Solubility of Ca(5)(PO(4))(3)OH in the System Ca(OH)(2)-H(3)PO(4)-H(2)O at 5, 15, 25, and 37 °C

Solubility isotherms of hydroxyapatite, Ca(5)(PO(4))(3)OH (OHAp), prepared by titrating a boiling aqueous suspension of Ca(OH)(2) with 0.5 M H(3)PO(4), were determined in the ternary system Ca(OH)(2)–H(3)PO(4)–H(2)O at 5, 15, 25, and 37 °C in the pH range 3.7–6.7 by equilibration with dilute H(3)PO(4) solutions. The solubility product K(s), determined as a function of temperature by a generalized least-squares procedure from 41 experimental points, is given by the equation [Formula: see text] The values of K(s) and its dispersion at 25 and 37 °C are 3.04 (0.25) and 2.35 (0.27) × 10(−59). There is a maximum in K(s) near 16 °C, which may be due to the form of temperature dependence found earlier for the stability constants of the ion pairs [Formula: see text] and [Formula: see text]. The relative positions of the isotherms show that OHAp has a negative thermal coefficient of solubility. Thermodynamic functions for the dissolution of the salt are reported. The solubility data previously reported by others for OHAp at 25 °C were reviewed. The solubility products obtained by three of these investigators were comparable with our value of 3.0 × 10(−59); their data were reevaluated by the method described here. We conclude that the best value for the solubility product at 25 °C is 4.7 (2.0) × 10(−59).