Complex ions in molten salts

Consider complex ion formation in the CdClj-KCl system, and let it be assumed for the moment that a CdCl complex ion is formed. If such complex ions were formed in an aqueous solution of CdClj and KCl, they would exist as little islands separated from other ions by large expanses of water. In fused salts, there are no oceans of solvent separating the ions. Thus, a Cd " ion would constantly be coming into contact on all sides with chloride ions, and yet one singles out three of these CP ions and says that they are part of (or belong to) a CdCIJ complex ion (Fig. 5.54). It appears that in the absence of the separateness possible in aqueous solutions, the concept of complex ions in molten salts is suspect As will be argued later, however, what is dubious turns out to be not the concept but the comparison of complex formation in fused salts with complex formation in aqueous solutions. 

An Electrochemical Approach to Evaluating the Identity of Complex Ions in Molten Salt Mixtures... 

Can One Determine the Lifetime of Complex Ions in Molten Salts ... 

Ideas on complex ions in molten salts tend to vary with the time at which they were published. In the first half of the century, there seemed no doubt that complex ions in molten salts were distinct entities and, it was implied, they were permanent. Later, there was doubt as to our ability to identify complex ions in molten salts. Thus, it was argued, there is no difficulty in accepting the existence of discrete ions in aqueous solutions because each ion is a separate entity, and there are many solvent... 

These two views can be described in terms of a time t, representing the time an ion such as remains bonded to a ligand, such as Br". In the two extreme views just given, t would be minutes or even hours for long-lived complexes and zero for the concept that distinguishable complex ions in molten salts do not exist. 

Such problems can be tackled by spectroscopic means, as shown later. Raman spectra, in particular, would indicate new lines having characteristic frequencies when Br is added to CdlNOjjj in KNOj-LiNOj, and in the preceding section it has been shown that an analysis of the variations of the electrode potential of Cd(N03)2in KNOj-LiNOjWith Cr addition has given reason to believe in complex ions in the cases quoted. However, there is a nifty electrochemical method that allows one to also obtain the lifetime of the individual ions and hence remove doubt as to the real existence of complex ions in molten salts. 

D. Inman and J. O M. Bockris, Complex Ions in Molten Salts A Galvanostatic Study, Trans. Faraday Soc. 57 2308 (1961). 

D. Inman, Complex Ions in Molten Salts A Potentiometric Study of the Halide Complexes of Cadmium in Molten Equimolar NaN03-KN03 at 250 °C, Electrochim. Acta 10 (1965) 11-21. 

A survey of recent developments in molten salts covers structures, physical properties, uses, and their reactions. Spectroscopic techniques for detecting and identifying complex ions in molten salts are also reviewed, together with the effects of complex ions on the activity, heats of mixing, and other physical properties. The information that can be obtained from Raman and i.r. spectroscopy is emphasized in a further review of molten salts covering types of reaction, ion solvation, and interionic vibrations in nitrates, halides,... 

The formation of complex ions in molten salts is considered elsewhere. However, in certain cases, where the complex ions themselves are electroinac-tive, they can dissociate to form electroactive species. The kinetic effects which are then manifested may be detected and quantified by electroanalytical... 

From the standpoint of this comparison (Fig. 5.54), it is seen that the concept of a complex ion in a molten salt is at least as tenable as that of an ion with a primary solvation sheath (Section 2.4) in aqueous solutions. Whatexperimental evidence exists for complex ions in fused salt mixtures To anwer this question, one must discuss some results of investigating the structure of mixtures of simple ionic liquids. 

High-temperature ionic solvents are known to contain relatively high total concentrations of cations (e.g. in the KCl-LiCl eutectic, the concentration of Li+ is approximately equal to 8.5 mol kg-1 of the melt). Usually, cation-anion complexes in molten salts are characterized by co-ordination numbers of the order of 4-6. This means that the maximal consumption of acidic cations does not exceed 0.4-0.6 mol kg-1 in diluted solutions with concentrations close to 0.1 mol kg-1. This estimate is considerably lesser than the initial concentration of acidic cations in the pure melt. In the case of the KCl-LiCl eutectic melt, this consumption is only of the order of 5-7%, and the value of NMe+ in equation (1.3.16) may be assumed to be constant. Therefore, for each ionic solvent of the second kind (kind II) the denominator in equation (1.3.16) is a constant which characterizes its acidic properties. We shall define p/L = -log /L to be the relative measure of acidic properties of a solvent and call it the oxobasicity index of ionic melt [37, 162, 181]. Since the direct determination of the absolute concentration of free oxide ions in molten salts is practically impossible, the reference melt should be chosen— for this melt, /L is assumed to be 1 and p/L = 0. The equimolar KCl-NaCl... 

