Kinetics and mechanism of the oxidation of [N-(2-hydroxy-ethyl)ethylenediamine-N,N′,N′-triacetatocobalt(II)] by vanadate ion

The kinetics of oxidation of CoIIHEDTA {HEDTA = N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid} by vanadate ion have been studied in aqueous acid in the pH range 0.75–5.4 at 43–57 °C. The reaction exhibits second-order kinetics; first-order in each of the reactants. The reaction rate is a maximum at pH = 2.1. A mechanism is proposed in which the species [CoIIHEDTA(H2O)]− and VO2+ react to form an intermediate which decompose slowly to give pentadentate CoIIIHEDTA(H2O) and VIV as final products. The rate law was derived and the activation parameters calculated: ΔH* = 26.96 kJ mol−1 and ΔS* = −311.08 JK−1 mol−1.