Phase Equilibria of Aqueous Ternary Systems Li2SO4 + Na2SO4 + H2O and Na2SO4 + K2SO4 + H2O at 303.2 K

The phase equilibria of ternary systems Li2SO4 + Na2SO4 + H2O and Na2SO4 + K2SO4 + H2O at 303.2 K were investigated by using the isothermal dissolution method. The solubility, density, and refractive index of the systems were measured. There are two ternary invariant points and three crystallization regions corresponding to Li2SO4·3Na2SO4·12H2O, Li2SO4·H2O, and Na2SO4·10H2O in the system Li2SO4 + Na2SO4 + H2O at 303.2 K. There are two ternary invariant points and three crystallization regions corresponding to Na2SO4·10H2O, Na2SO4·3K2SO4, and K2SO4 in the system Na2SO4 + K2SO4 + H2O at 303.2 K. By comparing the phase diagrams of the ternary system Li2SO4 + Na2SO4 + H2O at different temperatures, one can find that the existence form of double salt changes from Li2SO4·3Na2SO4·12H2O to Li2SO4·Na2SO4. By comparing the phase diagrams of the ternary system Na2SO4 + K2SO4 + H2O at different temperatures, the double salt Na2SO4·3K2SO4 was not found at 273.2 K, while it was formed at 303.2 and 313.2 K. Meanwhile, the thermodynamic data of these two systems at 303.2 K were fitted by the Pitzer–Simonson–Clegg model, and the calculated values agree well with the experimental values.