The Stable Phase Equilibria of the Ternary Systems Na2SO4 + Rb2SO4 (Cs2SO4) + H2O at 298.2 K

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.