KCl-NaCl 0.500.50 equimolar mixture

Didtchenko and Rochow investigated the effect of the charge and radius of some metal cations [135] on the basic properties of molten lead meta-silicate, PbSi03 at 800, 850 and 900 °C. The said cations were added to the melts in the form of the oxides T120, PbO, CdO, ZnO, Bi2C 3. The melt basicity was stated to increase with the increase of the cation radius for doubly charged cations. 

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Concerning molten alkali-metal halides (which are referred to the solvents of the second kind) it should be emphasized that there is no levelling of acidic properties in them, and, therefore, it is possible to determine the relative strength of acids by measurements in these media. Nevertheless, the properties of strong bases are levelled to those of the oxide formed by the most acidic constituent cation of the melt. As a rule, it is assumed that this oxide is formed by the alkali metal cation of the smallest radius (i.e. Li+ in the KCl-LiCl eutectic, and Na+ in the KCl-NaCl equimolar mixture). 

Fig. 1.2.5. Results of potentiometric investigations of acidic properties of Cr202- in the molten KCl-NaCl equimolar mixture at 700 °C (a) potentiometric curves 1, 0.101 mol kg-1 of Cr202- + O2- (F, the dependence of the ligand number n against the initial concentration of the Lux base) 2, 0.080 mol kg-1 Cr202- + CO2- 3, 0.043 mol kg-1 Cr202- + 02 (b) fractions of oxocompounds of CrVI in diluted solutions (<0.05 mol kg-1) at different pO.

All known investigations of the relative acidic properties of ionic melts are based just on the determination of the equilibrium constants of reaction (1.3.6) in these media. Since the applicability of these data for estimating the equilibrium parameters and conditions was not checked experimentally, there was no reason to doubt the correctness of the use of equilibrium (1.3.6) as an indicator one for f estimations. Indeed, we shall now consider the data presented in Ref. [169], where the acidic properties of the KCl-LiCl eutectic at 600 °C were investigated using equilibrium (1.3.6). The shift of the oxoacidity scale as compared with the KCl-NaCl equimolar mixture was found to be close to 8. Such a considerable difference in acidities for KCl-NaCl and KCl-LiCl melts is unexpected, since the index of the Li20 dissociation constant (pK) ... 

In the general case, the problem of choosing the indicator equilibrium is still far from being solved. Similarly, experimental studies suggest that in the choice of the reference melt, the KCl-NaCl equimolar mixture is an appropriate reference medium, but this system does not exist in the liquid state at temperatures below 658 °C. 

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. 

The calibration plot for the membrane oxygen electrode at 800 °C has no characteristic features, in contrast with similar plots obtained at 600 and 700 °C the inflection point is located at pO 2 and the slope at low pO values is appreciably less than 2.3RT/F. The latter fact is seemingly caused by the elevated acidity of the Li+-containing melt in comparison, for example, with the KCl-NaCl equimolar mixture. 

A simple but reliable experimental routine was used by Combes, Vedel and Tremillon to investigate the pyrohydrolysis of the KCl-NaCl equimolar mixture [240]. For this purpose, a mixture of HC1 and H20 obtained by passing inert gas through aqueous solutions of HC1 of certain concentrations was supplied into the inner space of the potentiometric cell with an indicator oxygen electrode (NilNiOlYSZ). The measurements of the equilibrium oxide-ion concentrations, performed by the potentiometric method, permitted calculation of the equilibrium constants of reaction (3.6), and determination of their thermal dependence ... 

The dependence of the dissociation constant (pK) of carbonate ion in the molten KCl-NaCl equimolar mixture was found by Combes et al. [310] to be expressed by the following equation,... 

Combes et al. reported investigations of the equilibrium constant of reaction (1.2.4) in the molten KCl-NaCl equimolar mixture [119, 180, 240], The experimental routine consists in addition of a weight of O2- donor to the melt afterwards the equilibrium e.m.f. of the cell with the indicator oxygen electrode (NilNiOlZr02) is measured and the e.m.f. values are determined at partial pressures of water vapour from 0.0013 to 0.011 atm. The calculations of the equilibrium constant of reaction (1.2.4), performed using the obtained e.m.f. values, show that in the temperature range 737-817 °C the dependence of the pK of the equilibrium (1.2.4) upon the inverse temperature is expressed by the following equation ... 

