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

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 sp = [ Ca 2 + ] ⋅ [ HPO 4 2 - ] ⋅ γ Ca 2 + ⋅ γ HPO 4 2 - = 2.49 ± 0.05 × 10 - 7   mol 2 × 1 - 2 ) was obtained when (i) the ion activity coefficients, γ i , were calculated with the Debye-Hückel equation, log   γ i = - A Z i 2 I / ( 1 + B α i I ) + 0.0626   I ; 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 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 NaHPO 4 - appears to have significant concentrations in physiological fluids.