Solubility of β-Ca3(PO4)2 in the System Ca(OH)2-H3PO4-H2O at 5, 15, 25, and 37 °C.

Solubility isotherms of beta-tricalcium phosphate, β-Ca(PO4)2, prepared by heating mixtures of CaCO3 and CaHPO4 above 800 °C, were determined in the ternary system Ca(OH)2-H3PO4-H2O at 5, 15, 25, and 37 °C in the pH range 6.0-7.5 by equilibration with dilute H3PO4 solutions. The results indicate that β-Ca3(PO4)2 has a negative thermal coefficient of solubility. The solubility product, K s , was determined as a function of temperature by a generalized least-squares procedure; the resulting equation is log K s = - 45723.26 / T + 287.4536 - 0.546763 T ; the values of K s and its dispersion at 25 and 37 °C are 1.20(0.056), and 0.283(0.011) × 10-29. Thermodynamic functions for the dissolution of the salt at the four experimental temperatures are reported. When treated as an adjustable constant, the Ca/P ratio in these β-Ca3(PO4)2 solutions was found to have the value 1.514(0.010), confirming that the stoichiometry of the high temperature form of this salt is correctly indicated by the above formula. The computed isotherms at 25 °C for several calcium phosphates are compared; it is shown that β-Ca3(PO4)2 is, next to Ca5(PO4)3OH, the most stable at pH's above 6.36. The singular points of the β-Ca3(PO4)2 isotherm with those of CaHPO4 · 2H2O and CaHPO4 are at pH's 5.88 and 6.36, respectively.