Theory of electrolyte crystal growth. The parabolic rate law

Abstract

Normally the growth of sparingly soluble electrolyte crystals in aqueous solution follows a parabolic rate law, rate (c -c)2. At non- equivalent concentrations, and in general, rate (fl1' -K1)2 where 11 = ionic product, K5 = solubility product, and V = ci. + for the electro- lyte It is shown how this can be explained by the Burton-Cabrera-Frank theory adapted for electrolytes, assuming that the ions are adsorbed in electroneutral (equivalent) amounts on the crystal surface. The absolute rate is controlled by the integrating process and the activation energy corresponds to the removal of one water molecule from the hydration shell of the cation in the case of 1,1 and 1,2 electrolytes. For 2,1 and 2,2 electrolytes the ratio between the activation energy and the energy of removal of a water molecule from the inner hydration shell is between 1.4 and 1.7.