The effect of stannous fluoride, sodium fluoride and stannous chloride on the crystallization of dicalcium phosphate dihydrate at constant pH

Abstract

A kinetic study has been made of the crystal growth of dicalcium phosphate dihydrate (DCPD) seed crystals from supersaturated solution at constant pH. A pH value of 5.6 was chosen in order to be able to compare the results with a previous investigation in which the growing crystals were in contact with a supersaturated solution whose pH was allowed to vary during the growth experiments. The rate of crystallization has been found to follow an equation of the form — dN dt = kSN 2 , where N represents the number of moles/1 of solid to be precipitated before equilibrium is reached, k is the rate constant and S is proportional to the number of active growth sites. In contrast to the additive sodium fluoride, both stannous fluoride and stannous chloride are effective inhibitors of crystal growth. The degree of inhibition, expressed in terms of the relative values of the rate constant k, can be interpreted in terms of an adsorption process at the active crystal growth surface sites.