Transformation of Hydroxyapatite to Fluorapatite

When solid hydroxyapatite and solid calcium fluoride were equilibrated at 25C with dilute solutions of HCl, analysis of the solutions yielded an average value of 118.8 for the negative logarithm of the ion‐activity product for fluorapatite (10 pCa + 2 pF + 6 pPO 4 ). This value compares favorably with values of 118.4, 119.2, and 120.9 reported by other investigators for the negative logarithm of the activity solubility‐product constant for fluorapatite. A plot of the negative logarithm of the activity‐product of (H) 3 (PO 4 ) against the negative logarithm of the activity‐product of (Ca)(OH) 2 in solution yielded a regression line with a slope of −1.52, which is close to the theoretical value of −1.50 for fluorapatite in a solution saturated with calcium fluoride and is greater than the theoretical value of −1.67 for hydroxyapatite. The results indicate that at least the surface of the hydroxyapatite had been converted to fluorapatite during the equilibration with solid calcium fluoride.