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A study of the solubility of synthetic hydroxyapatite. Sleeman, Kenneth Jack


study was made of the solubility of synthetic hydroxyapatite (Ca₁₀(PO₄)₆(OH₂). The basic calcium phosphate was synthesized in CO₂ free systems, over a pH range of 5.0 to 8.0 using reaction temperatures of 40°, 60°, and 90°C, and reaction periods of 24 and 96 hours. The study was divided into two phases. In one phase the pH of the suspensions, and the calcium and phosphorus concentrations in solution were measured after precipitation and after redispersing the preparations in water. The application of solubility criteria to these studies showed that the differences in solubility obtained by synthesis with varying reaction period and temperature were not brought about by conditions of super saturation. Solubility was found to decrease with increased synthesis reaction period and increased temperature of synthesis. With a given temperature and synthesis reaction period the hydroxyapatite preparations had a uniform solubility. Solubility changed only when the conditions of synthesis were changed. X-ray diffraction analysis of the solid phase indicated that the solubility of hydroxyapatite decreased with increasing crystallinity. Examination with the electron microscope, however, showed that increased crystallinity was not due to increasing crystal size. Owing to the smallness of crystals, it was not possible to examine the hydroxyapatite with a petrographic microscope, and hence it was not possible to detect the presence of amorphous phases. It is possible, then, that small amounts of amorphous substances were present in the solid phase, and, as a result the apparent degree of crystallinity was altered. In the second phase, studies of the Ca:P mole ratios of the preparations showed that Mole ratios were not constant for any mode of synthesis. Further studies indicated that adsorption was of no, or at best, minor importance in determining the Ca:P ratios of hydroxyapatite. Isomorphous substitution with OH¯ions replacing PO₄¯ ions, and H₃O⁺ ions replacing Ca⁺⁺ ions might have occurred, but it was not possible to detect solubility differences which should result from this substitution. Hence it was not possible to correlate solubility with composition of hydroxyapatite. Even though the hydroxyapatite appeared to be of variable composition, it was found that this basic calcium phosphate had a definite pKsp value in the pH range of 5.0 to 8.0.

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