Solubility of Ca5{P04)3OH in the System Ca{OH)2-H3P04-H20 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 equationlogK s = − 8219.41 / T − 1.6657 − 0.098215 T . 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 pairsCaH 2PO 4 +andCaHPO 4 0. 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 .