All the articles dealing with the NH₄H₂PO₄-KH₂PO₄-H₂O system^1-9 report a series of solid solutions as the equilibrium solid phases. Some comparisons are possible and they are shown on Fig. 19 and Fig. 20. For the 273 K data, Fig. 19, the values for the KH₂PO₄ content reported by Askenasy and Nessler¹ are larger than those reported by Polosin and Ozolin⁴. This is true also for the binary solubility of KH₂PO₄ in water at 273 K, where the 15.5 100w_i value is larger than the 11.74 100w_i value recommended by IUPAC.¹⁰ It, therefore, seems likely that there is a systematic error in the analytical values for potassium and, consequently, the data in Ref.¹ are rejected. The values at 298 K, Fig. 2, agree fairly well and may be tentatively accepted. The NH₄H₂PO₄-MH₂PO₄-H₂O [M = Rb,¹¹ Cs,¹² Tl¹³] sytems are characterized by the formation of solid solutions. Earlier¹⁰ it was noted that the results in Refs. 11 and 12 were affected by unreliable analytical procedures and are, therefore, rejected. No independent solubililty data have, as yet, been reported for these systems.

Experimental Data:   (Notes on the Nomenclature)