Electrical conductivity of molten salts

Molten salts are a special class of liquids because they consist of positive and negative ions without being in a dielectric medium like a neutral solution. The relatively high electrical conductivity of molten salts demonstrates the predominant ionic character of these liquids [41,42,44,48],... 

The electrical conductivity of molten salts can be expressed in two ways equivalent conductivity A (ohm-1 cm2 cquiv ) and specific conductivity k (ohm-1 cm-1), and between these terms there is the relation... 

The electrical conductivity of molten salts has been used to elucidate the structure of the salts. Generally the variation of that property was measured as a function of composition and temperature. The isotherm of the ideal equivalent conductivity of a binary system is additive, and it can be calculated by the relationship... 

The electrical conductance of molten salts is the easiest transport property to... 

Electrical conductivity of molten salts is of considerable interest both from practical as well as theoretical points of view. By means of conductivity data, conclusions on the structure and transport theories of molten salts may be tested. Furthermore, the current and energy efficiencies of electrolytic processes are closely related to electrical conductivity of the electrolyte. 

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. 

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. 

A new model for electrical conductivity of molten salt mixtures was introduced by Eellner (1984). This model should also define the ideal conductance of molten mixtures. 

Figure 8.1. Series (a) and parallel (b) model of the electrical conductivity of molten salt mixture.

Dissociation model of electrical conductivity of molten salt mixtures... 

The model of electrical conductivity of molten salt mixtures based on incomplete electrolytic dissociation of components was proposed by DanCk (1989). The dissociation degree of a component is affected by the presence of second component. Consequently, the dissociation degree of both components in the system is not constant, but changes with composition, affecting the concentration of the conducting particles in the electrolyte. This effect is caused by interactions of components, given by the nature of the repulsive forces between ions, determining their actual coordination sphere. 

With respect to the ability to dissolve non-conducting materials, the measurement of electrical conductivity of molten salts, especially of fluorides, at high temperatures requires often the use of metallic conductance cells with low cell constant. When measuring with conductance cells of this type, resistances of the order of some tenths of ohm are measured, which requires the use of precise resistance bridges. For the metallic conductance cells with low resistance capacity, it is necessary to determine the dependence of the measured resistance on the current frequency used and the measurement must be realized at a frequency at which the resistance does not change anymore. 

The electrical conductivity of molten salts is again an important feature and has been reported for many salts. The methodology has been described many years ago by Tomlinson [261] and by Sundheim [262] and much more recently by Nunes et al. [263]. The critically compiled conductance data in [214] and many of the subsequently reported data are in terms of the specific conductance k, which like the viscosity follows an Arrhenius-type expression ... 

Until now there has not been any general theory which explains the experimental results of electrical conductivity in molten salts. Some attempts at conductivity calculations following structure models of liquids have been made, but the results are not satisfactory. 

Vasu (1972a,b) used the results of the above-mentioned works for the calculation of contact correlation function in molten salts on the basis of a double hard core model, which describes better the real situation in molten salts. This contact correlation function was applied in the calculation of viscosity and electrical conductivity of molten alkali... 

Olteanu and Pavel (1995) presented theoretical premises of the dissociation model for electrical conductivity in molten salt mixtures. The authors gave a versatile numerical method together with its corresponding computing procedure and provided an easier and more precise way of calculation. Eliminating the sum (x a + X2 2) from Eqs. (8.25) and (8.26), one obtains a relation between a and o 2 independent of molar fractions... 

In 1963 Dr. Danbk joined the Institute of Inorganic Chemistry of the Slovak Academy of Sciences in Bratislava, of which he was the director in the period 1991-1995. His main field of interest was the physical chemistry of molten salts systems in particular the study of the relations between the composition, properties, and structure of inorganic melts. He developed a method to measure the electrical conductivity of molten fluorides. He proposed the thermodynamic model of silicate melts and applied it to a number of two- and three-component silicate systems. He also developed the dissociation model of molten salts mixtures and applied it to different types of inorganic systems. More recently his work was in the field of chemical synthesis of double oxides from fused salts and the investigation of the physicochemical properties of molten systems of interest as electrolytes for the electrochemical deposition of metals from natural minerals, molybdenum, the synthesis of transition metal borides, and for aluminium production. 

The electrical and thermal conductivity of molten salts is very high. 

Iwadate, Y, Igarashi, K., Mochinaga, J., and Adachi, T. (1986) Electrical conductivity of molten charge asymmetric salts PrClj-NaCl, PrClj-KCl, and PrClj-CaClj systems. J. Electrochem. Soc, 133, 1162-1166. 

Flow measurements. Flow rates are measured in molten-salt systems with orifice or venturi elements. The pressures developed across the sensing element are measured by comparing the outputs of two pressuremeasuring devices. Magnetic flowmeters are not at present sufficiently sensitive for molten-salt service because of the poor electrical conductivity of the salts. 

Electrochemically, the system metal/molten salt is somewhat similar to the system metal/aqueous solution, although there are important differences, arising largely from differences in temperature and in electrical conductivity. Most fused salts are predominantly ionic, but contain a proportion of molecular constituents, while pure water is predominantly molecular, containing very low activities of hydrogen and hydroxyl ions. Since the aqueous system has been extensively studied, it may be instructive to point out some analogues in fused-salt systems. 

Some 30 years ago, transport properties of molten salts were reviewed by Janz and Reeves, who described classical experimental techniques for measuring density, electrical conductance, viscosity, transport number, and self-diffusion coefficient. 

The composition of the electrolyte is quite important in controlling the electrolytic deposition of the pertinent metal, the chemical interaction of the deposit with the electrolyte, and the electrical conductivity of the electrolyte. In the case of molten salts, the solvent cations and the solvent anions influence the electrodeposition process through the formation of complexes. The stability of these complexes determines the extent of the reversibility of the overall electroreduction process and, hence, the type of the deposit formed. By selecting a suitable mixture of solvent cations to produce a chemically stable solution with strong solute cation-anion interactions, it is possible to optimize the stability of the complexes so as to obtain the best deposition kinetics. In the case of refractory and reactive metals, the presence of a reasonably stable complex is necessary in order to yield a coherent deposition rather than a dendritic type of deposition. 

According to H. W. Foote and L. H. Levy, soln. of the alkali chlorates in other molten salts gave f.p. indicating that the mol. wt. under those conditions are normal. A. Rosenheim and 0. Liebknecht48 found the mol. wt. of acid by the f.p. method depend on the concentration, and that the acid in dil. soln. is a monobasic acid, and in cone. soln. the acid is polymerized and exists as a dibasic acid, H2l206. E. Gro-schuff and P. Walden also showed that the electrical conductivity of iodic acid soln. corresponds with a monobasic acid, as is also the case with chloric and bromic acids. The monobasic acid, therefore, can be represented by A. Kekule s type of formula with iodine univalent or C. W. Blomstrand s type of formula, with iodine quinquevalent and iodic anhydride, i.e. iodine pentoxide will be represented ... 

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