Classic solvosystem concept

The preconditions of another approach to the treatment of acid-base concepts can be found in the classic solvosystem concept described above. Careful reading of this concept shows that the acid-base definition connects the terms acid and base only with the process of autodissociation of a molecular solvent or of one capable of ionization. Nevertheless, it is obvious that acid-base interactions can occur in those solvents, which are not able to form acid and base owing to a dissociation process. Aprotic solvents may serve as a typical example of solvents of such a kind another case of solvents incapable of the acid-base autodissociation takes place if we consider the Lux acid-base equilibria in molten oxygen-free media. Therefore, in relation to any given acid or base there exist two kinds of solvents of differing auto-dissociation ability with the formation of the said acid or base [36, 37, 44, 45]. 

Acid-base processes in molten ionic media are hardly described in the framework of the classic solvosystem concept. The reason for the seeming principal distinction of ionic melts from room-temperature molecular solvents consists in the limitations contained in the solvosystem concept. The main reason is that the division of substances into acids and bases is performed in relation to their reactions with the products of a molecular solvent autodissociation and the degree of this auto-dissociation, in the pure solvent is negligible. On the contrary, an ionic melt is a case of a completely ionized solvent, and this is the fact that does not allow one to apply the solvosystem concept fruitfully for studies of acid-base interactions in this kind of liquid media. 

If we use the classic solvosystem concept to describe acid-base reactions in the ionic melt of NaBF4, completely dissociated into the constituent ions Na+ and BF4, we shall have considerable difficulties, since the concentrations of Na+ and BF4 cannot be varied over a wide range without essential changes of the solvent composition. On the other hand, equilibrium (1.1.34) at 420 °C is characterized by a pK value of 1.8 [30, 31], and if we believe it to be the acid-base dissociation of the melt-solvent then the electron-deficient particle BF3 will demonstrate acidic properties in the melt, whereas fluoride ion F will be the solvent base. 

The above speculations allow us to determine the statements of the classic solvosystem concept which should be generalized for the case of non-dissociated and ionic solvents ... 

Moreover, it should be emphasized that the classic solvosystem concept considers the auto-ionization process to be primary, whereas substances dissolved are referred to as acids and bases by the formation of their adducts with cations and anions of the solvent. 

High-temperature iotuc solvents essentially differ from the above-described solvents, because of considerably higher degree of self-ioruzation, and, because in such media, ions, but not molecules, are the main components. Due to this reasoning, the use of the classic solvosystem concept to characterize add-base equilibria as dependent on self-... 

It should be emphasized that the classic solvosystem concept considers the self-ionization process to be primary, whereas the resultant substances such as acids and bases are obtained on the basis of their reaction with solvent or its constituent components. However, the practice of investigation of acid-base equihbria in solutions consists of studies of donor-acceptor reactions with transfer of a definite component (acid or base) in different solvents irrespective of the fact whether it is able to dissociate with the formation of a... 

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