EFFECT OF FLUORIDE ON APATITE FORMATION FROM CA-4(PO4)(2)O IN 0.1 MOLL-1 KH2PO4

The effect of fluoride on the hydrolysis of tetracalcium phosphate (TT CP; Ca-4(PO4)(2)O) 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 initia l pH was between 4.5 and 5.4, depending on the solutions, and the pH r apidly increased and was kept constant between 7.3 and 6.5. An increas e in KF concentration tended to lower the pH in the final stage of hyd rolysis. The calcium concentration was considerably lower than the pho sphorus concentration throughout the reaction. The fluoride concentrat ion decreased shortly after the start of hydrolysis. The hydrolysis of TTCP in 0.1 mol l(-1) KH2PO4 proceeded to form hydroxyapatite via DCP D when the KF concentration was low. The hydrolysis product was a calc ium-deficient non-stoichiometric hydroxyapatite with a Ca/P ratio of a bout 1.5. With an increase in the KF concentration in the 0.1 mol l(-1 ) KH2PO4 solution, TTCP directly transformed into hydroxyapatite conta ining F- ions or fluorapatite and with improved crystallinity. The add ition 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 HPO42- i ons in the structure. The formula for the hydrolysis product of TTCP i n the presence of fluoride can be expressed as follows: Ca10-x(HPO4)(x )(PO4)(10-x)(OH)(2-x-y)F-y.