One prominent feature of the systems containing ammonium and potassium salts is the formation of solid solutions. Because of the isomorphy in the NH₄H₂PO₄-KH₂PO₄-H₂O system,^2,4 the evaluator suspects that articles reporting solid NH₄H₂PO₄ in the presence of potassium salts, and vice versa, are in error, especially when reporting the nature of the solid phases. In the title system, the stable diagonal NH₄H₂PO₄-KNO₃-H₂O was studied by Bergman and Bochkarov.^1,3 From their data a reasonable phase diagram may be constructed, Fig. 25, at least with respect to solutions in equilibrium with two solid phases. But, as mentioned above, there is an uncertainty with respect to the precise nature of the solid phases. The work reported by Karnaukhov⁷ may be used to illustrate this uncertainty. In this article, solid solutions between each of the components and the compound NH₄NO₃K₃KNO₃ are reported. Furthermore, the "invariant points" reported in Ref. 10 cannot be fitted on Fig. 25. The evaluator surmises that the "modifications" of KNO₃ and NH₄H₂PO₄^10,11 (and even ones not reported in the respective binary systems (see pp 11-14) and, e.g., Ref. 5) reflect the difficulties in stating precisely the nature of the equilibrium solid phases. Another article⁶ also reports the occurrence of solid solutions between ammonium and potassium phosphates as well as chlorides in the NH₄⁺, K⁺ || H₂PO₄^–, NO₃^–, Cl^– -H₂O system which can be formed by adding another component to the title system. Apparently identical systems were studied by Pozin et al.^8,9 However, these data cannot be compared with each other because different sections were studied: NH₄H₂PO₄-KNO₃-KCl-H₂O in Ref. 6 and NH₄H₂PO₄-NH₄NO₃-KCl-H₂O in Refs. 8 and 9.

Experimental Data:   (Notes on the Nomenclature)