There are two reports^1,2 that give data for the NH₄H₂PO₄-(NH₄)₂HPO₄-K₂SO₄-H₂O system. One article² reports the composition of solutions that are saturated simultaneously with two or three solid phases, and have the molar ratio of N/P = 1.5. The temperature range of 262 K to 318 K is covered. Isotherms at 268 K, 273 K and 283 K are also given. The other article¹ reports the contents of N, P₂O₅ and K₂O in solutions saturated at 273 K by unspecified solid phase(s). The evaluator has found that the molar ratio of N/P in these solutions is very close to 1.3. Therefore, no direct comparison of the data in these two articles can be made. Furthermore, in each of these articles there are statements that raise questions. In the work of Tishchenko and Kuznetsova¹ the time allowed for equilibration was rather short and the attainment of equilibrium was not established experimentally. Babenko and Andrianov² reported the simultaneous crystallization of ammonium phosphates and potassium sulfate. This is surprising in view of the well-known ability of ammonium and potassium phosphates to form so-called β-solid solutions.^3-9 Further doubt about the composition of the solid phase is cast by the work of Fokina¹⁰ who observed these solid solutions formed between ammonium and potassium dihydrogenphosphates in her study of the NH₄H₂PO₄-H₃PO₄-K₂SO₄-H₂O system.
          Thus, more experimental work needs to be reported for the NH₄H₂PO₄-(NH₄)₂HPO₄-K₂SO₄-H₂O system before solubility data for this system can be considered as recommended values.

Experimental Data:   (Notes on the Nomenclature)