5.1 Solubilities in the NH3-H3PO4-HNO3-H2O system This system can be prepared by the addition of HNO3 to the NH3-H3PO4-H2O ternary system. Flatt et al. published solubility data for the quaternary system at 25 °C and 50 °C.1,2 However, these data of Flatt and co-workers cannot be critically evaluated because there are no other published solubility data with which they can be compared.
If CaO is added to the above quaternary system, the quinary system NH3-H3PO4-HNO3-CaO-H2O is formed. Flatt, Brunisholz and co-workers published solubility data for this quinary system.3,5 The main interest of these authors was the region where Ca(H2PO4)2·H2O crystallizes. They also reported the existence of a double salt Ca9(NH4)4H32(PO4)18·10H2O [no registry number available]. So far as comparisons can be made, the solubility data in Refs. 1-5 are fairly consistent with each other.
The system NH3-H3PO4-CaO-H2O is a quaternary subsystem of the quinary system discussed in the preceding paragraph. Flatt and co-workers published solubility data for this system at 0 °C, 25 °C and 50 °C.6,7 Orekov and Slobodkina8 published solubility data for this system at 45 °C, 60 °C, 75 °C and 90 °C in the region where the NH3/H3PO4 ratio is 0.5. The data of Flatt et al. at 50 °C can be compared with those of Orekov and Slobodkina at 45 °C. However, the two sets of data do not agree with respect to the identity of the phases and the composition of the solutions present at equilibrium. Orekov and Slobodkina8 did not find the double salt but did find anhydrous CaHPO4 as one of the equilibrium solid phases. Flatt's group7 report the crystallization field of the double salt diminishing with increasing temperature, but still present at 50 °C, and their CaHPO4 exists as a dihydrate. Obviously, more experimental work is necessary before this system can be evaluated.