An infrared and x-ray diffractometric study of the incorporation of fluoride into hydorxyapatite under conditions of the cyclic variation of pH.

The solid products from the interaction of hydroxyapatite with supernatant solutions of sodium fluoride under conditions where the pH of the supernatant have been cyclically varied between the limits pH 4 and pH 7 were examined by chemical analysis, infrared spectrophotometry, X-ray diffraction and thermogravimetry. During the initial lowering of the pH from 7 to 4, there is a dissolution of the hydroxyapatite, followed or accompanied by a precipitation of calcium fluoride formed by reaction of the Ca²⁺ ions released with supernatant fluoride ions. On raising the pH from 4 to 7, this latter phase reacts with supernatant H₂PO₄^? ions to form fluoroapatite. Subsequent cyclic variations in the pH increasingly yield a more homogeneous product. The efficiency of fluoride incorporation into the hydroxyapatite depends on the number of complete fluctuation cycles employed. The results are discussed in terms of the dynamic equilibrium existing at the solid-liquid interface.