], [S₂O₃^2–] and [S₄O₆^2–]. At relatively low concentrations of S₂O₃^2–(0.01 mol dm^–3) and in the presence of appreciable concentrations of Fe(CN)₆^4– and S₄O₆^2–(0.01 mol dm^–3] the kinetics depend directly upon [Fe(CN)₆^3–] and [RuO₂·xH₂O] and inversely upon [Fe(CN)₆^4–]. Both sets of kinetics can be rationalised using an electrochemical model of redox catalysis in which a reversible reduction reaction [Fe(CN)₆^3–+ e^–→ Fe(CN)₆^4–] is coupled to an irreversible oxidation reaction [S₂O₃^2–– e^–→½S₄O₆^2–), by a dispersion of RuO₂·xH₂O microelectrodes. At high concentrations of S₂O₃^2– this model predicts that the kinetics of Fe(CN)₆^3– reduction are controlled by the rate of diffusion of the Fe(CN)₆^3– ions to the RuO₂·xH₂O particles. The kinetics observed at low concentrations of S₂O₃^–2 are predicted by the electrochemical model. assuming that the Tafel slope for the oxidation of S₂O₃^2– to S₄O₆^2– on the RuO₂·xH₂O particles is 56.4 mV decade^–1.