The hydroxyapatite ion activity product in acid solutions equilibrated with human enamel at 37 degrees C

Five samples of powered enamel were each sequentially equilibrated 5 times at 37 degrees C with either 4 or 17 mmol/l H3PO4, in contact with air, and the ionic activity product for hydroxyapatite (IHA) estimated. There was evidence for preferential release of Mg and Na, especially in the first equilibrations. In two experiments, raised values of IHA were observed in the first equilibration but otherwise IHA was reasonable constant within experimental error and was much closer to the solubility product of hydroxyapatite than many previous estimates, mainly at 25 degrees C, suggest. The mean value of IHA for the majority fraction, averaged over all samples, was 1.7 (+/- 0.7) x 10(-58). Non-apatitic solids formed in all systems but solubility appeared nevertheless to be controlled by an apatitic phase: either the enamel mineral itself or apatite reprecipitated during the course of equilibration. High values of IHA reported previously may be due to use of conditions favouring dissolution only of more soluble factions or to metastability artifacts associated with control of solubility by non-apatitic phases.