Solubility behavior of carbonated apatite in the presence of solution fluoride

The objective of this study was to quantitatively understand the solubility properties of carbonated apatite in the presence of solution fluoride. The first project of this research was to assess the applicability of the Metastable Equilibrium Solubility (MES) concept for the study of CAP solubility behavior over a range of pH and a wide range of solution fluoride concentrations, and to examine the hypothesis that in the presence of solution fluoride, a surface complex with the stoichiometry of fluorapatite (FAP) governs the MES. The findings of this study demonstrated that the CAPs investigated exhibit the MES distribution phenomenon in solution media of varying pH and fluoride concentrations. The experimental MES data obtained with these CAP preparations at the lower pH (4.5) and at higher solution fluoride levels (> 0.1 ppm) suggested that in the presence of solution fluoride the MES governing surface complex may be an entity possessing a stoichiometry of FAP. Previous studies have shown that the MES behavior of CAPs may be described by a surface complex with the stoichiometry of hydroxyapatite (HAP) and that the magnitude of the MES is determined by the CAP crystallinity. In the presence of solution fluoride, employing the results that a surface complex of FAP stoichiometry controls the dissolution of CAPs, the MES distributions of CAP preparations of various solubilities can be determined. In this study, it has been found that there is a linear relationship between the CAP solubility and the CAP crystallinity parameter and that the slope of the CAP mean pKhap and the mean pKfap are essentially parallel. Knowledge of the lowest concentrations (thresholds) of solution fluoride which influence the MES of CAPs is critical in understanding the mechanism and stoichiometry controlling the solubility of CAPs at extremely low solution fluoride levels. The major finding of this third study is that the influence of fluoride on the CAP solubility becomes asymptotically negligible as solution fluoride is reduced to ultra-low levels, i.e., < 0.02 ppb. Also, at these low fluoride levels (i.e., < 0.02 ppb), the solubility governing function is consistent with a surface complex possessing the HAP stoichiometry.