The equimolar mixture KCl-NaCl

Now let us to estimate the equilibrium molarity of the constituent acidic cations in the melt, e.g., the eutectic KCl-LiCl melt (0.4 0.6) contains 8.5 mole of Lfr per 1 kg. Usually the ionic complexes in melts are characterized by the coordination number 4-6. For the solution of O of the 0.1 mole/kg concentration, the maximum possible quantity of fixed LU concentration may be estimated as 0.4-0.6 mole/kg, i.e., efficiently lower than 8.5. In this case the change of actual LF concentration is approximately equal to 5-7% and m,.[5+ in this case may be suggested as constant. Therefore, for each melt the sum in the denominator of [10.4.12] is the constant reflecting its acidic properties. So, pli= -loglj is a measure of melt acidities and may be denoted as the oxobasicity index of the melt. Since the determination of the absolute concentration of free O is practically impossible one should choose the standard melt , for which Ij is conditionally equal to 1 and pl =0. It is reasonable to choose the equimolar mixture KCl-NaCl as the standard melt , since this melt is most frequently investigated. Further, one should choose standard equilibria and formulate the non-ther-modynamic assumptions which usually postulate that the constant of the standard equilibrium calculated using absolute oxide ion eoncentrations remains flie same for all other melts. 

Barbin et al. determined thermal dependence of LijO solubility in equimolar mixture KCl-NaCl by isothermal saturation technique in temperature range of 973-1073 K N=0.107-5.221/T, in this range the solubility varied from 0.52 to 0.86 mol%. In earlier work by Kaneko and Kojima, the solubility at 973 K was lower (0.31 mol%). The solubility of lithium oxide was close to that for BaO. 

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. 

The equimolar KCl-NaCl mixture and the ternary CsCl-KCl-NaCl (0.455 0.245 0.30) eutectic are shown to be convenient reference melts. The obvious advantage of the latter melt is its low melting point ( 480 °C), which allows its use over a wider temperature range. 

Most of the experiments were carried out using the eutectic mixture LiCl(59 mole %)-KCl(41 mole %). In order to examine the influence of the alkali cations of the solvent two other electrolytes were used the equimolar mixture NaCl-KCl and the eutectic mixture LiCl(59.5 mole %)-CsCl(40.5 mole %). The experiment were carried out over the temperature range 675-995 K in the LiCl-KCl and LiCl-CsCl mixtures and over the range 950-1095 K for the NaCl-KCl mixtures. For experimental details see (8, 9]. In the present paper the potentials are given vs the reference system Cl2/Cl [10]. 

In the present part of the work we studied alloy formation upon the deposition of hafiiium on a niobium cathode from a molten equimolar mixture of NaCl and KCl... 

Polovov, I.B., Vasin, B.D., Abakumov, A.V. et al. (2005) Corrosion of vanadium in molten equimolar mixture of NaCl and KCl. Proceedings of the Seventh International Symposium on Molten Salts Chemistry and Technology, Toulouse, France, Vol. 2, pp. 905-908. 

The classification system described earlier is limited to the simplest kinds of individual melts and is not intended to include mixtures. However, molten mixtures of these different classes of compounds are often more practical solvents than the melts of the individual compounds, due to their much lower melting points and other favorable properties, and this system of classification can usually be extended to these mixtures. For example, the very popular molten LiCl-KCl eutectic mixture is simply a binary ionic melt, whereas molten NaN03-KN03-LiN03 is a ternary polyanionic melt. Interestingly, the equimolar molten mixture of the simple ionic salt NaCl (a) and the molecular compound A1C13 (d) produces a simple polyanionic salt melt (b) composed of Na+ and A1C14 ions ... 

The solubility of MgO in the equimolar NaCl-KCl mixture at 1000 K was found to be very low. The solubility of other alkaline-earth oxides in the equimolar NaCl-KCl melts are low, but are considerably higher than that of MgO and do increase in the sequence MgO < CaO < SrO < BaO. However, in earth alkali metal chloride-rich melts. 

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.

Combes and Tremillon [154] studied the oxoacidic properties of tungsten(VI) oxide and the solubility of calcium tungstate in a molten equimolar KCl-NaCl mixture at 1000 K. A potentiometric cell with the membrane indicator electrode Ni,NiOlYSZ was used for the detection of the equilibrium oxide ion concentration. Investigation of the equilibria taking place in CaW04 solutions in KCl-NaCl allowed them to determine the solubility of CaO in the said melt at 1000 K as 0.084 mol%. The solubility of Scheelite (CaW04) was determined to be 10-3 5 mol kg-1, and the equilibrium constant of reaction (1.2.38) was estimated as 1010. 

To determine the acidic properties of vanadium oxocompounds, we performed potentiometric titration of V2O5 in an equimolar mixture of molten KCl-NaCl at 700 °C with known weights of Na2C03 and NaOH (see Fig. 1.2.9a) [157]. The addition of initial weights of V205 to the molten... 

As for germanium(IV) oxide, this was considered to be slightly soluble in the molten chloride—similar to Si02—and its acidic properties were expected to be too weak. However, one feature was not mentioned. While considering aqueous solutions we can see that Ge02 possesses appreciable solubility in water. For example, at 100 °C approximately 1.0 g of Ge02 is dissolved in 100 g of water, and this allows us to assume that under the experimental conditions conventional for molten chlorides this oxide will be sufficiently soluble. Indeed, this is the case for the molten equimolar KCl-NaCl mixture. 

In contrast, germanium oxide was found to be appreciably soluble in the molten equimolar KCl-NaCl mixture and its solutions show weak acidic properties. Its acid-base titration using NaOH as a titrant results in the appearance of a small pO drop at the equivalence point ( 1 pO unit), which corresponds to the ligand number value —0.5. This allows us to conclude that the acid-base titration runs in one step, and the final product is di-germanate ion, Gc202... 

The cause of the decrease of the calculated pK value is the competitive action of the cation acid Li+, whose acidic properties are sufficiently strong, even on the background of the equimolar KCl-NaCl mixture [141]. In other words, the said shift of the constant value may be considered as the difference of constants of the following equilibria ... 

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. 

There is no the upper limit to the basicity in the molten KCl-LiCl (0.41 0.59) eutectic mixture, and the shift of the acidity scale against the equimolar KCl-NaCl mixture is approximately equal to 8 [169]. 

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) ... 

If the f) values obtained in Refs. [169, 170] may be attributed to the primary medium effect of O2-, they allow us to consider the above-mentioned melts as extremely aggressive acidic media, analogous to anhydrous sulfuric acid in comparison with water (H0 -log y0ff+ = -11.1) [174], In particular, such melts should dissolve appreciable quantities of solid oxides, e.g. MgO or NiO, up to concentrations of the order of 1 mol kg-1. For example, MgO is practically insoluble in the equimolar KCl-NaCl mixture (pP = 9.33 0.06) [175], but should be characterized by values of the order of pP 1 (since 9.33 — 8 1) in the KCl-LiCl eutectic that corresponds to the value of the solubility product of the order of 10 1. For the KCl-NaCl-CaCl2 melt, the solubility-product value should be close to 1. On the contrary, most metal oxides which are slightly soluble in the KCl-NaCl melt were shown by... 

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