The dehydrations of Ca(H2POa)2 " H20 and Mg(H2PO4)2 HzO were examined with a simultaneous TG, DTG, DTA and EGA method, partly under conventional, and partly under quasi-isothermal and quasi-isobaric conditions. It was found among others that the dehydrations of the examined compounds took place gradually between 100 and 500 ~ a series of intermediates being formed. The end-products were [Ca(PO3)2 ]8 and Mg(PO3)2. The reactions of these hydrates with KC1 were also examined. The many subsequent part reactions could be described by the following overall equation: 3 Ca(H2PO~)~ + 2 KCt = Ca2P20~ + Ca(PO3)~ + 2 KPO~ + 2 HC1 + 5 H20It is noteworthy that the reactions do not lead to the formation of orthophosphates, but stop after the liberation of 2 moles of HC1.All knowledge which concerns the dehydration or other reactions of Ca(H2PO~) z 9 9 HzO is important from both theoretical and practical points of view. This method has therefore been investigated by many researchers . It was found with the help of various methods, among others thermal analysis, that the thermal decomposition of Ca(H2PO~)2"H20 is a very complicated polycondensation process, in the course of which, depending on the experimental conditions, CaH2PzO7, Ca3(HP2OT)2, Ca(HaP207)2 and many other forms of polyphosphates are formed as intermediates, partly in amorphous, and partly in crystalline form. The end-product of the decomposition is [Ca(PO~)2]3, which may be amorphous or of crystalline structure.It was recently also found [2,6,17,19,21] that in the course of the decomposition of Ca(H2PO4) z 9 HzO "free phosphoric acid" is also formed 9 The curve representing the changes in the amount of phosphoric acid displays a double maximum and shows a parallel course with the DTG curve representing the rate of dehydration [19,21 ]. Under given conditions about the first maximum at 280 ~ the presence of "free phosphoric acid" corresponding to about 15 % of the P.,Oa was detected, while in the vicinity of the second maximum at 380 ~ the detected amount of "free phosphoric acid" corresponded to 3 ~ of the P205 in relation to the sample amount.The dehydration process of Mg(H2PO4)2 " 2H~O is less known, but it can be assumed that it has many common features with the decomposition of Ca(H~PO4)2 9 H20.
