Solubility of CaHPO₄ · 2H₂O in the System Ca(OH)₂-H₃PO₄-H₂O at 5, 15, 25, and 37.5 °C

Solubility isotherms for CaHPO₄ · 2H₂O, dicalcium phosphate dihydrate, DCPD, in the ternary system Ca(OH)₂ - H₃PO₄ - H₂O were determined at 5, 15, 25, and 37.5 °C in the pH range 3.5 - 7; the relative positions of the isotherms indicate that DCPD has a negative thermal coefficient of solubility. The solubility product, K , of DCPD and the stability constants K and K for the ion pairs $$ and $$ , respectively, were obtained as functions of temperature by the use of a generalized least squares procedure subject to three condition functions - constancy of the solubility product, electrical neutrality in the solution, and congruent dissolution of the solid. The equations obtained are The existence of a maximum in K in the neighborhood of 25 °C is plausible on the basis of available thermodynamic data for DCPD. Thermodynamic functions are reported for the solution of DCPD and for the association of the ion pairs.