The kinetics of dissolution of calcium oxalate monohydrate; a constant composition study

Abstract

The dissolution of calcium oxalate monohydrate, COM, has been studied by a constant composition technique as a function of concentration, temperature, fluid dynamics and in the presence of crystallization inhibitors. The method enables kinetic data to be obtained at very low undersaturation with a precision hitherto unobtainable. A striking change in the mechanism of dissolution is observed as the undersaturation is reduced. At high undersaturation the first-order dependence of the rate of dissolution on the undersaturation suggests the diffusion controlled process normally assumed for dissolution reactions. At very low undersaturations, however, the effective order of reaction appears to approach the value of 2, proposed for a surface controlled reaction. Moreover, the two regions of undersaturation show markedly different dependencies on changes in temperature, hydrodynamics and upon the addition of adsorbing molecules. The theory of Burton, Cabrera, and Frank, as applied to dissolution processes, predicts a change in reaction order from n = 1 to n = 2 as the undersaturation is decreased. This paper represents the first indication of such a change in reaction order for the dissolution of an electrolyte.