The quaternary system NH₃-H₃PO₄-H₂SO₄-H₂O may be formed by the addition of H₂SO₄ to the ternary system NH₃-H₃PO₄-H₂O. There are four reports^1-4 that contain solubility data for this quaternary system. The solubility data in these four reports agree reasonably well with each other after discounting a systematic error in two of the reports.^1,2 Therefore these solubility data are considered to be reliable. Furthermore, the existence of (NH₄)₂H₃PO₄SO₄ [25993-60-1] is also considered to have been established as well as the fact that it is congruently soluble in water over a temperature range of 273 K to 373 K.
          In two reports^5,6 the system is described as NH₄H₂PO₄-(NH₄)₂HPO₄-(NH₄)₂SO₄-H₂O. The Evaluator believes that a comparison can be made of the solubility data for this system and that of the system described in the preceding paragraph. Such a comparison can be made at 273 K, 298 K and 323 K. To do this, the isotherms were depicted in tetrahedral coordinates and then projected on planes perpendicular to an edge of the NH₃-H₃PO₄-H₂SO₄ triangle.¹⁵ Such a projection gives absolute solubility values, the shape and position of individual crystallization fields, and any scatter in the reported data. Using this procedure, all the data in Refs. 1-4,6 are in satisfactory agreement with each other. The data reported by Kaganskiy and Babenko⁵ and by Uno³ agree with each other but there is significant scatter in the data near the crystallization surface.
          Fokina et al.,⁷ studied the NH₄H₂PO₄-H₃PO₄-K₂SO₄-H₂O system. Such a system can be considered as a section through the NH₃-H₃PO₄-H₂SO₄-K₂O-H₂O system where the molar ratio H₂SO₄/K₂O = 1. Solid solutions between ammonium and potassium dihydrogenphosphates exist as solid equilibrium phases in this system. The ability of these two dihydrogenphosphates to form such solid solutions (designated as β-solid solution) has been proved in studies of the NH₄H₂PO₄-KH₂PO₄-H₂O system.^8-14

Experimental Data:   (Notes on the Nomenclature)