Solubility of CaHPO4 · 2H2O in the Quaternary System Ca(OH)2 - H3PO4 - NaCl - H2O at 25 °C.

Abstract

Solubility of CaHPO₄ · 2H₂O was determined in the quaternary system Ca(OH)₂ - H₃PO₄ - NaCl - H₂O at 25 °C in the pH range 4.39-6.38; ionic strengths of the saturated solutions varied from 0.00485 to 0.545, Satisfactory constancy in the solubility product, ( $K_{\text{sp}}=\left[{\text{Ca}}^{2+}\right]\cdot \left[{\text{HPO}}_4^{2-}\right]\cdot {\gamma }_{{\text{Ca}}^{2+}}\cdot {\gamma }_{{\text{HPO}}_4^{2-}}=2.49\pm 0.05\times {10}^{-7}{\text{mol}}^2\times 1^{-2}$ ) was obtained when (i) the ion activity coefficients, γ _i , were calculated with the Debye-Hückel equation, $\text{log }{\gamma }_i=-A{Z}_i^2\sqrt{I}/\left(1+\text{B}{\alpha }_i\sqrt{I}\right)+0.0626I$ ; the value 0.0626 for the coefficient in the linear term was derived from the solubility data by utilizing a statistical procedure, and (ii) formation of an ion pair ${\text{NaHPO}}_4^{-}$ was taken into account; a statistically derived value for the stability constant of this ion pair is 7.0 ± 2.4 1 × mol^-1. The ion pair ${\text{NaHPO}}_4^{-}$ appears to have significant concentrations in physiological fluids.