Kinetics and equilibria in the redox reaction between ethylenedinitrilotetra-acetate complexes of iron(III) and vanadium(III) in aqueous solution

Abstract

The kinetics of the reaction [Fe^III(edta)]^–+[V^III(edta)]^–⇌[Fe^II(edta)]^2–+[V^IVO(edta)]^2–+ 2H⁺(edta = ethylenedinitrilotetra-acetate) have been studied at 25.0 °C in the forward direction at pH 5.4–7.1 and in the reverse direction at pH 2.5–4.0. Similar studies have been made on the corresponding system involving iron complexes with (trans- 1,2-cyclohexylenedinitrilo)tetra-acetate. Direct measurements of the equilibrium constants at pH 4.2–5.6 yielded values similar to those calculated from the kinetic results. The mechanism is believed to be of the inner-sphere type. At pH ⩽ 1 the reaction [Fe^III(edta)]^–+[V^III(edta)]^–→ Fe²⁺+ V^IV+ H₅edta⁺ occurs, the consumption of hydrogen ion and the form of vanadium(IV) depending on [H⁺]. The kinetics of this reaction show that as well as a bimolecular redox process, probably of the inner-sphere type, two other reaction paths occur. These involve the dissociation of one of the reactant complexes followed by a redox reaction between the cation so formed and the other edta complex.