Oxidation of nickel(II) ethylenediaminetetraacetate by carbonate radical

Abstract

Reactions of the carbonate radical (CO₃^•–), generated by photolysis or by radiolysis of a carbonate solution, with nickel(II) ethylenediaminetetraacetate [Ni^II(edta)]^2– were studied at pH 11.4 and ionic strength (I)= 0.2 mol dm^–3. Time-resolved spectroscopy and kinetics for the transients were studied using flash photolysis whereas the stable products arising from ligand degradation of the complex were ascertained by steady-state radiolysis experiments. From the kinetic data it is suggested that CO₃^•– reacts initially with [Ni^II(edta)]^2– to form a five-co-ordinated Ni^III species [k=(4.5 ± 0.4)× 10⁷ dm³ mol^–1 s^–1] which undergoes aquation with a first-order rate constant of (1.1 ± 0.3)× 10³ s^–1 to give [Ni^III(edta)(H₂O)]^–. The latter reacts with iodide with a rate constant of (5.5 ± 0.4)× 10⁵ dm³ mol^–1 s^–1. At the alkaline pH used [Ni^III(edta)(H₂O)]^– undergoes transformation to give a radical intermediate which reacts with [Ni^II(edta)]^2– to give ligand degradation products of which glyoxylic acid and formaldehyde were detected.