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ION–ION INTERACTIONS 271

Fig. 3.23. The comparison of the experimentally
observed mean activity coefficients of HCI and those
that are calculated from the Debye–Hückel limiting
law.

solutions, albeit about solutions of extreme dilution. The success of the model is so
remarkable and the implications are so wide (see Section 3.5.6) that the Debye–Hückel
approach is to be regarded as one of the most significant pieces of theory in the ionics
part of electrochemistry. It even rates among the leading pieces of physical chemistry
of the first half of the twentieth century.
The Debye–Hückel approach is an excellent example of electrochemical theory.
Electrostatics is introduced into the problem in the form of Poisson’s equation, and
the chemistry is contained in the Boltzmann distribution law and the concept of true
electrolytes (Section 3.2). The union of the electrostatic and chemical modes of

Fig. 3.24. An electrode immersed in an ionic solution is
often enveloped by an ionic cloud [see Fig. 3.11] in which
the excess charge density varies with distance as shown
in (b).