Kinetic Studies of the Decomposition of Tris(oxalato)cobaltate(III) Ion in Binary Solvents of Water with Organic Solvents

Abstract

The kinetic studies of the decomposition of tris(oxalato)cobaltate(M) ion [Co(C204)8]3- were made in the binary solvents of water with methanol (MeOH), ethanol (Et0H), t-butyl alc o hol (t-BuOH), dimethyl sulfoxide (DMSO), and acetonitrile (AN) at an ionic strength of O.2 mol dm-3 and at 25--40°C. In all solvents the rate of decomposition of the cobalt(III) complex ion was described as d[[Co(C204)3]3-]= k obs[[Co (C2O4)3]3-], where (1/2)kobs=k0+kh[H+]+k2h [H+]2 in the solutions of the neutral to acidic region. The values of ko, kh, and kth were determined under various conditions. The ko value was independent of the sorts of solven t of the binary-aqueous media, and also of the mole fraction of solvents. On the other hand, th e values of kh and km increased in the following order: t-BuOH<Et0H<Me0H<DMSO<AN at a mole fraction of 0.2, where the activation parameter s were determined for the each reaction path of ko, kh, and k2h. They were also dependent on the mole fraction of solvents u s e d and increased with an increase in the amount of the organic constitute. The activationent ropy aelΔh for the ko path was negative or very small if positive in all solvents, while ΔSh and Δ2h were, in general, very large in all solvents. The results obtained were discu s s ed in terms of (1) the permittivity of solvents, ( 2 ) the stability of the solvated proton (HSalv+), and ( 3 ) the stability of the proposed intermediate species with solvation.