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.

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