Electrical conductivity of ideal and real solutions

The study of electrical conductivity of molten salts is one of the indirect methods used for the determination of molten salts structure and of component interaction in molten mixtures. The change in composition of a molten mixture is often accompanied by structural changes, which affect the dependence character of the electrical conductivity on composition. Consequently, an analysis of this dependence should provide some information regarding the present ionic species and their arrangement in the melt. Supplementary information, i.e. concerning the formation and decomposition of complex ions, the character of the cation-anion bond, and the character of conductivity, cationic, anionic, electronic, etc., can be obtained from analysis of the dependence of the activation energy on composition. 

Two factors principally determine the shape of isotherms of the electrical conductivity of binary mixtures  

In order to get some information on the possible structure of the given molten system from the conductivity measurement, a suitable reference state should be defined. Since conductivity is a scalar quantity, no ideal behavior is given by definition. However, there were several attempts in the literature to present a model of electrical conductivity of molten salts, which would describe satisfactorily the course of the conductivity dependence on composition. 

Theoretical interpretation of the concentration dependence of equivalent conductivity for simple binary mixtures was first presented by Markov and Shumina (1956). It should be emphasized that this theory, even when considering simple structural aspects, represents rather a method of interpretation of the experimental data than a genuine picture of the structure of the melt. In molten salts generally only ions and not molecules are present, hence the conception of Markov and Shumina (1956) is to be considered also from this aspect. Their theory is based on the assumption that the electrical conductivity of a mixture of molten salts varies with temperature like pure components. In this respect, general character of the electrical conductivity dependence on composition, indicating the interaction of components in an ideal solution, could be expected. 

In a mixture of univalent salts of the type AX-BX, the following interactions should be present AA, BB, AXB, and BXA. The last two interactions are equal, thus they can be written as 2AXB. Considering that the probability of the interactions mentioned is proportional to their molar fractions, Markov and Shumina derived a relation for the composition dependence of the equivalent conductivity in a mixture of molten salts in the form 

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