Heats Of Mixing Aqueous Electrolytes. XII. Reciprocal Salt Pair Na+, Li+.dvr.Cl-, SO42-

The heats of mixing aqueous solutions of all combinations of the reciprocal salt pair Na⁺, Li⁺ || Cl⁻, SO₄²⁻ have been measured at 25°. In one set of experiments, the initial solutions had the same molal ionic strength (I = 1, 3, or 6 mol/kg). In another set of experiments, the initial solutions had the same number of equivalents per kilogram of solvent (E = 1, 3, or 6 mol/kg). For charge-asymmetric mixtures, the magnitude of constant E mixings is about the same as that of the constant I mixings. Young\u27s cross-square rule is quite accurate at I = 3 mol/kg and E = 3 and 6 mol/kg, marginal at I and E = 1 and fails definitely at I = 6 mol/kg. The constant E mixing process is preferred because the cross-square rule holds at higher concentrations. The predictions of the cross-square rule based on both constant I and constant E mixings are surprisingly accurate at these high concentrations in view of the fact that sodium and sulfate as well as lithium and sulfate ions have rather strong interactions