Excess Free Energies and Heats of Mixing in Certain Molten Salt Mixtures

The third‐ and fourth‐order terms of the perturbation theory of Reiss, Katz, and Kleppa for the excess free energies and heats of mixing of molten salts have been calculated for a particular choice of ``test salt.'' Up to the fourth order, the expressions have the form X1X2Pδ2+X1X2(X1−X2)Qδ3+(X1X2R+X1X2(X1−X2)2S)δ4+···, which is consistent with and rigorously justifies the concentration dependence of the observed heats of mixing of alkali nitrates. The equations can be derived for salts having a much more general cation—anion interaction potential than was stated by Reiss, Katz, and Kleppa.