The kinetics of crystallization and dissolution of barium oxalate dihydrate and cadmium oxalate trihydrate

Abstract

The kinetics of crystallization and dissolution of barium oxalate dihydrate and cadmium oxalate trihydrate have been studied at 25°C by following the changes in conductivity when supersaturated or subsaturated solutions are inoculated with seed crystals. In additive-free solutions, the growth of barium oxalate dihydrate seed crystals follows a rate law second order with respect to supersaturation, and a surface-controlled reaction is proposed. The same kinetic equation may be used to interpret the growth of this salt from supersaturated solutions containing a variety of phosphonate additives. However in these experiments the reaction is preceded by an initial fast surge in the concentration—time profiles. The rate of growth of cadmium oxalate trihydrate crystals varies linearly with supersaturation and this can be explained in terms of a surface diffusion model for growth. The dissolutions of both barium oxalate dihydrate and cadmium oxalate trihydrate follows a rate law in which the rate of reaction is proportional to the undersaturation and evidence is presented for a liquid transport rate-determining step for the dissolution reactions. Although both the crystallization and dissolution of cadmium oxalate trihydrate follow similar first order kinetic equations the appreciable differences in the values of the rate constants suggest that they are not reciprocal processes.