The formation of complex ions is an important problem for the study of the structure and properties of molten salts. Several physicochemical measurements give evidence of the presence of complex ions in melts. The most direct methods are the spectroscopic methods which obtain absorption, vibration and nuclear magnetic resonance spectra. Also, the formation of complex ions can be demonstrated, without establishing the quantitative formula of the complexes, by the variation of various physicochemical properties with the composition. These properties are electrical conductivity, viscosity, molecular refraction, diffusion and thermodynamic properties like molar volume, compressibility, heat of mixing, thermodynamic activity, surface tension. 

The most important feature of fast electrode processes is that they usually involve the discharge of an adsorbed layer of the electroactive ions. These adsorption processes are complicated by the fact that in molten salts the electroactive ions are usually complex ions. In this case it is possible to presume [126] that the rates of charge transfer for different electrode processes are determined by... 

Bredig M. A., The Experimental Evidence for Complex Ions in Some Molten Salt Mixtures , in Molten Salts. Characterization and Analysis, G. Mamantov, ed., Marcel Dekker, New York, 1969, p. 55. 

The concept of complexing in molten salt mixtures is somewhat different from that for solutes in aqueous or organic solvents. In water solutions, every ion is separated from the other ions by the solvatation wrapping of water. Ions, simple or complex, are mutually influenced only by weak forces and each ion behaves independently. 

In molten salts, the positively and negatively charged ions are in close contact and the interaction forces are great. The heats of formation of complex anions, except for the stabile ions like [804] , [NOs]", etc., are in the order of a few kJ mol . These values are lower than that of the activation energy of diffusion, which in these melts is... 

The crucial problem in niobium deposition in molten salts is the presence of oxygen in the electrolyte, because it is extremely difficult to prepare a melt free of 0 ions, especially in the case of industrial applications. It was formerly assumed that the presence of ions might decrease the quality of Nb coatings or prevent the formation of Nb coatings completely. Therefore a great part of the research efforts was concentrated on the influence of ions on the reduction mechanism of Nb and the formation of niobium oxofluoro-complexes in the melt. 

Molten salts are systems in which the components are ions but which are electrically neutral on a local scale. Coulombic forces are long range in molten salts and electrical screening is important. The complexity of these systems depends on the nature of the ions, that is, whether they are monoatomic or polyatomic. For polyatomic ions, other electrostatic forces may be involved. 

This reaction is shifted appreciably to the right, compared with the interactions with other halide ions. This fact is explained in the context of the HSAB concept the formation of complexes of A1 with Cl-, Br-, or I- ions is less favourable than of the complex fluoroaluminates, since fluoride ion is the hardest halide base and Al3+ ion is referred to as the strongest hard acids. The F- and O2- ions seem to possess closely similar hard basic properties in molten salts. 

As noted before, solubility of covalent organic compounds in molten salts seems to require proton interaction between acidic groups of the solute and anions of the solvent. For un-ionized solutes this may well involve structures analogous to the solvent-bridged ion-pair complexes postulated for concentrated aqueous solutions ... 

A method has been developed for calculating stabilities of complex ions in binary molten salt mixtures, using accurate enthalpy of mixing data as bases. It was possible to discriminate between two possible models for the dissociation of molten cryolite ... 

A number of unique difficulties pertain to oxidation states of metal ions encountered in molten salt solutions. For example, for first-row transition metals, the highest oxidation state prevailing is often +3, as in the case of Fe and Cr. Frequently, for chlorides in particular, the +3 state compounds are volatile at suitable operating temperatures and, hence, their solutions are thermally unstable.Other problems encountered include rapidly dispropor-tionating states, the formation of oxyhalides, and precipitation of complexes by reaction with the melt. While redox reactions per se involve very fast charge transfer steps, these may occur at the extremes of the range of electrochemical stability, thus leading to concomitant solvent melt decomposition. Nevertheless, suitable processes such as Fe /Fe on vitreous carbon in chloride melts can be employed to determine the effective electrochemical areas of electrodes where diffusion coefficients are accurately known. ... 

The complexing role of fluoride ions in the reduction of refractory metals is now well known for the metal recovery [12]. The technology of extracting tantalum in molten salts is based on the formation of K2TaF7, obtained by reaction of HF on the oxide Ta205 extracted from raw materials [14] before the reduction of this compound by sodium in the liquid phase ... 

Zirconium is prepared from ZrCLj by the Kroll process (chemical reduction of the chloride by hquid magnesium at 900 °C), but the electrochemical route in molten salts is considered as a good alternative. As for Ta and Nb, pure chloride melts are not appropriate for the electrowinning to yield the metal because of too complex cathode process. The addition of fluoride ion source (KF) allows to stabilize Zr ions in the form of K2ZrFg which is reduced in one step to Zr [23]. 

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