Alkali metal halide melt

In every pure alkali metal halide or earth alkali metal halide melt, the cation is surrounded by anions in the first coordination sphere and has cations in the second coordination sphere. This arrangement is caused by the coulombic forces between cations and anions. 

Alkali metal halide melts are characterized by a quasi-crystalline structure originating by dilatation of the crystal structure and by the occurrence of different kinds of positional disordering. Cations and anions are preferably surrounded by ions of the opposite charge... 

The changes in coulomb energy, when substituting one alkali metal cation by another one in a binary alkali metal-earth alkali metal halide melt, will tend to give negative k versus 512 plot slopes, which are decreasing when the size of the common alkali metal cation is increasing. 

Watson and Perry [172], and Picard et al. [173] studied the processes of pyrohydrolysis of alkali metal halide melts doped with ZnCl2 at 600 °C and established that these melts were more acidic than the KCl-LiCl eutectic. 

We shall consider a single alkali-metal-halide melt, in which carbonate ion is dissolved in the form of an alkali metal carbonate. The carbonate-ion dissociation reaction occurs in such a solution as... 

The potentiometric cell construction was similar to that in equation (2.4.28) NaOH and KOH were used as Lux bases for obtaining calibration plots. The behaviour of the membrane oxygen electrode agrees qualitatively with the results obtained in studies of other alkali metal halide melts (Fig. 2.4.13). All the calibration plots are characterized by an inflection point whose position shifts to higher pO values as the melt temperature lowers. At 700 and 800 °C changes in the character of the potential-determining process are observed at pO = 3, at 600 °C the inflection takes place at pO = 4 (Table 2.4.5). It is obvious that such a shift is caused by the peroxide function of the gas membrane oxygen electrode. 

Smirnov, Korzun and Oleynikova investigated the pyrohydrolysis of individual alkali metal halide melts, excluding the lithium and rubidium salts [258] over a wide temperature range. The equilibrium on which the calculations are based on is ... 

Eckstein, Gross and Rubinova reported the use of halide derivatives of silicon for the purification of alkali metal halide melts from oxygen impurities [293] by means of the interaction of SDQ with the mixtures by the reaction ... 

The simplified hole model was shown to describe the data on C02 solubility in the alkali metal halide melts with good accuracy. The entropy changes in the process of dissolution are close to — 1 J mol-1 K-1, which agrees with the data of Novozhilov [311], and the solubility data obtained for the molten chlorides are in good agreement with Ref. [311]. An interesting fact was revealed— the solubility of C02 increased by four times upon the addition of a small concentration of Ni2+ ( 10-3 mol kg-1), introduced into molten NaCl as an admixture. A study of the kinetics of the dissolution process showed that the rate of C02 dissolution in alkali metal halide melts was defined by the rate of transfer of C02 from the gaseous phase into the liquid, but not by the diffusion and convection of the dissolved molecules in the melt. 

Apart from the above-mentioned investigations performed over sufficiently wide temperature ranges, there are some studies of carbonate-ion dissociation performed at a single temperature in alkali metal halide melts of the same cation composition and of individual alkali halides. 

Fig. 2.5.10. The values of pK of equilibrium (1.2.3) in different individual alkali metal halide melts at 800 °C (for the sodium salts the data were obtained at 830 °C).

Thus, in the present part we have considered the key features of the main methods used for the determination of oxide solubilities in molten salts. Such an analysis of their advantages and drawbacks will be very helpful for the consideration and understanding of the data on metal-oxide solubility in alkali-metal halide melts which will be discussed in Part 7. 

Essential deviations from the theoretical predictions are observed in the saturated solutions of oxides formed by large cations such as PbO, BaO, SrO, which possess appreciable solubilities in alkali-metal halide melts. The experimentally obtained relative thermal coefficient of solubility of lead oxide in the CsCl-KCl-NaCl eutectic melts is approximately three times as high as the theoretical one. This may be explained by the distinction of the electron structure of lead from other studied metals (lead belongs to p-elements) and because of this, lead drops out of all the found regularities. Another possible reason consists in the closeness of the melt temperature (600 or 700 °C) to the melting points of lead oxide (886 °C), and the complete solubility of PbO in all the studied chloride melts is very appreciable [359]. 

