Interaction of Citric or Hydrochloric Acid with Calcium Fluorapatite: Precipitation of Calcium Fluoride

The interaction of citric acid (H(3)Ci) with calcium fluorapatite(Ca(10) F(2)(PO(4))(6)) was explored for two reasons: (i) to determine the role of the acid in the dissolution process and hence in the mechanism of tooth fluoridation and (ii) to determine whether there is any formation of calcium citrate. It was found that the concentration of calcium or fluoride ions is not stoichiometric with respect to that of phosphate ions in the solution and the stoichiometric deficiency of the amount of fluoride ions in the solution is twice that of the calcium ions and this demonstrates that some calcium fluoride is precipitated. The interaction may be represented by the following chemical equation (pH approximately 2.5): where x (presently < 1) is related to the amount of CaF(2) that has precipitated and may be calculated from the experimental ratio of Ca to P in the solution. In order to establish that the interaction occurs generally with all acids, the reaction of hydrochloric acid with fluoroapatite was studied, and exactly analogous behavior was observed. These facts are also in accord with the solubility of fluorapatite and calcium fluoride. When the amounts of Ca and P in solution are corrected for the precipitated CaF(2), the ratio of Ca to P becomes stoichiometric (= 1.67). Preliminary X-ray analysis of the reacted fluorapatite showed that it contained calcium fluoride. The precipitated CaF(2) may act as a reservoir for the subsequent fluoridation of the mineral and may inhibit bacterial action in the mouth. The concentration of citrate ions does not change in solution, and no formation of calcium citrate is observed.