Solvosystem concept

As is seen from equation (1.1.16), the solvosystem concept is appropriate for describing acid-base interactions in any molecular solvent with a relatively small ionic product and, consequently, slight auto-ionization. Also, it may be used for the description of interactions in covalent melts mercury, zinc and aluminium halides should be mentioned among these. In relation to the terms acid and base , this definition is more common than those formulated by Arrhenius or BrOnstcd and Lowry, although there are charge limitations on acid and base an acid of solvent is a cation particle whereas a base one is an anion. 

Let us consider examples of non-aqueous solvents working in complete agreement with the solvosystem concept. In this respect, antimony(III) chloride and bromide have been studied. According to results of Refs. [23, 24]... 

However, the cases considered are the exceptions rather than the rule. Practical use of the solvosystem concept is laboured since for the systems similar to equation (1.1.19) identification of the ions formed by autoionization and measurement of their equilibrium concentrations are very difficult. Also, the systems mentioned have low dielectric constants that create additional obstacles for the investigations the first being the incomplete dissociation and formation of ionic associates even in diluted solutions. For example, it is known that in liquid sulfur dioxide the following acid-base interaction takes place [26] ... 

Nevertheless, the isotopic exchange examinations of liquid S02 with the use of 180 [27] showed that the equilibrium (1.1.32) did not take place. From the law of mass action, it means that in equation (1.1.32) the degree of ionization in liquid S02 is extremely low, and the isotopic exchange cannot be revealed even by isotopic techniques. High-temperature melts-solvents differ from those described above, first by their considerably higher degree of auto-ionization in such media, ions but not molecules are the main components. For this reason the use of the solvosystem concept to characterize acid-base equilibria as being... 

To conclude this section, it should be emphasized that the solvosystem concept is restricted to self-ionizing solvents it is, therefore, not suitable for completely describing ionic solvents and those which have no ionizing ability. 

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. 

Nevertheless, the behaviour of oxide-containing melts as a kind of background for acid-base reactions can be described in a manner similar to the solvosystem concept. However, for this purpose the solvosystem concept should be generalized in some ways. As mentioned above, the main object in the basis of Franklin s solvosystem concept is a molecular solvent prone to auto-ionization, and this process results in the formation of small concentrations of cations and anions of the solvent. Water, glycol, different spirits and some other room-temperature liquids are to be mentioned as typical examples of such solvents. 

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. 

However, the practice of investigations of acid-base equilibria consists in studies of the reactions with the transfer of definite particles (acid or base) in different solvents, irrespective of the fact whether they are prone to dissociation with the formation of given acid (base), or not. In order to give the most general formulation of the solvosystem concept we should divide the solvent in relation to the initially chosen definition of acids and bases (definition of acidic and basic particles). 

Therefore, the generalized solvosystem concept proposed above is applicable to the description of protic and aprotic solvents, although just for this case this does not allow us to obtain basically new results. As for the case of ionic melts, it is more important, since the theoretical basis of acid-base interactions in this kind of solvents is not developed so extensively. Nevertheless, there are many results on oxoacidity obtained in ionic solvents of the first and the second kinds, which have been treated in a proper way. 

Hence, on the basis of the generalized solvosystem concept it is possible to formulate the main distinctive features of different kinds of high-temperature ionic melts as media for acid-base interactions by the Lux-Flood method. 

The analysis of the literature data, taking into account the generalized solvosystem concept introduced in Part 1, gives a possibility of revealing the... 

Such solvent-dependent behavior of dissolved substances requires the development of definitions for solvent system, which will help in dividing substances into adds and bases depending on their action in the presence of solvents and on concentration of components formed due to the intrinsic self-dissociation process of a given solvent. The most known definition of such kind is definition for solvents system or solvosystem concept by Franklin. According to it, the process of self-ionization of an ionizing solvent L can be written as follows ... 

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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