The chemical equilibrium of phosphate aqueous solutions is discussed. To obtain the concentration of the species in solution as a function of pH, we used an acid-base equilibria system, complex formation equilibria, and balance mass equations; and to resolve the non-linear equation system we used a computer program PV-WAVE with ZEROSYS subroutine function. It will be demonstrated that the chemical composition of the calcium phosphate precipitate depends decisively on the species concentration in solution. Also, the pH and the reaction conditions for the selective precipitation of hydroxyapatite are critical. A small variation of these conditions allows either the coprecipitation of other phosphates or no precipitation at all. The solubility variation of the phosphates as a function of the solution ionic force is also discussed. Finally, proper reaction conditions are proposed for hydroxyapatite growth starting from saturated solutions of calcium phosphates.

Keywords: Calcium phosphates, Hydroxyapatite, Precipitation, Computer simulation, Phase diagrams