The effect of citric acid addition on the formation of sol–gel derived hydroxyapatite

In order to improve the gelation in sol–gel preparation of hydroxyapatite by using Ca(NO3)2 and PO(OH)3−x(OEt)x precursors, citric acid was selected as an enhancing gelation additive. Hydroxyapatite derived from the mixed precursor solutions (Ca/P molar ratio of 1.67) with citric acid showed a different reaction path from that without citric acid. The addition of citric acid led to formation of amorphous as-prepared powder, the powders calcined at 650 °C contained three phases: hydroxyapatite, carbonated hydroxyapatite and a small amount of β-tricalcium phosphate. The powders calcined at 800 °C contained only a single phase of oxy-hydroxyapatite. It is suggested that citric acid plays a role in enhancing gelation through a strong coordination ability of Ca ions with citrate groups.