Effects of pH and reaction temperature on hydroxyapatite powders synthesized by precipitation

Hydroxyapatite (HA) was synthesized through the precipitation method and different processing parameters (Ca/P molar ratio, pH, and reaction temperature) were varied to investigate their influence on the HA formation. No HA powder was obtained at pH 10 and 25 °C, even when using a Ca/P ratio as high as 2.2. However, HA powders were successfully produced at pH above 11, 25 °C, and Ca/P ratio of 2.2. At pH 10 and 25 °C, the concentration of the H⁺ ions in the reaction solution increased and so did the Ca loss, resulting Ca-deficient hydroxyapatite (CdHA) formation. While HA was formed instead due to a lower Ca loss when the pH was increased to 11 and 11.3. As the reaction temperature was increased to 70 and 90 °C, the HA formation occurred regardless of the pH because of the decreased solubility of HA in the solution.