Dehydration and Condensation of Calcium Hydrogenphosphate Dihydrate

Abstract

The crystal structure of calcium hydrogenphosphate dihydrate (CaHPO₄·2H₂O) was analogous to that of gypsum (CaSO₄·2H₂O). Its dehydration process was not so simple as that of CaSO₄·2H₂O, because 0.5 mol of condensation water was present besides 2.0 mol of crystallization water.
TG-DTA curves of the dihydrate (heating rate, 5-10°C/min) showed three endotherms and weight losses of three steps, which occurred at the temperatures of 140, 160 and 185°C. This finding indicated that the 2.0 mol of crystallization waters had unequal bond strengths to CO+ ions. X-ray diffraction data of samples obtained by the above heating showed only two phases: the dihydrate and the anhydrate, but the formations of di- and tri- phosphate groups were detected in samples heated to about 180°C and 300°C, respectively, from paper chromatographic analysis. Therefore, it was found that the dehydration of the dihydrate, the condensations of the anhydrate and diphosphate proceeded simultaneously parallel to one another. Those di- and tri- phosphate groups were fairly stable so that γ-Ca₂P₂O₇ crystallized completely before about 600°C.
Although the weight loss of the sample heated at 100°C reached an equilibrium state corresponding to elimination of about 0.5 mol of crystallization water and the formation of some intermediate phase was suggested, the existence of a new phase was not confirmed from X-ray diffraction patterns.