Thermodynamic Properties of Molten Sodium‐Lead Alloys: EMF Measurements and Interpretation

Chemical potentials of Na as a function of composition in liquid NaPb alloys were determined in the temperature range 380–480°C by an emf technique. A solid β‐Al2O3 electrolyte and a coulometric titration were used to vary the composition. The variations of the logarithm of the activity coefficient of Na with composition exhibit three inflection points, at XNa ˜ 0.5, 0.66, and 0.80. The inflections at 0.5 and 0.80 correspond to maxima, and the inflection at 0.66 corresponds to a minimum in the Darken excess stability function. The entropy, excess stability, and other related functions, evaluated across the entire composition range, reflect the unusual properties of liquid NaPb solutions. A possible interpretation of these features might be related to the simultaneous presence of more than one valence states of Pb.