A Theoretical Consideration on Critical pH in Enamel Demineralization

—Why the Critical pH is Estimated to be Around 5.3—

Abstract

 This article briefly reviews the history of the concept of critical pH for enamel demineralization, and its theoretical background with some comments on enamel acid erosion. It is known that the critical pH is dependent on not only the acidic pH but also calcium and phosphate ion concentrations in the acidic medium. Moreover, the critical pH is influenced by the solubility of enamel; i. e., the thermodynamic solubility product. This study estimated the critical pH values at different Ca²⁺ and phosphate concentrations using calculation formulas for degree of saturation (DS) of acidic mediums regarding enamel and pure hydroxyapatite minerals, where mathematical algorithms were employed including dissociation constants of relevant chemical reactions, electrical neutrality, mass balance of the ions such as calcium and phosphate, and so on. Representative calculations based on calcium and phosphate concentrations measured in plaque fluid found the critical pH value to be around 5.15. Similarly this calculation found quite a wide range of critical pH value for different enamel solubility products, with 5.02 for the least soluble and 5.81 for the most soluble enamel minerals, respectively. The calculated critical pH values are discussed in relation to the actual demineralizing plaque in vivo, where ionizable calcium and phosphate substances are present in plaque. These substances will be ionized when acid is produced and increase Ca²⁺ and phosphate ion concentrations, resulting in lower critical pH than the theoretically calculated pH. Moreover, the actual rate of enamel dissolution in vivo at the critical pH would be substantially very low due to the inherent nature of the solubility product of apatite.