Kinetics of the Dissolution of Human Dental Enamel in Acid

¹ Miami Valley Laboratories, Procter and Gamble Company, Cincinnati, Ohio

The kinetics of the heterogeneous process of dissolving the solid, human dental enamel, in acidic buffers have been studied and the results expressed in the form of a general differential equation. The solubility rate of enamel in acid solutions was measured by using a differential method to determine the initial rate and using an integral method to determine the approach to equilibrium. The dissolution involved a reaction between the hydrogen ion of the solution and the major constituent of enamel, an apatitic calcium phosphate. The effects of temperature and agitation showed that this reaction was diffusion-controlled. The initial rate of dissolution was a function of the hydrogen-ion concentration and, in the presence of buffers, also depended on the concentration of undissociated acid, which could furnish hydrogen ions to the site of the reaction at the enamel surface. Values calculated from an equation derived for the initial dissolution rate as a function of the hydrogen-ion concentration, buffer concentration, and buffer dissociation strength agreed well with experimental values. The initial rate was decreased by the presence in solution of the reaction products—calcium and phosphate ions, of all cations, and of fluoride ion. A mechanism for this retarding action was proposed, involving the formation of a coating on the enamel surface from the reaction products reprecipitating or precipitating with added or foreign ions.