Measurement and Correlation of Solid–Liquid Equilibria of Quaternary System KCl–KH2PO4–(NH2)2CO–H2O and Its Ternary Subsystem KH2PO4–(NH2)2CO–H2O at 303.15 K

The phase equilibria of the quaternary system KCl–KH₂PO₄–(NH₂)₂CO–H₂O and its ternary subsystem KH₂PO₄–(NH₂)₂CO–H₂O at 303.15 K were studied by the isothermal dissolution equilibrium method, and the phase diagrams were plotted. The equilibrium solid phase was identified by Schreinemaker’s wet residue method and X-ray diffraction (XRD). The ternary subsystem is a simple cosaturated system, the phase diagram contains one saturation point, two univariate curves, and three crystallization regions. A comparison of the phase diagrams of ternary subsystem at different temperatures shows that the cocrystallization region of KH₂PO₄ and (NH₂)₂CO decreases with the increase of temperature. The phase diagram of the quaternary system shows that the system is a simple cosaturated system, including one invariant point, three univariant curves, and three crystallization regions. Based on the Pitzer–H–W model, the lacking particle interaction parameters were regressed using the solubility data of the ternary systems and used to calculate the solubility data. By comparing the experimental value with the calculated value, the results both indicated that the experiment and theoretical calculation have good consistency, and the Pitzer–H–W model is suitable for the electrolyte and neutral molecule solution systems.