Kinetic radiochemical study of cobalt(II) ion substitution in zinc(II) EDTA in aqueous buffer

Abstract

The kinetics of Co(II) ion substitution in Zn(II)-EDTA chelate was studied experimentally at concentrations of zinc ion and zinc-EDTA chelate of the order of 10 −3 M, considerably in excess of cobalt ion at 5 × 10 −5 M, and over a pH range of 4·4 to 6·4 at 20, 25, and 30°C. The reaction was followed by the rate of incorporation of radiotracer 60Co ion in the EDTA chelate. Under these conditions and in a medium of 0·05 M acetate buffer, the reaction was found to be quasi first order in cobalt ion and the dependence of the rate of the forward reaction of the solution variables was given, to within 6·6 per cent on the average, by the rate equation: ▪ where L represents the ligand EDTA and the rate constants have the values at 20°C of: κ 1 = 5·15 × 10 −8/sec, κ 2 = 0·122 1./mole per sec, and κ 3 = 20·7 1./mole per sec. Activation energies corresponding to the three rate constants were obtained by repeating some experiments at 25 and 30°C and assuming that the same reaction mechanism applied at the higher temperatures and the experimental values were 20, 16·5 and 6·4 kcal/mole. The direct substitution of cobalt(II) ion in Zn-EDTA chelate contributed a major part of the overall process except at lower pH where protonation of chelate and subsequent dissociation was appreciable. Reaction involving the species CoOH + was also indicated. Comparison of the kinetics of the Co(II)-ZnEDTA reaction with previous studies of Co(II)-Co(II)EDTA and Zn(II)-Zn(II)EDTA isotopic exchange showed that the cross-substitution was much slower than other reactions involving the zinc-EDTA chelate and occurred at a rate comparable to Co(II) exchange. It seems possible that the mechanism of the cross exchange reaction is similar to that involved in the Co(II)-Co(II)EDTA and Zinc(II)-Zinc(II)EDTA exchange reactions.