Oxoacidity scale

For ionic melts, we should mention that solvents of the second kind have been studied more intensively than those belonging to the first kind. Also, the treatment of the results former media is simpler. According to the data obtained on equilibrium constants, the general oxoacidity scale may be presented to a sufficient accuracy in the following manner (Fig. 1.1.3). The extremely wide range of liquid state of ionic liquids presented there forces us to picture solvents which cannot coexist at the same temperature in the same scale, but a similar situation is observed for protic solvents (liquid ammonia, water). So, the destruction of nitrate melts runs at temperatures near... 

C whereas the minimum of the melting-point plot for the KC1-NaCl binary system is achieved at the component ratio of 1 1 and corresponds to a temperature near 658 °C. Naturally, such a considerable difference in temperature should cause appreciable changes in the acidic/basic properties of both substances arranged in one conjugate pair. Therefore, we shall consider further in Part 3 the general oxoacidity scales corresponding to the definite temperatures 600, 700 and 800 °C. 

Fig. 1.1.3. The general oxoacidity scale of ionic melts. The positions of the conjugated acid-base pair corresponds to the KCl-NaCl eutectic at 700 °C.

The correct quantitative data can be obtained only for molten alkali metal halides and, mainly for the molten KCl-NaCl eutectic as the reference melt for oxoacidity scales. These data are collected in Table 1.2.3. 

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) ... 

Considerations similar to those lying at the basis of the determination of the acidic properties of ordinary room-temperature molecular solvents [174] can be used to determine the relative acidic properties of high-temperature ionic melts-solvents. To estimate the positions of melts within the general oxoacidity scale, it is necessary to choose the indicator equilibrium and the reference melt. The basicity of the standard solution of a strong base in the reference solvent according to the definition should be equal to zero (0). 

OXOACIDITY SCALES FOR MELTS BASED ON ALKALI- AND ALKALINE-EARTH METAL HALIDES... 

Oxoacidity scales for melts based on alkali- and alkaline-earth metal halides 119... 

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.

To estimate the relative oxoacidic properties of molten alkali-metal halides, the carbonate-ion dissociation reaction was studied for the following sets of melts at 800 °C KCl-KBr-KI [183], CsCl-CsBr-CsI [197], NaCl-NaBr-Nal (830 °C) [198]. Also, in order to arrange all the melts in the oxoacidity scale at 800 °C, the solubility of MgO was determined for the following melts CsCl, KC1, NaCl, KCl-NaCl, KCl-LiCl, CsCl-KCl-NaCl, which allowed the estimation of the oxobasicity indices by the solubility method. 

The oxoacidity scale of molten salts at 800 °C is presented in Fig. 1.3.5. The oxoacidic properties of the chloride melts are stated to be close, whereas those of the bromide ones differ appreciably. As a rule, the bromides are more acidic than the corresponding chlorides. Similar to the single chloride melts, the molten iodides are of the same acidities, which are weaker that those of the chloride melts. Molten bromides are found to possess higher acidities in the chloride-bromide-iodide sequences for potassium and... 

Further development of the problem of general oxoacidity scales in molten salts may be connected with finding more suitable indicator equilibria, and obtaining new data on buffer solution acidities in various melts that will allow the calibration of different oxide ion sensors against 12 values. 

Another peculiarity of the alkali metal chlorides doped with alkali metal chlorides appears in the values of the slopes of the peroxide section they are appreciably lower than 2.3RT/F. The behaviour of the Pt(02)lYSZ membrane oxygen electrode in chloride melts based upon alkali- and alkaline-earth metal chlorides is similar to that in the KCl-LiCl. In this case, the changing position of the inflection point of the E-pO calibration plot agrees with the positions of these melts doped with acidic cations (Ba-Sr-Li-Ca) in the general oxoacidity scale. 

From the viewpoint of the general oxoacidity scale, it is of interest to continue investigations of phenomena such as the levelling of acidic and basic properties by ionic solvents and the effect of temperature on the reciprocal arrangement of ionic melts in the oxoacidity scale. 

Oxoacidity scales of halide melts at different temperatures... 

The principal shape of the melt oxoacidity scale is presented in Fig. 9.2.3. 

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).]...

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