Debye–Hückel Theory

Abstract

According to the state of the art, it is impossible to determine the activity or the activity coefficient of an ion. However, one knows how to approach their numerical values, at least for the sufficiently dilute solutions in ions, solutions qualified of weak ionic strengths. This calculation is possible thanks, notably, to the Debye–Hückel relations. Hence, the Debye–Hückel’s theory is of utmost interest not only, evidently, from the theoretical standpoint but also from the practical one. In order to illustrate this point, the following example of the electrochemical methods of chemical analysis may be mentioned. Some of them respond, indeed, to the activities of species to which they are sensitive. Moreover, from the theoretical standpoint, the Debye–Hückel relations are greatly useful as soon as calculations of the “concentrations” of the ions must be refined.

In this chapter, the setting up of the Debye–Hückel theory is given and its validity is thoroughly discussed. It is particularly the case for some simplifications accompanying the theory which, despite their occurrence, do not preclude its validity, at least in some conditions. It can be said, indeed, that the Debye–Hückel works better than it is awaited!

Some other relations which can be considered as being extensions of the previous ones are also given in the chapter. They are devised in order to be useful in the cases of solutions of less weak ionic strengths as those to which Debye–Hückel relations can legitimately apply.

A brief review concerning the forces exerting between atoms and molecules precede these considerations in order to better specify the range in which Debye–Hückel laws apply and why.