Electroanalytical chemistry in molten salts

Charles L. Hussey The University of Mississippi, University, Mississippi 

Molten salts or ionic liquids (also referred to as fused salts by some authors) were among the very first media to be employed for electrochemistry. In fact, Sir Humphrey Davy describes electrochemical experiments with molten caustic potash (KOH) and caustic soda (NaOH) [1] as early as 1802 A wide variety of single molten salts and molten salt mixtures have been used as solvents for electroanalytical chemistry. These melts run the gamut from those that are liquid well below room temperature to those melting at more than 2000°C. The former present relatively few experimental challenges, whereas the latter can present enormous difficulties. For example, commercially available Teflon- and Kel-F-shrouded disk electrodes and Pyrex glass cells may be perfectly adequate for electrochemical measurements in ambient temperature melts such as the room-temperature chloroaluminates, but completely inadequate for use with molten sodium fluoroaluminate or cryolite (mp = 1010°C), which is the primary solvent used in the Hall-Heroult process for aluminum electrowinning. 

This chapter is aimed at the inexperienced researcher who desires to carry out electroanalytical measurements in molten salts and seeks introductory information about the experimental details associated with the use of these solvents. It is intended to complement the chapters appearing elsewhere in this volume that discuss conventional molecular solvents and supporting electrolytes and various electroanalytical techniques. 

It is important to recognize that not all inorganic melts are ionic. In fact, a great many substances are categorized as molten salts that do not deserve this distinction. 

The classification system described earlier is limited to the simplest kinds of individual melts and is not intended to include mixtures. However, molten mixtures of these different classes of compounds are often more practical solvents than the melts of the individual compounds, due to their much lower melting points and other favorable properties, and this system of classification can usually be extended to these mixtures. For example, the very popular molten LiCl-KCl eutectic mixture is simply a binary ionic melt, whereas molten NaN03-KN03-LiN03 is a ternary polyanionic melt. Interestingly, the equimolar molten mixture of the simple ionic salt NaCl (a) and the molecular compound A1C13 (d) produces a simple polyanionic salt melt (b) composed of Na+ and A1C14 ions  

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Fung, K. W. and Mamantov, G., Electroanalytical Chemistry in Molten Salts-A Review of Recent Developments, in Advances in Molten Salt Chemistry, J. Braunstein, G. Mamantov, and G. P. Smith, Editors. 1973, Plenum Press New York. p. 199. 

Hussey, C. L., Electroanalytical Chemistry in Molten Salts, in Laboratory Techniques in Electroanalytical Chemistry, P. T. Kissinger and W. R. Heineman, Editors. 1996, Marcel Dekker New York. p. 511. 

ELECTROANALYTICAL CHEMISTRY IN MOLTEN SALTS—A REVIEW OF RECENT DEVELOPMENTS... 

This chapter is concerned with electroanalytical chemistry in molten salts. Electroanalysis has been applied extensively for in situ determination and characterization of solutes in molten salt solvents. This field has been reviewed previously either alone or in conjunction with other topics. In this review we shall stress more recent developments (1965-1971) in several important solvent systems. We shall exclude the discussion of transport properties and related topics, the electrical double layer, and the rates of charge-transfer processes, since the reviews of these topics " are quite up to date. 

Electroanalytical Chemistry in Molten Salts—A Review of Recent Developments 201 2.2. Methodology... 

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