Lux bases in ionic melts

As repeatedly mentioned in the previous parts, the correctness of the constants obtained for the Lux acid-base equilibria depends essentially on the correctness of the assumption about the completeness of dissociation under the experimental conditions of the strong Lux bases, which are used for potentiometric titration of the studied acids and for calibration of the potentiometric cell with the indicator oxygen electrode. For the latter factor, the accordance to the corresponding Nernst equation of the potentialdetermining process at the indicator electrode is less important than the completeness of the oxide-ion donor s dissociation with the formation of O2-. 

Oxygen electrodes in ionic melts. Oxide ion donors 

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Part 5. Investigations of Dissociation of Lux Bases in Ionic Melts... 

The investigation of the equilibria occurring in the dissociation processes of oxide ion donors (Lux bases) in ionic melts is of great practical importance... 

On the other hand, dissolution of the said bases in ionic melts in an inert atmosphere should cause their complete dissociation according to Le Chatelier s Law, since the partial pressures of conjugated acidic gases (e.g. H20, C02, 02) are clear to be zero and the constituent cations of the melt fix the formed oxide ions. The degree of a Lux base s decomposition is defined by the equilibrium concentration and acidity of the constituent cations of the melt, the temperature, the solubility of the conjugate acid-gas in the melt, and its concentration as a component in the inert gas over the melt. 

K2Cr207 in KNO3 is put down at a constant rate into a melt containing a Lux base. The e.m.f. values of the potentiometric cell are registered and printed using a recorder. This method may be employed to analyse ionic melts basicity in industrial molten systems since it allows one to estimate the concentration of bases in the melts studied, to an accuracy of the order of 3.5%. However, the thermodynamic parameters cannot be estimated on the basis of the e.m.f.-time plots obtained, since the e.m.f. recorded is not referred to the equilibrium state. 

It is obvious that the expression enclosed in the brackets by the author of the present book is nothing but the primary medium effect of O2- expressed via the difference in the values of the equilibrium constants of equation (1.3.6) for the media compared the molten equimolar KCl-NaCl mixture, which was chosen as a reference melt, and for which pKHa/H20 was found to be 14 at 700 °C, and the melt studied. As to the physical sense of the common acidity function Cl, this is equal to the pO of the solution in the molten equimolar KCl-NaCl mixture, whose acidic properties (oxide ion activity) are similar to those of the solution studied. Moreover, from equation (1.3.7) it follows that solutions in different melts possess the same acidic properties (f ) if they are in equilibrium with the atmosphere containing HC1 and H20 and Phc/Ph2o — constant. This explanation confirms that the f function is similar to the Hammett function. Therefore, Cl values measured for standard solutions of strong bases in molten salts allow the prediction of the equilibrium constants on the background of other ionic solvents from the known shift of the acidity scales or the f value for the standard solution of a strong Lux base in the solvent in question. According to the assumption made in Refs. [169, 170] this value may be obtained if we know the equilibrium constant of the acid-base reaction (1.3.6) in the solvent studied. 

The oxoacidic properties of an ionic melt can be characterized by the oxobasicity index defined as the difference of the indices (negative logarithms) of absolute concentrations of oxide ions in standard solutions of a strong Lux base in the reference melt and in the melt studied. 

We have repeatedly emphasized that the most reliable data on Lux acid-base interactions taking place in ionic melts can be obtained using the potentiometric method, which provides detection of the equilibrium oxide-ion concentrations over wide pO ranges. These investigations imply the use of indicator electrodes reversible to O2- in cells with the liquid junction. Literature sources contain much information devoted to studies of the features of reversibility of the following kinds of oxygen electrodes ... 

As mentioned earlier, investigations of the oxygen electrode operation in different ionic melts are numerous. The reversibility of the oxygen electrode is reported practically in every paper concerned with the study of Lux acid-base interactions in ionic melts. 

On the basis of the facts given above, one may consider the following reagents to be strong (completely dissociated) Lux bases for oxoacidity studies in ionic melts ... 

Another definition used in ionic melts concerns donor-acceptor processes with the transfer of oxide ions,. Since 1939, when Lux proposed to define acids as oxide ion acceptors and bases as donors of such kind of acid-base interactions became to be known as... 

Therefore, melts-solvents of the first kind are of interest in the following scientific aspects determination of the acid-base product of the ionic solvent and estimation of the upper limit of acidity of these solvents (such as nitrates, sulfates). The decrease of stability of the solvent acid can be used for the stepwise decomposition of acidic solutions of cations and synthesis of complex oxide compounds and composites by coprecipitation [53-56], It is possible to obtain complex oxides containing alkali metals by precipitation of multivalent metal oxides with the alkali metal oxide as a strong Lux base, as was reported by Hong et al., who used 0.59LiNO3-0.41LiOH mixed melt to obtain electrochemically active lithium cobaltate, LiCo02 [57]. 

Examinations of the interactions of ionic halide melts-solvents with gaseous reagents of acidic or basic character, which include high-temperature hydrolysis of molten alkali-metal halides, their purification from oxygen-containing admixtures, and studies of the dissociation of strong and weak Lux bases (such as COf-, OH-, 0 ) in melts of different acidity. 

The Lux acid-base interactions in oxygen-containing ionic melts have been studied more extensively than those in the media of the second kind. The complications are caused, first by the superimposition of the acid-base equilibrium of the melt-solvents themselves (autodissociation), which, according to what we have said in Part 1, belongs to the solvents of the first kind. 

The Lux acid-base equilibria and the acidic properties of ionic melts are examined using different methods, which can be divided into equilibrium and kinetic ones. The indicator method is one of the simplest equilibrium methods described in the literature. The essence of this method consists in Ref. [68]. A conventional acid-base indicator is added to melts at temperatures... 

The described levelling of the acidic properties in molten nitrates makes them unavailable for the determination of the constants of Lux acid-base equilibria where the strongest acids take part. The melts based on molten alkali metal halides are the most convenient solvents for this purpose. Since the most essential physico-chemical properties of ionic melts (density, charges and ionic radii) are close,1 application of the data obtained in molten chlorides for the description of acid-base equilibria in molten nitrates is more correct than is the use of a similar approach to room-temperature molecular solvents, since the properties determining their acid-base properties are numerous (e.g. dielectric constants, donor-acceptor properties). 

Homogeneous Lux acid-base equilibria similar to equation (0.1) have not been considered earlier as convenient indicator reactions allowing the determination of the oxobasicity indices of ionic melts. Nevertheless, the equilibrium constant of such an equilibrium, pATL, in the melt studied should be expressed as a function of the concentrations and the oxobasicity index in the following manner (the 1 1 1 stoichiometry is assumed)... 

The necessity of passing gaseous oxygen immediately through ionic melts imposes some limitations on the utilization of oxygen electrodes of the first kind for the investigations of the Lux acid-base equilibria in these media. In particular, these electrodes cannot be recommended for the following cases ... 

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