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 [ CaHPO 4 0 ] and [ CaH 2 PO 4 + ] , 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 ln  K s = - 8403 .5 / T + 41 . 863 - 0 . 09678 T ​ln  K x = - 51090 / T - 341.14 + 0 . 5880 T ​ln  K y = - 19373 / T - 122.81 + 0 . 1994 T The existence of a maximum in K s 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.