Hydrolysis of alpha-tricalcium phosphate in NaF solutions

Controlling the incorporation of Na and fluoride in apatite-based dental restoratives and analogs of bone formed under physiologic conditions may be desirable in controlling their long-term in vivo responses. Formation of these analogs was investigated. Hydrolysis of alpha-tricalcium phosphate (alpha-TCP) was carried out at physiologic temperature in NaF solutions and the solids product(s) were analyzed. alpha-TCP hydrolysis was complete within 48 h and the only solid products identified were apatites. Changes in solution chemistry were determined at 48 h and 4 weeks. Na and Ca concentrations were determined by spectrometric methods, P and F concentrations were determined by ion chromatography; pH values were obtained. The extent of fluoride and sodium incorporation in the apatites formed varied depending on the NaF concentration of the solution. Both increased with increasing NaF concentration. Apatite compositions ranged from Ca9.06(HPO4)0.94(PO4)5.06OH1.06 for hydrolysis in water to Ca9.28Na0.56(HPO4)(x)(PO4)(6-x)F0.920Hy for hydrolysis in 0.1 M NaF solution. The proportions of PO4, HPO4 and OH in Na- and F-containing apatites are indeterminate but obey the equation (y = constant+x). The (Ca+Na)/P ratio of the apatite formed by alpha-TCP hydrolysis in 0.1M NaF approaches 1.67 indicating the vacant Ca sites become almost completely filled by the Na ions. The surface areas of these apatites initially remain constant with increasing NaF concentrations; at elevated concentrations apatite surface areas decrease with increasing NaF concentration. This is consistent with the removal of Na from solution by its incorporation in the apatites, not by its adsorption onto their surfaces.