Seven articles report solubility data in the NH₄H₂PO₄-urea-H₂O system^1-7. Four of these articles^1-3,7 present data on solutions in equilibrium with several solid phases. Two of these articles^1,2 also contain sets of isotherms. The rest of the data^4-6 deal with the solubility along sections having constant N/P₂O₅ ratios. The data of Blidin¹ are scattered and the description of the solid phases at 25 °C is not convincing. In fact, a reasonable phase diagram, Fig. 13, can be constructed only if based on the data of Polosin^2,3 and Polyakov, et al.⁶ Then, too, α- and γ- forms of urea are reported for solutions having higher urea contents and at higher temperatures^2,3 and a β- form of NH₄H₂PO₄ is mentioned by others⁴. However, such polymorphs seem to be inconsistent with the boundaries of the crystallization fields in the phase diagram. Therefore, any evaluation must await further investigation of the solubility of the polymorphs. Furthermore, the solubility polytherm on sections having a constant N/P₂O₅ ratio reported by Sarbaev, et al.⁷ and by Polyakov, et al.⁶ differ substantially from each other. Once again, further independent data are needed before any evaluation can be made. The heavy lines in Fig. 13, below, represent phase equilibria while the lighter lines represent isotherms at the temperatures indicated.
          Kummel and Fahsl⁸ studied the closely related NH₄H₂PO₄-CO(NH₂)₂·H₃PO₄-H₂O system.

Experimental Data:   (Notes on the Nomenclature)