Oxygen-containing melts

Acid-base processes in oxygen-containing melts are more complex than those in oxy-gen-less ones, since they are accompanied by competitive equilibria of own acid-base autodissociation of the melt-solvent. The coexistence of acidity and basicity carriers into melt is the characteristic feature of oxygen-containing melts making them similar to low-temperature molecular solvents with own acid-base equilibrium. 

there exist some principal features due to relatively high temperatures of flie liquid state. Own acids of the melts are often unstable or volatile, therefore, acidic solutions, 

The so-called kinetic methods of melt acidity determination, which will be considered below, are based just on this reaction. The reaction [10.4.9] leads to the decrease of nitronium concentration and the rate of NO2 emission decreases until essentially constant acid concentration is determined by a sequence of consecutive measurements. This concentration is the upper limit of acidity of the melt. The thermal dependence of upper limit of acidity in nitrate melts can be easily explained on the basis of the increase of the melt temperature, which leads not only to the reduction of nitronium stability but also to the elevation of process [10.4.9] rate. Sulphate melts have the upper limit of acidity too, it seems connected with limited and low solubility of SO3 at elevated temperatures, such assumption may be confirmed by results.  

3 The effect of the ionic soivent composition on acid-base equilibria 

The equilibrium parameters of Lux acid-base reactions in ionic media (solubility products of oxides and acid-base equilibrium constants) are essentially affected by the acidic properties of the molten alkaline halide mixtures, i.e., they are dependent on the constituent cation acidities. Therefore, one should consider the reverse problem - the estimation of the basicity indices of ionic melts on the basis of the calculated equilibrium constants. 

Shams El Din et al. reported potentiometric investigations of the reversibility of Pt(C 2) gas oxygen electrode in molten KNO3. Na202 was 

Oxygen electrodes in ionic melts. Oxide ion donors 

290-320 °C temperature range. Hence, from the obtained results it can be concluded that Pt(02) gas oxygen electrode shows a good reversibility to oxide ions, and that the potential-determining process at the electrode corresponds to equation (2.4.3). 

Keenan and Williamson studied the use of the gas platinum oxygen electrode in the KN03-NaN03 equimolar mixture at 350 °C with an oxygen-free atmosphere over the melt [221]. The investigation was performed by the calibration of the electrode with quantities of carbonate in a carbon dioxide 

At low concentrations of hydroxide ions (10-6-10-4 mol kg-1), the slope of the E-pO plot for the gold oxygen electrode is equal to 1. 15RT/F. The authors consider that the stabilization of oxide ions in the nitrate melts is caused by the presence of traces of water and silica, and that the latter dissolves in the melt owing to the interaction of the nitrate melt with the Pyrex glass from which the container for the melt is made. This means that the stabilization of oxide ions is nothing but the formation in the melt of silicate and hydroxide ions. If such a stabilization is absent in solutions, then oxide ions are oxidized by 

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Approximate values of z for different electrodes reversible to oxide ions in some oxygen-containing melts... 

Na20 and NaOH (KOH) are suitable for all halide- and oxygen-containing melts at temperatures near and above 500 °C ... 

V.I. Minenko, S.M. Petrov and N.S. Ivanova, The Use of Reversible Oxygen Electrode in Oxygen-Containing Melts, Izv. Vysschikh Utchebn. Zav., Chem. Metallurgia N7 (1960) 10-13. 

So, the behavior of oxygen-containing melts as solvents for acid-base reactions can be described by a manner similar to the solvosystem concept. However, for this purpose the solvosystem concept should be generalized in some relations. The main object lying in the basis of the Franklin s solvosystem concept is a molecular solvent prone to self-ionization with formation of small concentrations of cations and anions of the solvent. Water, spirits, and some other room-temperature liquids are the typical examples. 

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