Effect of calcium ions on transformation brushite to hydroxyapatite in aqueous solutions

The conversion of brushite into hydroxyapatite (HAP) by hydrolysis has been studied in alkaline solutions at pH 10.8 and temperature of 39 °C with and without addition of calcium ions. Morphology of particles was analyzed by SEM. It was shown that the precipitation of hydroxyapatite crucial depends on temperature and pH. The stoichiometric hydroxyapatite as thermodynamically the most stable form of calcium phosphates was obtained in the solution without addition of Ca2+ ions after long-time hydrolysis only. In pure aqueous solution at applied pH, the surface nucleation of HAP and gradual transformation of brushite by dissolution–precipitation mechanism were confirmed. The addition of Ca2+ ions into solution causes the acceleration of brushite conversion to calcium deficient HAP forms.