Solubility of calcium fluoride and fluorapatite by solid titration

Abstract

Objectives: The solubility isotherms, S, of two compounds detected after topical fluoride treatment - calcium fluoride (CaF2) and fluorapatite (FAp) - are of fundamental interest in saliva chemistry and in the context of reduction of acid dissolution of teeth, whether through the process of caries or from exogenously ingested acids. Solid titration has shown its reliability and reproducibility for complicated systems that are not suitable for study by the traditional excess-solid method. The primary aim of this work was to ascertain S[CaF2] and S[FAp]. Methods: Solid titration was used for CaF2 (pH 2.3-9.5) and FAp (pH 2.8-5.1) in 100 mM KCl solution at 37.0 ± 0.1 °C, and further to determine the apparent solubility of hydroxyapatite (HAp) in the presence of 1 mM fluoride (pH 3.2-4.8). Results: Peculiar results were obtained at first which were attributed to the adsorption or reaction of fluoride with the reaction vessel glass surface interfering with the intended solution equilibria. Wax-lined glass apparatus resolved this problem. The solubility isotherm of CaF2 was then as theoretically expected at pH < 8, but above this point a new solid species (CaFOH) was postulated to account for the data. The position of FAp was as expected relative to HAp, being about 0.63× less soluble. FAp was the only detectable equilibrium solid at pH 3.2, 3.6 and 4.1. The apparent solubility of HAp was depressed somewhat by the presence of 1 mM fluoride. Conclusion: The solid titration method was again found to be reliable once glass interferences were eliminated. The interaction of fluoride with borosilicate glass may have affected other work in the field; such work may therefore require re-evaluation. The S[FAp] is very similar to that of HAp determined by solid titration. Excess-solid method results are strongly discrepant from the present determination and may not be reliable, primarily due to lack of solution speciation data for that calculation. © 2007 Elsevier Ltd. All rights reserved.