Mechanisms of Enamel Dissolution in Acid Buffers

A method of analyzing enamel dissolution data has been presented. It is based on the consideration of simultaneous diffusion and chemical reactions of the various ionic and molecular species involved. The technique permits a critical test of examination of various physical models with experimental data. The method has been applied to Gray's data on the initial rates of enamel dissolution in acidic buffers. The results of the analyses of two models show that a solution diffusion-controlled mechanism, involving dissolution from a hydroxyapatite surface in equilibrium with the solution adjacent to the surface, best explains all the data. Both pH effects and buffer-concentration effects are quantitatively in agreement with this model. Common ion effects are also satisfactorily accounted for, and rate determinations in several acidic buffers agree well with the theory. At very high initial rates of dissolution, the theory indicates that conversion to dicalcium phosphate may become possible. That conversion actually takes place remains uncertain because of the uncertainties in the values of the parameters used in the theoretical calculations.