Solutions in molten salts

Separation of the tetrahalides can be achieved from their solutions in molten salts (K2ZrF6, NaCl, SnCh) or under pressure (10-70 atm). However, distillation of the adducts of the tetrachlorides with POCI3 is a more convenient route. Borohydrides and aUcoxides of Zr and Hf are more volatile, but their complicated syntheses and high air-sensitivity hmit their usefulness. 

The fact that the values of the relative thermal coefficients of solubility are close to the ideal ones shows that the deviations from ideality of the behaviour of the metal-oxides in their saturated solutions in molten salts are not so considerable, and that these deviations depend in the same proportion on the temperature changes. This conclusion may be substantiated by the concentration dependence of the activity coefficients of cations in molten chlorides, which is discussed in Ref. [81] the activity coefficients of metal cations in diluted solutions are practically unchanged up to concentrations in the order of 0.1 mol kg-1. 

Thus, treatment of metal-oxide solutions in molten salts with carbon dioxide may be proposed as a promising method to monitor the crystal-size and composition of complex oxide materials. 

Blander M., Thermodynamic Properties of Molten Salt Solutions in Molten Salt Chemistry, M. Blander, ed.. Interscience, New York, 1964, p. 127. 

The electromotive series that we have been discussing is for aqueous solutions. Similar series can be established for other solutions including solutions in molten salts. These series are all similar, but are not identical. Differences of order are quite numerous. For the most part, however, these arise where the potentials are close together, where differences in order are less significant. Sometimes, however, more serious differences occur. 

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. 

Corrosion Resistance. Zirconium is resistant to corrosion by water and steam, mineral acids, strong alkaUes, organic acids, salt solutions, and molten salts (28) (see also Corrosion and corrosion control). This property is attributed to the presence of a dense adherent oxide film which forms at ambient temperatures. Any break in the film reforms instantly and spontaneously in most environments. 

Bismuthides. Many intermetaUic compounds of bismuth with alkafl metals and alkaline earth metals have the expected formulas M Bi and M Bi, respectively. These compounds ate not saltlike but have high coordination numbers, interatomic distances similar to those found in metals, and metallic electrical conductivities. They dissolve to some extent in molten salts (eg, NaCl—Nal) to form solutions that have been interpreted from cryoscopic data as containing some Bi . Both the alkafl and alkaline earth metals form another series of alloylike bismuth compounds that become superconducting at low temperatures (Table 1). The MBi compounds are particularly noteworthy as having extremely short bond distances between the alkafl metal atoms. 

Typical values of self-diffusion coefficients and mutual diffusion coefficients in aqueous solutions and in molten salt systems such as (K,Ag)N03 are of the order... 

I.D. Efros, M.F. Lantratov, About decomposition voltage of potassium fluorotantalate in molten salts solutions, Metallurgiya, Moscow, 1965 p. 284 (in Russian). 

In Table 25 the values of E Lo - in molten salt (eutectic LiCl + KC1 melt) are compared with AE in aqueous solutions (relative to the value of Eo=0 for a pc-Pb electrode in a surface-inactive aqueous electrolyte). According to these data, the difference of AEaf in aqueous electrolytes and molten salts is not very high to a first approximation, it can be assumed that the quantity in square brackets in Eq. (61) has the greatest... 

Because of the inherent technical difficulties, investigations of transport properties in molten salts are much less common than those of aqueous solutions. However, interpretation of the phenomena seems to be even simpler in molten salts where water is not involved. Molten salt systems are considered to be the simplest liquid electrolytes. Data have been compiled largely due to the great efforts of the Janz group." "... 

The electrolysis temperature affects the electrolyte conductivity, the overpotential, and the solubility of the electrodeposit in aqueous as well as in molten salt systems. The effect of temperature is particularly important in the latter case. The lower limit of the temperature of operation is set by the liquidus temperature of the bath and the solubility of the solute. Generally, the temperature chosen is at least 50 °C above the melting temperature of... 

