Calcium phosphate system in saliva-like media

Abstract

Despite the well documented importance of carbon dioxide dissolved in physiological media, consideration of its effect on calcium phosphate solubilities has been neglected, an omission now shown to be serious. A solid titration technique has been developed to avoid a number of the difficulties inherent in solubility studies of calcium phosphates; this employed a graphical method based on pH and calcium-ion electrode potential to identify breakpoints. The solubility isotherm at 37.0 °C for hydroxyapatite has thus been determined for the chosen saliva-like medium. A numerical model of the system provided corroboration for the results obtained. An approximately 10-fold increase in the calcium-carrying capacity of that medium was then demonstrated with CO₂ present at a fixed partial pressure of 0.035 bar, a result not explicable in terms of known equilibria. Two new complexes, CaH₂PO₄H₂CO⁺₃(β= 54) and CaH₂PO₄HCO₃(β= 43), are postulated to account for the data.