Solid–Liquid Equilibrium in the NH4Cl + RbCl + H2O and NH4Cl + CsCl + H2O Systems at 298.15 K

Ammonium ion is of the same charge as alkali ions and with similar ionic radius as potassium, rubidium, and cesium ions. Solid solutions are very common between ammonium and alkali salts, which is of great interest for understanding the geology, soil, and environment fate of these elements. In this study, the solid–aqueous equilibrium in the NH4Cl + RbCl + H2O and NH4Cl + CsCl + H2O systems was determined at 298.15 K. For each system, two incontinuous solid solutions, i.e. (NH4,Rb)Cl(α) and (Rb,NH4)Cl(β), or (Cs,NH4)Cl(α) and (NH4,Cs)Cl(β), presented, and their composition and structure were studied using Schreinemakers’ wet residues analysis, XRD, and Raman characterization. The equilibrium between these solid solutions and aqueous solutions were well modeled using an integrated solution equilibrium calculator program, in which a Pitzer-Simonson-Clegg model and a subregular model were implemented for describing the nonideality of aqueous phase and solid solution. According to the experimental and thermodynamic modeling results, NH4Cl can dissolve RbCl and CsCl in the solid state (NH4,Rb)Cl(α) or (NH4,Cs)Cl(β) with limited solid solubilities of 0.18 or 0.051 in mole fraction, respectively. RbCl or CsCl can also dissolve NH4Cl in the solid state (Rb,NH4)Cl(β) or (Cs,NH4)Cl(α) with limited solid solubilities of 0.37 and 0.095 in mole fraction, respectively.