Kinetics and mechanism of the redox behaviour of the ethylenediaminetetraacetatoferrate(III)–sulfite system in aqueous solution

Abstract

The decomposition reaction of [Fe^III(edta)(SO₃)]^3– has been studied as a function of total sulfite, [Fe^III(edta)^–], oxygen concentration, pH and temperature using spectrophotometric, potentiometric and ion-chromatographic techniques. The catalysed autoxidation of sulfite is significantly slower for [Fe^III(edta)]^– than for aquated iron(III) ions. The reaction exhibits a strong oxygen dependence, and the participation of oxygenated complexes is suggested to account for this effect. The major redox products are sulfate and [Fe^II(edta)]^2–, which is rapidly reoxidized to [Fe^III(edta)]^– in the presence of oxygen. Various possible redox mechanisms are discussed, and the results are compared to corresponding data for the non-chelated iron(III) system.