Effect of carbonate and phosphate ratios on the transformation of calcium orthophosphates

Complexes among phosphate, carbonate and calcium have been synthesized by a designed hydrothermal method. Effects of carbonate and phosphate ratios on the transformation of calcium-orthophosphates were investigated. With X-ray diffraction measurement the synthesized product at the low (0.15) and the high (1.8) molar ratio of PO43−/CO32− is calcium phosphate hydrate at pH 9.0, and hydroxylapatite (HAp) at pH 8.0, respectively. Fourier transform infrared spectroscopy of product at the high ratio (1.8) of PO43−/CO32− shows that the CO32− peaks disappear, and the strong peaks at 1412 and 1460 cm−1 are assigned to the vibrations of PO43− in HAp. 31P nuclear magnetic resonance spectra of products at the low (0.15–0.6) to the high (1.2–1.8) ratios of PO43−/CO32− are obtained at 2.9 and 2.7 ppm, respectively. Molar ratios of PO43−/CO32− are effective on the reduction of carbonate activity during the formation and infiltration events of calcium-phosphate surface precipitates, and are subsequently enclosed during HAp formation.