Molten salts complex formation

Thermodynamic data show that the stabilities of the caesium chloride-metal chloride complexes are greater than the conesponding sodium and potassium compounds, and tire fluorides form complexes more readily tlrair the chlorides, in the solid state. It would seem that tire stabilities of these compounds would transfer into tire liquid state. In fact, it has been possible to account for the heats of formation of molten salt mixtures by the assumption that molten complex salts contain complex as well as simple anions, so tlrat tire heat of formation of the liquid mixtures is tire mole fraction weighted product of the pure components and the complex. For example, in the CsCl-ZrCU system the heat of formation is given on each side of tire complex compound composition, the mole fraction of the compound... 

Molten salts are characterized by the formation of discrete complex ions that are subjected to coordination phenomenon. Such complex ions have specific compositions that are related to the rearrangement of their electronic configuration and to the formation of partially covalent bonds. The life time of the coordinated ions is longer than the contact period of the individual ions [293]. 

In most cases, the formation of complexes in molten salts leads to an increase in the molar volume relative to the additive volume. This phenomenon is usually explained by an increase in bond covalency. Nevertheless, the nature of the initial components should be taken into account when analyzing deviations in property values, as was shown by Markov, Prisyagny and Volkov [314]. In particular, this rule applies absolutely when the system consists of pure ionic components. The presence of initial components with a significant share of covalent bonds leads to an S-shaped isotherm [314]. 

Electro-conductivity of molten salts is a kinetic property that depends on the nature of the mobile ions and ionic interactions. The interaction that leads to the formation of complex ions has a varying influence on the electroconductivity of the melts, depending on the nature of the initial components. When the initial components are purely ionic, forming of complexes leads to a decrease in conductivity, whereas associated initial compounds result in an increase in conductivity compared to the behavior of an ideal system. Since electro-conductivity is never an additive property, the calculation of the conductivity for an ideal system is performed using the well-known equation proposed by Markov and Shumina (Markov s Equation) [315]. 

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. 

This means that addition of elemental E to alkali metal polychalcogenide fluxes (200-600°C) will promote the formation of longer chains as potential ligands, when such molten salts are employed as reaction media for the preparation of polychalcogenide complexes. Speciation analysis for polychalcogenides in solution has been performed by a variety of physical methods including UV/vis absorption spectroscopy, Raman spectroscopy, Se, Te and Te NMR, electron spin resonance and electrospray mass spectrometry. 

Ccllman s reagent, 704 Complex formation, of molten salts, 377-378 Complex solids, 253—263 Concentration, and stability, 590-593 Conductivity... 

The deviations from ideal behavior can be calculated for all thermodynamic and transport functions of mixtures of molten salts, and these reflect the phenomena which take place at mixing ion associations, formation of complex ions, etc. 

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. 

Bredig [21] presumed that in a mixture of molten salts the variation of the interaction parameter X [see Eq. (8)] makes evident the formation of complex ions. Thus, in the KCl-CdCl2 and RbCl-MgCl2 systems it is observed variations of the X parameter and with the composition of the melt, and the minimum points of those variations correspond to the composition of complex ions. However, the most convincing data on the formation of complex ions can be obtained from measurements of the enthalpy of mixing [22],... 

Using that classification, Redkin modified Biltz and Klemm s table (see Table 3). That classification was confirmed by Nakamura and Itoh [52], who found that the specific conductivity of molten alkali chlorides increases monoton-ically with decreasing cation radius, as shown in Figure 6. In the case of chlorides of alkaline earth metals there is a break at CaCl2, and the specific conductivity decreases dramatically when going to Mg2+ and Be2+. This break may be attributed to complex formation among the component ions in those molten salts [53], This means that in the molten alkali halides there are free ions and no complex formation, a fact confirmed by Raman spectroscopy [52],... 

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. 

Complex Formation Molten salts provide a medium in which the concentration of anionic/ligands can be much higher than is possible in aqueous solutions. The concentration of the chloride ion in concentrated aqueous hydrochloric acid is about 12 M, for example, in contrast. 

Effects of Additives. Selected additives to molten nitrate systems offer possibilities of specific acid-base reactions or formation of specific complexes with the various chemical species. Acids and bases are conveniently denoted in oxyanionic molten salt systems by the Lux-Flood definition (18, 19). Acids are defined as compounds capable of removing oxide ions from the melt, while bases are defined as compounds capable of donating oxide ions to the melt. Examples of various acidic and basic species may be found in the general review articles (14, 15,... 

The process is most efficient when the ratio NaCl CaC2 is 2, and this is explained as being due either to the formation of a complex in solution between Na+ and C2 or to the slow production of the metal followed by its dissolution in the molten salt. The two further processes were studied at 830 and 930 °C ... 

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... 

Among the pentavalent elements, the most important are niobium and tantalum. Niobium is an excellent material for surface treatment of steel materials for chemical industry due to its high hardness and corrosion-resistance in wet acidic conditions. Nowadays, niobium is also used for the preparation of superconductor tapes and it is used in other branches of industry, for instance in nuclear technology and metallurgy. Tantalum is also of similar importance. For these applications, it is necessary to prepare high purity metal. Molten salt electrolysis, as an alternative process to classical thermal reduction, provides niobium and tantalum with required quality. In order to optimize these processes, it is necessary to know details of both complex formation and redox chemistry of the species present in the melts. 

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. 

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. 

Fluorine is produced by electrolysis of molten salts on carbon anodes including KF-21TF at about 100°C, potassium bifluoride at about 250°C, and fluoride salts at about 1000°C. The decomposition potential of molten potassium bifluoride is 1.75 V at 250°C, a value close to that estimated thermodynamically [80]. The kinetics of the anodic process is characterized by a Tafel slope of 0.56 V per decade, j), = 1 x 10 A/cm [81], and by a complex reaction mechanism involving the formation of fluorine atoms on carbon. During the electrolysis, C-F surface compounds on the carbon anode are formed via side reactions. Intercalation compounds such as (CF) contribute to the anodic effect in the electrochemical cell, which can be made less harmful by addition of LiF. 

There is a trend towards the formation of polynuclear complexes in the solutions of oxocompounds of Group V of the Periodic Table in molten salts,... 

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. 

Another question arises why are the saturated solutions of fine dispersed oxides so stable, and the processes of the re-crystallization very slow This is explained by cluster formation in the molten salts. The transfer of oxide from smaller particles to larger ones is considerably retarded since it consists not in the replacement of Me2+ and O2- ions from one particle to another, but in the formation of MeX2 n and KtmOm 2 complexes. Khokhryakov and Khokhlova reported the formation of mixed complexes of composition MeOX - in the molten salts, such complexes preventing precipitation of the metal-oxide to the solid phase [352],... 

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