Preparation and Solubility of Hydroxyapatite.

Two portions of a synthetic hydroxyapatite (HA), Ca5OH(PO4)3, fully characterized by x-ray, infrared, petrographic, and chemical analyses, were heated at 1,000 °C in air and steam atmospheres, respectively. Solubility isotherms for these two samples in the system Ca(OH)2-H3PO4-H2O were determined in the pH range 5 to 7 by equilibrating the solids with dilute H3PO4 solutions. Both samples of HA dissolved stoichiometrically. The activity products ( Ca + + ) 5 ( OH - ) ( PO 4 ≡ ) 3 and their standard errors-obtained by a least squares adjustment of the measurements (Ca and P concentrations and pH of the saturated solutions) subject to the conditions of electroneutrality, constancy of the activity product, and stoichiometric dissolution - were 3.73 ± 0.5 × 10-58 for the steam-heated HA and 2.51 ± 0.4 × 10-55 for the air-heated HA. Allowance was made in the calculations for the presence of the ion pairs [CaHPO4]0 and [CaH2PO4]+. The higher solubility product for the air-heated HA is ascribed either to a change in the heat of formation brought about by partial dehydration or to a state of fine subdivision resulting from a disproportionation reaction. The solubility product constants were used to calculate the points of intersection (i.e., singular points) of the two HA solubility isotherms with the isotherms of CaHPO4 · 2H2O and CaHPO4; it was found that the pH's of the singular points for the air-heated HA were a full unit higher than those of the steam-heated preparation. Conditions are described for the precipitation of HA crystals suitable for solubility measurements.