Effect of fluoride on apatite formation from Ca4(PO4)2O in 0.1 mol L(-1) KH2PO4

The effect of fluoride on the hydrolysis of tetracalcium phosphate (TTCP; Ca4(PO4)2O) was investigated in 0.1 mol l(-1)KH2PO4 containing 0-83 mol l(-1) KF. Characterization of the final apatite phase formed by the hydrolysis was made with X-ray diffraction and SEM. The initial pH was between 4.5 and 5.4, depending on the solutions, and the pH rapidly increased and was kept constant between 7.3 and 6.5. An increase in KF concentration tended to lower the pH in the final stage of hydrolysis. The calcium concentration was considerably lower than the phosphorus concentration throughout the reaction. The fluoride concentration decreased shortly after the start of hydrolysis. The hydrolysis of TTCP in 0.1 mol l(-1) KH2PO4 proceeded to form hydroxyapatite via DCPD when the KF concentration was low. The hydrolysis product was a calcium-deficient non-stoichiometric hydroxyapatite with a Ca/P ratio of about 1.5. With an increase in the KF concentration in the 0.1 mol l(-1) KH2PO4 solution, TTCP directly transformed into hydroxyapatite containing F- ions or fluorapatite and with improved crystallinity. The addition of fluoride in the solution initially accelerated the formation of apatite. However, the layer of newly formed apatite adhering to the TTCP particles retarded TTCP dissolution; as a result, hydrolysis was delayed. IR analysis showed that the apatite phase contained HPO2-4 ions in the structure. The formula for the hydrolysis product of TTCP in the presence of fluoride can be expressed as follows: Ca10-x(HPO4)x(PO4)10-x(OH)2-x-yFy.