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_s, of DCPD and the stability constants K_x and K_y for the ion pairs $[{\text{CaHPO}}_4^0]$ and $\left[{\text{CaH}}_2{\text{PO}}_4^{+}\right]$, 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 $\begin{array}{c}\text{ln\hspace{0.17em}}{K}_s=-\text{8403}.5/T+\text{41}.\text{863}–\text{0}.\text{09678}T\\ \text{ln\hspace{0.17em}}{K}_x=-\text{51090}/T-341.14+\text{0}.\text{5880}T\\ \text{ln\hspace{0.17em}}{K}_y=-\text{19373}/T-122.81+\text{0}.\text{1994}T\end{array}$