Solubility and surface properties of finely divided hydroxyapatite

Abstract

Synthetic hydroxyapatite (OHAp), Ca5(PO4)3OH, was prepared by slow addition of phosphoric acid to a boiling Ca(OH)2 suspension. The solid, precipitated at a pH of about 12, was stabilized by prolonged boiling. The Ca/P ratio of the product was 1.69 +/- 0.04 and it had a specific surface area of 26 m2/g. Samples of OHAp were equilibrated with phosphoric acid solutions. Within the pH range studied, 5 to 7, the solubility product, (Ca)5(PO4)3(OH), was found to be 6.3 +/- 2.1 X 10-59 at 25 deg C. Substantial deviations of the overall dissolution reaction from OHAp stoichiometry were observed and are attributed to nonstoichiometric reactions occurring at the surface. A linear dependence of excessive phosphorus on the surface with decrease in pH was found. Estimates on the extent of deviation from stoichiometry caused by the surface reaction are given.