Solubility of Ca5{P04)3OH in the System Ca{OH)2-H3P04-H20 at 5, 15, 25, and 37 °C

Abstract

Solubility isotherms of hydroxyapatite, Ca5(PO4)3OH (OHAp), prepared by titrating a boiling aqueous suspension of Ca(OH)2 with 0.5 M H3PO4, were determined in the ternary system Ca(OH)2–H3PO4–H2O at 5, 15, 25, and 37 °C in the pH range 3.7–6.7 by equilibration with dilute H3PO4 solutions. The solubility product Ks, determined as a function of temperature by a generalized least-squares procedure from 41 experimental points, is given by the equation logKs=−8219.41/T−1.6657−0.098215T.The values of Ks 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 Ks near 16 °C, which may be due to the form of temperature dependence found earlier for the stability constants of the ion pairs and .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.