Asymmetrical reciprocal molten salt systems: excess enthalpies of the liquid mixture (Na+ + K+ + F− + SO42−)

The excess enthalpies HE and the limiting partial enthalpies of liquid mixtures of the (Na+ + K+ + F− + SO42−) molten-salt reciprocal system were measured with a high-temperature calorimeter by an improved drop method. From HE's measured for the two diagonals: (NaF + K2SO4) and (KF + Na2SO4), we obtained the enthalpy difference ΔHo associated with the characteristic exchange reaction: 1 2 Na 2 SO 4 ( l ) + KF(l) = NaF(l) + 1 2 K 2 SO 4 ( l ) . This value is in good agreement with that calculated from thermodynamic quantities for the four pure salts. We propose a theoretical approach to interpret the mixing mechanism in a (A+ + B+ + X− + Y2−) asymmetrical reciprocal system: it makes possible an estimate of the excess thermodynamic quantities from those for the four common-ion binary mixtures and from ΔHo. This model has been applied to (NaF + K2SO4) and (KF + Na2SO4) and the results obtained were compared with the experimental excess enthalpies.