Studies of the process of hydroxide ion dissociation performed over a wide range of initial concentrations of oxide ions and partial pressures of H20 demonstrate that in molten alkali metal halides this process does not comply with the generally accepted equation (1.2.4) [232, 321], The dissociation process of OH-, both in the KCl-LiCl eutectic and in the molten KCl-NaCl equimolar mixture, was found to proceed according to the scheme ... 

Before describing the results of our experiment it should be noted that the e.m.f. value measured in the solutions of the initial Lux base at temperatures of 500 °C and higher coincides with the final e.m.f. value after passing the wet gas, both for the KCl-LiCl eutectic and the KCl-NaCl equimolar mixture. This shows that, in the melts studied, the hydroxide ions are completely dissociated with the formation of O2- (according to the potentiometric results). The traces of water remaining in the melts after the bubbling of dry inert gas have no essential effect on the process of dissociation of hydroxide ions. 

As to the problem of completeness of hydroxide ion dissociation in the atmosphere of inert gas, which is not purified from water traces, the situation is as follows. By taking into account the value of the equilibrium constant of reaction (2.5.68), which is equal to 0.006 at 700 °C in the molten KCl-NaCl equimolar mixture, one can estimate the degree of dissociation of hydroxide ion as 0.995 ( 1) at the partial pressure of water vapour near 10-5 atm. (this is the usual concentration for gaseous extra-purity A), 0.857 at 0.001 atm. (for pure Ar) and 0.376 at 0.01 atm. (which is appropriate for nitrogen of technical quality). This means that only extra-pure argon can be used without preliminary purification for the creation of an inert atmosphere over the melts for oxoacidity studies, where the degree of hydroxide-ion dissociation approaches 1 (unity). 

The experimental results and those derived from the potentiometric titration of lead cations with additions of NaOH in the molten KCl-NaCl equimolar mixture at 700 °C are typical examples of the division of the potentiometric titration curves into sections for unsaturated and saturated solutions. We shall now consider the data in Table 3.6.1 and Fig. 3.6.3. 

Experimental and derived results of potentiometric titration of PbCl2 (0.050 mol kg-1) in the molten KCl-NaCl equimolar mixture at 700 °C... 

All the routines described for the determination of the thermodynamic (concentration) parameters in metal oxide solutions include some indirectly obtained values. For example, the equilibrium concentration of metal cations is calculated proceeding from the quantity of the oxide-ion donor consumed for titration (precipitation). Direct determination of the concentration of metal cations in the melt (if it is possible) allows one to obtain more correctly the obtained solubility product values. Our paper [332] reports a method for correction of the solubility product values for oxides on the basis of the potentiometric titration data. The modification of the standard routine consists of the simultaneous use of two indicator electrodes, one of which is the membrane oxygen electrode and the other is a metal electrode, reversible to the cations the oxide consists of. This routine was used to estimate the solubility products of copper(I) and nickel(II) oxides in the molten KCl-NaCl equimolar mixture at 700 °C. Investigation of Cu20 by the proposed method is of considerable importance since, as will be shown further, the process of dissociation/dissolution of copper(I) oxide in molten alkali-metal halides differs from the generally accepted one which was considered, e.g. in Ref. [119]. 

The silver electrode, which is a silver wire with a platinum top immersed in a 0.2 mol kg-1 solution of AgCl in the molten KCl-NaCl equimolar mixture is used as reference electrode [332]. The metal electrode is made as a plate of the metal (2-3 X 1 cm), whose oxide solubility is determined with the platinum top immersed in the solution studied. 

As is seen from Fig. 3.7.1 the pO values at the excess of the studied cation are sufficiently low, and owing to the enhanced acidic properties of the KCl-LiCl melt, the increase in the melt acidity results in a considerable increase in the oxide solubility, so that CoO, which is practically insoluble in the molten KCl-NaCl equimolar mixture, becomes appreciably soluble with a sharp pronounced section of the unsaturated solution (see Fig. 3.7.1, curve 3). The existence of the said section allows us to calculate the dissociation constant of CoO in the molten KCl-LiCl eutectic at 700 °C using the potentiometric data for three initial points of the calibration curve (the corresponding treatment results are collected in Table 3.7.2), and its average value is presented in Table 3.7.3. The fourth point of the titration curve is the boundary one between the saturated and unsaturated solution, and therefore, it is available for calculations of the values of both the dissociation constant and the solubility product. 

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