Solubility of synthetic calcium hydroxyapatites

Abstract

The solubilities of synthetic calcium hydroxyapatites [Ca(OH)A], Ca₅(PO₄)₃OH, are determined in the ternary system Ca(OH)₂+ H₃PO₄+ H₂O at 25°C in the pH range 5–7 by equilibration with dilute H₃PO₄ solutions. The thermodynamic solubility product K_sp of precipitated Ca(OH)A, prepared by titrating a boiling aqueous suspension of Ca(OH)₂ with H₃PO₄, is (3.2₅± 0.2₉)× 10^–58 mol⁹ dm^–27. Ca(OH)A prepared by solid state reaction of CaCO₃ and CaHPO₄ and precipitated Ca(OH)A heated at high temperatures in a water vapour atmosphere showed no constant K_sp. After being washed, these samples equilibrate according to the usual solubility product principles with a K_sp value of (3.4₄± 0.3₆)× 10^–59 and (2.6₀± 0.2₁)× 10^–59 mol⁹ dm^–27, respectively.

The influence of heat treatment and washing on the solubility behaviour is discussed. Deviations from solubility behaviour which accords with the usual solubility product principles are caused by reactions occurring at the surface of the solid. The latter are induced by a high free energy of the solid and/or the presence of a fraction with high solubility associated with the Ca(OH)A sample.