Oxobasicity index

THE OXOBASICITY INDEX AS A MEASURE OF RELATIVE OXOACIDIC PROPERTIES OF HIGH-TEMPERATURE IONIC SOLVENTS... 

The oxobasicity index as a measure of relative oxoacidic properties... 

High-temperature ionic solvents are known to contain relatively high total concentrations of cations (e.g. in the KCl-LiCl eutectic, the concentration of Li+ is approximately equal to 8.5 mol kg-1 of the melt). Usually, cation-anion complexes in molten salts are characterized by co-ordination numbers of the order of 4-6. This means that the maximal consumption of acidic cations does not exceed 0.4-0.6 mol kg-1 in diluted solutions with concentrations close to 0.1 mol kg-1. This estimate is considerably lesser than the initial concentration of acidic cations in the pure melt. In the case of the KCl-LiCl eutectic melt, this consumption is only of the order of 5-7%, and the value of NMe+ in equation (1.3.16) may be assumed to be constant. Therefore, for each ionic solvent of the second kind (kind II) the denominator in equation (1.3.16) is a constant which characterizes its acidic properties. We shall define p/L = -log /L to be the relative measure of acidic properties of a solvent and call it the oxobasicity index of ionic melt [37, 162, 181]. Since the direct determination of the absolute concentration of free oxide ions in molten salts is practically impossible, the reference melt should be chosen— for this melt, /L is assumed to be 1 and p/L = 0. The equimolar KCl-NaCl... 

For developing a more general basis for the oxobasicity index (irrespective of the anion and cation compositions of ionic melts), the following scheme [37, 183, 184] should be analysed. 

The considerations stated above show that the oxobasicity index of ionic solvents is the concentration analogue of the primary medium effect for H+ in molecular solvents [174], which allows us to compare the acidic properties of different protolytic and aprotic solvents with respect to H+ as an acidic particle. 

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 oxobasicity index allows us to connect the instrumental values of pO obtained by performing a variety of experiments, with the values of the common oxoacidity function, ll, by a rather simple relationship... 

As follows from Part 1, the ionic melts based on molten alkali metal halides are referred to the solvents of the Second Kind (Kind II), and, therefore, the acid-base ranges for these media are half-open (see Fig. 1.1.1, scheme N3). Therefore, to form an idea of the relative oxoacidic properties of the studied chloride melts it is enough to know their oxobasicity indices. The necessary experimental parameters obtained at 600 °C are presented in Table 1.3.1. The data in this Table show that the KCl-LiCl eutectic melt possesses appreciable acidic properties, the corresponding oxobasicity index being equal to 3.2. 

Investigations of oxide solubilities in the molten KCl-LiCl eutectic at 700 °C have been reported in Refs. [162, 189, 190]. Oxide solubilities in this melt were found to be considerably larger than those in the equimolar KCl-NaCl mixture at the same temperature (see p. 296). The value of the p/VMeO shift as compared with the KCl-NaCl was calculated to be close to 3.4 logarithmic units, this value being the oxobasicity index, p/Kci-ucts = 3.4. It is remarkable that the shifts are so close to each other, which demonstrates the applicability of the parameter introduced for the melt acidity estimations—at least, for calculations of the solubility parameters. Aside from the solubility investigations, the equilibrium constant of the addition of O2- to dichromate ion, Cr202- in the KCl-LiCl eutectic at 700 °C was performed in Ref. [163]. The shift of pK value in this melt as compared with the KCl-NaCl equimolar... 

According to Fig. 1.3.4, in order to determine the oxobasicity index of NaBr with respect to the CsCl-KCl-NaCl eutectic it is necessary to know, at least, two experimental magnitudes. The first set is the oxobasicity index of NaCl, and the difference of pK of CO dissociation in molten NaBr and NaCl. The second set is the difference of pKof CO > dissociation in molten CsBr-KBr-NaBr and CsCl-KCl-NaCl and the difference of pPMeo in molten CsBr-KBr-NaBr and NaBr. The former case is obviously more convenient since the investigation of the oxide solubility products in molten bromides and iodides is more complicated than the determination of the pK of CO in the said melts. 

Fig. 1.3.4. Probable variants of the oxobasicity index determination in the melts of different...

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. 

The data of Table 3.6.4 lead to the conclusion that the use of BaO instead of alkali-metal hydroxides for the titration results in an overestimation of the value of the solubility product. This seems to be caused by Ba2+ cations possessing enhanced acidic properties which arise in the melt, thus leading to the fixation of some quantity of oxide ions from the melt and, consequently, to the increase in the solubility of the studied metal oxide. A similar conclusion may be drawn if we attribute the composition BaCl2-KCl-NaCl to the melt studied. As seen in Part 3, such a melt possesses enhanced acidity that causes an increase in metal-oxide solubilities. We can estimate the oxobasicity index of the melt KCl-NaCl + 0.05 mol kg-1 BaCl2 as +0.23, although this value is comparable with the accuracy of the solubility calculations. 

The solubilities of metal-oxides (on the molar-fraction scale) in chloride melts possessing practically the same oxobasicity-index values are close. This shows the negligibly small effect of low-acidic constituent cations of the melt on its acidity and the solubility product values owing to the levelling of their acidity by that of the most acidic cation of the melt. The solubilities of the oxides in bromide and iodide-melts are considerably lower than those in chloride melts. This may be explained by the stronger association of bromide and iodide ionic melts, and by weakening of the stability of the complexes formed by the hard bases which most cations belong to, with the intermediate (Br ) and soft (I-) bases. 

V.L. Cherginets, O.V. Demirskaya and T.P. Rebrova, Oxobasicity Index as a Measure of Acidic Properties of Ionic Melts Definition and Methods of Acidity Estimation, Molten Salts Forum 7 (2000) 163-166. 

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. 

The oxobasicity index for KCl-LiCl (0.4 0.6) compared with KCl-NaCl at 700"C lies within the range 3.4-3.7 log units. 

To determine the oxobasicity index, the values of scale can be used. The expression for pIl is veiy simple ... 

If E is the emf value measured in the studied melt and E is the value obtained in the reference melt and pi is the oxobasicity index of the reference melt than the oxobasicity index of the studied melt, pIl, is calculated according to the formula ... 

To demonstrate the possibilities of the discussed variants of oxoacidity determination, let us consider the corresponding results obtained in Li -based chloride melts. The method for HCI/H2O acid-base pair yields for KCl-LiCl eutectic value of Q (pl ) ca. 8, whereas the solubility method gives value of and the use of Cr207/CrO acid-base pair permits to estimate plKci-Lici 3.6. It is interesting that the oxobasicity index of 2CsCl-LiCl melt is found in the mentioned above paper to be ca 3.8. The presented comparison shows that the use of HCI/H2O leads to overestimation of oxoacidic properties of the chloride melts whereas the results of other approaches are practically coincident. 

These reactions were considered as indicator reactions. The corresponding pK values are included in Table 9.2.3. Increase in acid strength in iodide melt causes that the pK shifts coincide for both reactions. This magnitude may be treated as the oxobasicity index of molten sodium iodide, pl aj equal to -0.2. The negative sing means that the iodide... 

PIl s valne increases by 0.18. It is thrrs possible to estimate the oxobasicity index of melts, which differ in the content of the same acidic cation, using the following relation ... 

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