Critical Evaluation: Ammonium dihydrogenphosphate with Ammonium chloride, Potassium dihydrogenphosphate, Potassium chloride and Water (IUPAC SDS Volume 66)
8.4. NH4+, K+ || H2PO4-, Cl- - H2O

The NH4+, K+ || H2PO4-, Cl- - H2O system has been studied either in broader extent1,3 or in terms of the diagonal NH4H2PO4- - KCl - H2O.2,4 Formation of solid solutions between both dihydrogenphosphates and chlorides is the prominent feature of this system. The data in the different reports cannot be compared with each other because of different experimental conditions. The phase diagram of Ref. 2 - see Fig. 26 - can be accepted tentatively.

Figure
Solubility graph
FIG. 26. Solubility in the (NH4,K)H2PO4–KCl–H2O system.
References:

1 Askenasy, P.; Nessler, F.; Z. Anorg. Allg. Chem. 189, 305 (1930).
2 Polosin, V. A.; Shakhparonov, M. I.; Zh. Fiz. Khim. 21, 119 (1947).
3 Iovi, A.; Haiduc, C.; Rev. Roum. Chim. 16, 1743 (1971).
4 Fokina, Z. N.; Kornishina, E. N.; Kim, P. P.; Tekhnologiya Mineral'nykh Udobreniy 63 (1977).