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 [NiII(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 [NiII(edta)]2– to form a five-co-ordinated NiIII 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 [NiIII(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 [NiIII(edta)(H2O)]– undergoes transformation to give a radical intermediate which reacts with [NiII(edta)]2– to give ligand degradation products of which glyoxylic acid and formaldehyde were detected.