Effect of phosphate salts concentrations, supporting electrolytes, and calcium phosphate salt precipitation on the pH of phosphate buffer solutions

Abstract

A monobasic and a dibasic phosphate salt are usually added to water to form a standard phosphate buffer solution. The solution pH depends primarily on the concentration ratio R of the two salts, and secondarily on their concentrations and on the concentrations of the supporting electrolytes such as NaCl or KCl. Various ideal and non-ideal solution thermodynamic models are presented. The ideal solution model predicts that the pH should depend only on R, and underestimates the pH at physiological osmolarities by 0.3–0.5 pH units at both 298 K and 310 K, depending on the phosphate, NaCl, and KCl concentrations. Model predictions using the non-ideal extended Debye–Hückel (D–H) equation agree with the data up to ca. ±0.1 pH units at 298 K and 310 K. The pH predictions for the standard isotonic phosphate buffer saline, containing 2 mM of monobasic and 8 M of dibasic sodium phosphate, 137 mM of NaCl, and 2.7 mM of KCl agree well with the data. When CaCl 2 is added, up to 3 mM, a phosphate salt often precipitates, affecting the free Ca 2+ ion concentration, the phosphate ion concentrations, and the pH. The conditions for the formation at equilibrium of hydroxyapatite (Ca 5(PO 4) 3(OH)), or dicalcium phosphate dehydrate (CaHPO 4·2H 2O), or tricalcium phosphate (Ca 3(PO 4) 2), are predicted. The results are important in determining and predicting electrolyte concentrations for achieving desirable pH values while avoiding precipitation for various biological or biomedical applications.