Melts based on alkali metal halides

What we have said above especially concerns molten potassium halides, since the ionic radii of Ag+ and K+ are close. 

Characteristic features of the construction of the reference electrodes mentioned consist in the materials serving as current taps. Platinum is used with silver reference electrodes, but cannot be used for lead electrodes since the lead forms alloys with Pt and the current taps are destroyed by such interactions. This requires the use of iron or tungsten wires as intermediate current-tap materials. The potentials of the mentioned reference electrodes are highly reproducible, and they were extensively and thoroughly studied over a wide range of temperatures and concentrations in potentiometric cells of the following kinds  

Oxygen electrodes in ionic melts. Oxide ion donors 

Landresse and Duyckaerts reported the results of a potentiometric investigation of the membrane oxygen electrode NilNiOlYSZ in the KCl-LiCl eutectic at 660 °C [231]. The slope of the -pO calibration plot is approximately equal to. 5RTIF. The results of this study were used to estimate the solubility product of uranium oxide in the chloride melt. 

The work of some of us (Cherginets, Banik and Rebrova) is devoted to studies of the reversibility of various oxygen electrodes such as Pt(02)lYSZ and NilNiO, in the KCl-LiCl eutectic melt at 400, 500, 600 and 700 °C. The potentiometric cell for the measurements at 400 and 500 °C consisted of the lead reference electrode (PblPbCl2, 0.05 mol kg-1) and one of the indicator oxygen electrodes [232]  

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Pure oxygen-less melts contain no oxide ions in any form, and, therefore, such pure melts cannot serve as donors of O2-. The melts, which are solvents of the second kind, can affect acid-base interaction on their background in two manners by fixation of oxide ions entering in the melt and by solvation of the conjugate acid or base. However, the ionic solvents of the second kind, used in practice for different measurements and applied purposes, contain admixtures of oxide-ion donors, which are formed in the melt from initial admixtures of oxo-anions such as SO4-, COf- or OH-. The second way of appearance of oxide ion admixtures in molten media is characteristic of the melts based on alkali metal halides the process of high-temperature hydrolysis of the said halide melts results in the formation of hydroxide ions and, after their dissociation, of oxide ions ... 

A number of our works are devoted to the investigations of different kinds of acid-base equilibria in the ionic melts based on alkali metal halides in order to determine their oxobasicity indices at 700 and 800 °C. Unfortunately, none of the necessary experimental data have been published by other investigators. The equimolar KCl-NaCl mixture has been chosen to be the reference melt at these temperatures, although its oxoacidic properties differ by less than 0.1 from the CsCl-KCl-NaCl eutectic (see below), i.e. they are practically coincident. The solubilities of 11 metal oxides in the equimolar KCl-NaCl mixture are reported in Ref. [175]. Similar investigations in the molten CsCl-KCl-NaCl eutectic [188] allow us to conclude that the solubility products of the same oxide (in molar fraction scale) in both melts are close. This leads to the conclusion that both melts are suitable as reference ones, not only at 700 °C but also at other temperatures at which these media exist in the liquid state. 

All above-mentioned experiments have made a basis for the construction of the oxoacidity scale of melts based on alkali metal halides at 700 °C [195]. This scale is presented in Fig. 1.3.1. The studied ionic melts may be divided into three groups. The first group includes the melts of weak acidity, such as the KCl-NaCl equimolar mixture, the ternary CsCl-KCl-NaCl eutectic possessing the p/csCi-KCi-NaCi,s which is equal to —0.1, and the Nal melt. 

Fig. 1.3.1. Oxoacidity scale of ionic melts based on alkali-metal halides at 700 °C, constructed using p/L s indices for the chloride melts and p/Nai,h index for Nal.

High-temperature hydrolysis of melts based on alkali metal halides... 

Similarly, the spread of the calculated values of the equilibrium constant of the reaction described by the stoichiometry of equation (2.5.68) is very small at all initial molalities of the Lux base and all the partial pressures of water over the melt studied. Hence, the equilibrium (2.5.68) is more correct for the description of the dissociation process for hydroxide ions, at least, in the melts based on alkali metal halides than the generally accepted reaction (1.2.4). 

Investigations of metal-oxide solubilities in the melts based on alkali-metal halides now represent one of the most developed branches of oxoacidity studies, which is of great importance for a variety of practical applications [59, 181]. These applications include processes for the production of various functional materials with the use of different halide melts as media for synthesis (melts-solvents) or as starting materials (this concerns the singlecrystal growth). 

In view of the above-said it is necessary to investigate oxide solubilities in the melts based on alkali-metal halides and to establish regularities in metal-oxide-ionic melt systems . Such data are required for predicting the stability of melts doped with additions of metal cations, and of electrochemical cells containing chlorides of transition metals, under the action of oxide ions formed in the melt owing to oxidation or pyrohydrolysis of its constituent parts. 

The calculations show, however, that the solubilities of metal-oxides in the melts based on alkali-metal halides are considerably lower than the values predicted by the Le Chatelier-Shreder equation, i.e. the properties of the oxide solutions are characterized by negative deviations from ideality. This is caused by the fact that the sum of the interaction energies of the dissolved substance-dissolved substance system (non-dissociated oxide, Me2+-02 )... 

In order to estimate the effect of the cation composition of melts based on alkali-metal halides upon the metal-oxide solubilities, we investigated the... 

Solubility parameters of MgO (pFMgo) in the melts based on alkali-metal halides at 800 °C (molar fractions)... 

Our consideration of the data on metal-oxide solubilities in ionic melts based on alkali-metal halides shows that there is much experimental material, and an attempt at systematizing it is made in this book. Some regularities of metal-oxide solubilities in different melts and over a wide temperature range are obtained. 

For oxides of the type MeO, there exists a linear dependence of the solubility product index of the oxide against the inverse-squared radius of the metal cation. The slopes of these plots in the melts based on alkali-metal halides and alkaline-earth metal halides are stated to be approximately the same. This can give evidence that, at high temperatures in the order of 1000 K, the changes in solvation ability of the ionic melts, proceeding from one melt to another, are close for different cations with radii in the order of 0.1 nm (from 0.74 nm for Mg2+ to 1.38 nm for Ba2+). An increase in the melt temperature... 

Figure 9.2.5. Oxoacidity scale of melts based on alkali metal halides at 700°C, constructed using pl j, >f>dices (for chloride melts the pl indices were used). [Data from V. L. Cherginets and T. P. Rehrova, Doklady Physical Chemistry, 394, 42 (2004).]...

In order to estimate the effect of cation composition of melts based on alkali metal halides on the metal oxide solubilities, we investigated the solubihty of MgO in the following melts KCl and CsCl at 800°C, NaCl (830°C) and molten CsCl-KCl-NaCl and KCl-LiCl eutectic mixtures at 800°C. The obtained solubility parameters are included in Table 9.2.12, where the oxoacidic properties of the chloride melts increase from the top to the bottom. 

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