Solubility of hydroxyapatite as a function of solution composition (experiment and modeling)

Abstract

Solubility of hydroxyapatite in aqueous solutions and artificial blood serum was studied at 37 ℃. The obtained results demonstrate non-monotonous changes of the concentration of calcium ions in solution over time: the respective curves contain a maximum. This may be evidence in favor that dissolution of hydroxyapatite is an incongruent process. It was shown that equilibrium can be achieved in three weeks. The logarithm of activity of calcium ions exhibits a direct relation on the pH. Results of the determination of the chemical composition of solid phase obtained by XPS, EDX and ICP-OES methods confirm the incongruent nature of hydroxyapatite dissolution. In this study, we have performed a modeling of the chemical and phase equilibria in solution that served as a prototype of the blood serum with application to calcification of tissues. The concentrations of molecular–ionic forms containing calcium and hydrogen cations and phosphate anions were calculated, and the nonideality of the solutions was accounted for by the Debye–Hückel theory. The calculated degrees of salt supersaturation of different phosphates, which tend to crystallize in blood serum, demonstrate that hydroxyapatite is the most supersaturated phosphate at any values of calcium and phosphorus overall concentrations.