A proposed calcium hydroxyapatite (HA) reference material has been prepared in large quantity in accordance with the following reaction in solution at 100°C : 5CaO+3H3PO4--Ca5(PO4)3OH+H2O

Stoichiometric HA powder with an atomic Ca/P ratio of 1.67 was obtained as determined by chemical analysis. The X-ray powder diffraction technique was used to evaluate the crystallinity with the measured width at half height of the diffraction peaks. The infrared spectra of the HA powder were obtained to determine the structure and to detect any minor phases. The thermal analysis indicated thermal stability and no decomposition. The thermodynamic solubility product (K^sp) of this HA, expressed as - logK^sp (pK^sp), was determined to be 57.7 at 25°C. Three additional HA samples prepared by different methods were also characterized and compared.