Mechanism for the formation and growth of ionic precipitates from aqueous solution

Abstract

The rate of growth of slightly soluble ionic precipitates from aqueous solution is affected by the presence of a co-ordinated layer of water molecules clustered around a central ion. When the ions are small and highly charged, water molecules in the cluster are firmly held; in such solutions it is known that ion association takes place. In this paper, a mechanism for ionic growth is considered in which a preliminary association to form an uncharged complex, followed by a secondary association with free ions, precedes the incorporation of an ionic cluster at the surface site. Particular mechanisms for the formation of precipitates of barium and calcium sulphates, magnesium hydroxide and magnesium ammonium phosphate are formulated. Kinetic expressions derived for the growth reaction from the postulated mechanisms are found to agree with expressions derived from experiment.