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 PbCl
2–CdCl
2, PbCl
2–KCl and PbCl
2–NaCl have been carried out at temperatures ranging from 540° to 640°C. It was observed that the measured activities in the system PbCl
2–CdCl
2 showed small positive or negative deviations from Raoult’s law depending on its concentration. On the other hand, those of the systems PbCl
2–KCl and PbCl
2–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
NPbCl2>0.1 in the system PbCl
2–CdCl
2 and positive at
NPbCl2>0.4 in the systems PbCl
2–KCl and PbCl
2–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 PbCl
2–KCl is, for example, expressed by the relation
El=−
RT⁄
F∫[(
tPbCl2NKCl⁄2−
tKClNPbCl2)⁄
NKCl]
dln
aPbC12.
In order to calculate
El from this equation, important assumptions for salt transport numbers of the components PbCl
2 and KCl,
tPbCl2 and
tKCl, were employed. Good agreement is observed between the calculated and the measured values in the systems PbCl
2–KCl, PbCl
2–CdCl
2 and PbCl
2–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.
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