The components (solubilities and densities) in aqueous stable equilibria systems Cs 2 SO 4 + MgSO 4 + H 2 O and Rb 2 SO 4 + Cs 2 SO 4 + H 2 O at T = 298.2 K have been studied by the isothermal dissolution equilibrium method. The wet residue method and the X-ray diffraction method were used to confirm the solid phase compositions. The ternary system Cs 2 SO 4 + MgSO 4 + H 2 O is a complex saturated type, with the commensurate double salt Cs 2 SO 4 •MgSO 4 •6H 2 O formed at the investigated temperature. There are two two-salt cosaturation invariant points, with the coexisting solid phases corresponding to commensurate type saturation points E 1 (MgSO 4 •7H 2 O + Cs 2 SO 4 • MgSO 4 •6H 2 O) and E 2 (Cs 2 SO 4 •MgSO 4 •6H 2 O + Cs 2 SO 4 ), three monovariant curves, and three crystallization fields in the system of Cs 2 SO 4 + MgSO 4 + H 2 O. The ternary system Rb 2 SO 4 + Cs 2 SO 4 + H 2 O with solid solutions formed contains two cosaturated points, with the equilibrium solid phases of the saturation points F 1 and F 2 corresponding to [(Rb, Cs) 2 SO 4 ] + Cs 2 SO 4 and Rb 2 SO 4 + [(Rb, Cs) 2 SO 4 ], respectively, three monovariant curves, and three crystallization fields and is also a complex type at 298.2 K. Furthermore, under the investigated temperature, the Pitzer model was applied to calculate the solubilitites of these two ternary systems. Comparing the predicted solubilities with the experimental solubilities, the results both indicated good consistency.

To understand the crystallographic forms of salts in sulfate systems containing sodium and rubidium (cesium), the stable phase equilibria of ternary systems Na2SO4 + Rb2SO4 + H2O and Na2SO4 + Cs2SO4 + H2O at 298.2 K were studied with the isothermal dissolution method. The composition and crystallographic forms of the equilibrated solid phase was verified by Schreinemaker’s wet residue method combined with X-ray diffraction. Experimental results show that there is no double salt or solid solution formed. The stable phase diagram of Na2SO4 + Rb2SO4 + H2O at 298.2 K is characterized by one invariant point (H), two univariant curves, and two crystallographic forms (corresponding to Na2SO4·10H2O and Rb2SO4). The ternary system Na2SO4 + Cs2SO4 + H2O is of the hydrate type II, there are two invariant points (E1, E2), three univariant curves, and three crystallographic forms (corresponding to salts Na2SO4·10H2O, Na2SO4 and Cs2SO4). Affected by the unique structure of Cs+, both salt Na2SO4·10H2O and Na2SO4 have been found in Na2SO4 + Cs2SO4 + H2O system at 298.2 K. Furthermore, the density and refractive index of the equilibrated solution possess the analogous variation tendency and regularly change with the change of concentration.