Thermodynamic Activities and Potentials at Liquid Junction in the Molten Salts

Abstract

Thermodynamic activities in binary mixtures of molten salts have been measured by using the TIE method based on the principle of the concentration cell.
The measurements for the systems PbCl₂–CdCl₂, PbCl₂–KCl and PbCl₂–NaCl have been carried out at temperatures ranging from 540° to 640°C. It was observed that the measured activities in the system PbCl₂–CdCl₂ showed small positive or negative deviations from Raoult’s law depending on its concentration. On the other hand, those of the systems PbCl₂–KCl and PbCl₂–NaCl showed both remarkable negative deviations.
The liquid junction potentials have been obtained by combination of electromotive forces by the TIE method and the usual one with liquid junction. The liquid junction potentials were negative at N_PbCl2>0.1 in the system PbCl₂–CdCl₂ and positive at N_PbCl2>0.4 in the systems PbCl₂–KCl and PbCl₂–NaCl, each value being as small as less than 8 mV.
Based on the irreversible thermodynamics, the potential at the liquid junction of the concentration cell for the system PbCl₂–KCl is, for example, expressed by the relation
E_l=−RT⁄F∫[(t_PbCl2N_KCl⁄2−t_KClN_PbCl2)⁄N_KCl]dlna_PbC12.
In order to calculate E_l from this equation, important assumptions for salt transport numbers of the components PbCl₂ and KCl, t_PbCl2 and t_KCl, were employed. Good agreement is observed between the calculated and the measured values in the systems PbCl₂–KCl, PbCl₂–CdCl₂ and PbCl₂–NaCl. From these calculations, it was made clear that the TIE method is capable of determining the activities for the mixture of molten salts with high accuracy.