The growth of nonstoichiometric apatite from aqueous solution at 37°C: II. Effects of pH upon the precipated phase

Abstract

Apatite growth was studied at 37°C over a pH range 6.0 to 9.0 using the constant composition method. Observed changes in the total calcium and phosphate molar concentrations of the supersaturated solution during experiments, with effective titrant calcium/phosphate molar ratios of 1.67 or 1.40, were interpreted in terms of the formation of nonstoichiometric apatites having the formula Ca 10−u( HPO 4) 6−u( OH) 2−u The calcium/phosphate atomic ratio of the grown phase, verified by X-ray diffraction studies, was found to vary from 1.49 to 1.65 as the working solution pH changed from 6.0 to 9.0. At a pH of 6.0, it was estimated that protonation of surface phosphate would change the calcium/phosphate atomic ratio by only 0.02. It is therefore concluded that the nonstoichiometry was associated with that of the grown material. There was no indication of the existence of other types of calcium vacancies. The nonstoichiometric apatites from these seeded growth experiments were compared to those produced by coversion of an amorphous calcium phosphate phase.