Thus, the electronegativity values are linearly dependent on a parameter similar to the polarizing action of the cation. The dependence of the solubility on the Allred-Rochow electronegativity is divided into two sharply bounded and practically linear plots with close slopes for 3d-elements and for alkaline-earth metal-oxides (see Fig. 3.7.13). Since the slopes of these dependences are close, different positions of these plots may be explained by different relationships between the nuclear charge and Z for metals characterized by different electronic configurations. From the above-said it may be concluded that in high-temperature alkali-metal halide melts a correlation of metal-oxide solubilities with the crystal-lochemical radii of the cations is considerably simpler, i.e. it does not require the introduction of any corrections of the nucleus charge, such as Z. ... 

The said dependences of metal-oxide solubilities on the electronegativities are qualitatively similar for all the studied alkali-metal halide melts, there are no essential distinctions in their behaviour. 

The concentrations of the ionized and non-dissociated forms of the metal-oxides in their saturated solutions in the studied alkali-metal halide melts (on the molar-fraction scale) and the ratios of these forms are presented in Table 3.7.17. 

From the above-mentioned studies [374, 375] it can be concluded that, up to now, systematic solubility investigations in molten potassium halides have not been performed. This also concerns other alkali-metal halide melts, although these results would be very useful for the estimation of the effect of melt acidity and anion composition on metal-oxide solubilities at 800 °C. 

The solubilities of the metal-oxides in alkali-metal halide melts are stated to increase with increase in the parameter a, of the crystalline lattice of the oxides. This linear regularity applies for all the studied oxides possessing the cubic lattice, and can be observed for two oxides, ZnO and PbO, characterized by a hexagonal lattice. 

The solubility products of metal-oxides in alkali-metal halide melts containing such acidic cations as Li+, Ba2+, Sr2+ and Ca2+ are considerably higher than those in low-acidic melts—in particular, KCl-NaCl and CsCl-KCl-NaCl mixtures—and increase in the mentioned sequence of the alkali-and alkaline-earth cations. Changes in the melt composition cause practically the same changes in the solubility products, i.e. the changes of the oxoacidic properties of the melts are the main reason for the oxide solubility changes. 

It is believed that the most convenient method of the elimination of oxygen-containing impurities from alkali metal halide melts is carbohalogenation." " ... 

The presence of a suspension of carbon in alkali metal halide melts is not always desirable and this method cannot therefore be recognized as universal. 

The most attractive candidates for electrodes are thus a combination of an alkali metal (possibly an alkaline earth metal) anode with a halogen or chalcogenide cathode. These might well be immersed in a eutectic alkali metal halide melt electrolyte. 

In describing the properties of complex species in alkali metal halide melts and the electrochemical processes involving these species, the anionic complex should be considered integral with its outer-sphere (OS) cationic shell [1-4], In model calculations, the composition of this shell is chosen rather arbitrarily. However, calculations show that, in many cases, variations in the composition of the second coordination sphere in the model system radically changes the resulting correlations. Therefore, the task is to search for criteria that permit determining the composition of the dominant complex species in alkali metal halide. 

The results permitted to constmct the dependences of oxide solubilities -logSMeo and the molar surface area of the studied oxide. These dependences for some of the studied oxides are presented in Fig. 9.2.9. All dependences are close to Unear plots. Their slopes permit to estimate the surface energy values at the metal oxide-alkali metal halide melt interface boundary. The calculated values of the surface energy at the oxide-chloride melt interface boundary were calculated to be within 30-40 J m range. These values considerably exceed the corresponding parameters estimated for aqueous solutions of electrolytes thus giving rise to essential changes of the solubility with the component sizes. 

---