The stability of complex fluorides in molten salt solutions has also been widely investigated. The studies of cryolite and chiolite in NaF/AlF3 melts by cell methods (67) or by mass-spectrographic examination of vapor species (150) are typical. 

For the formation of HNO2 and O2, the following reactions due to the direct effect to the solute have been proposed in aqueous solutions [123], molten salts, and crystals [124], having the G-values of g 2 and g s2-... 

In aqueous solution, thorium exists as Th(IV), and no definitive data have been presented for the presence of lower-valent thorium ions in this medium. The standard potential for the Th(IV)/Th(0) couple has not been determined from experimental electrochemical data. The values presented thus far for the standard reduction potential have been calculated from thermodynamic data or estimated from spectroscopic measurements. The standard potential for the four-electron reduction of Th(IV) ions has been estimated as —1.9 V in two separate references 12. The reduction of Th(OH)4 to Th metal was estimated at —2.48 V in the same two publications. Nugent et al. calculated the standard potential for the oxidation ofTh(III) to Th(IV) as +3.7 V versus SHE, while Miles provides a value of +2.4 V [13]. The standard potential measurements from studies in molten-salt media have been the subject of some controversy. The interested reader is encouraged to look at the summary from Martinot [10] and the original references for additional information [14]. 

Typical values of self-diffusion coefficients and mutual diffusion coefficients in aqueous solutions and in molten salt systems such as (K,Ag)N03 are of the order of 10 m s and the coefficients do not usually vary by more than a factor of 10 over the whole composition range [1, 2, 15]. From measurements in pure ionic liquids we have learned that their self-diffusion coefficients are only of the order of 10 m s From this point of view it is interesting to investigate systems of ordinary and ionic liquids. Figure 4.4-3 shows the results of first measurements in the methanol/[BMIM][PF6] system, which can be seen as a prototype for a system in which an organic and an ionic liquid are mixed. 

Joseph J. Jordan Prof. Duke said that one of his motivations for exploring oxidation-reduction in molten salts was his desire to study separately the electron transfer process proper, which in aqueous solutions is invariably complicated and encumbered by overlapping proton transfer. 

A positive standard cell potential tells you that the cathode is at a higher potential than the anode, and the reaction is therefore spontaneous. What do you do with a cell that has a negative " gii Electrochemical cells that rely on such nonspontaneous reactions cire called electrolytic cells. The redox reactions in electroljdic cells rely on a process called electrolysis. These reactions require that a current be passed through the solution, forcing it to split into components that then fuel the redox reaction. Such cells are created by applying a current source, such as a battery, to electrodes placed in a solution of molten salt, or salt heated until it melts. This splits the ions that make up the salt. 

England et al. (1983) have shown that a variety of metal oxides having layered, tunnel or close-packed structures can be ion-exchanged in aqueous solutions or molten salt media to produce new phases. Typical examples are ... 

The three principal electrochemical methods are described by which fluorine can be directly introduced into organic compounds, namely electrolysis in molten salts or fluoride ion solutions, electrolysis in molten potassium fluoride/hydrogen fluoride melts at porous anodes, and electrolysis in anhydrous hydrogen fluoride at nickel anodes. Using examples from the past decade, it is aimed to demonstrate that electrofluorination in its various forms has proved to be an increasingly versatile tool in the repertoire of the synthetic chemist. Each method is described in terms of its essential characteristics, reaction parameters, synthetic utility, advantages and disadvantages, patent protection, and potential for commercial exploitation. The different mechanisms proposed to explain each process are critically reviewed. 

Although many nonaqueous solvent systems have been studied, the discussion here will be limited to a few representative solvents ammonia, a basic solvent sulfuric acid, an acidic solvent and bromine trifluoride. an aprotic solvent. In addition a short discussion of the chemistry taking place in solutions of molten salts is included. 

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