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ION–ION INTERACTIONS 285
Fig. 3.35. The variation of the ion size parameter with concentration
of NaCl.
The best illustration of the fact that a has to be adjusted is its concentration
dependence. As the concentration changes, the ion size parameter has to be modified
(Fig. 3.35). Further, for some electrolytes at higher concentrations, a has to assume
quite impossible (i.e., large negative, irregular) values to fit the theory to experiment
(Table 3.11).
Evidently there are factors at work in an electrolytic solution that have not yet
been reckoned with, and the ion size parameter is being asked to include the effects of
all these factors simultaneously, even though these other factors probably have little
to do with the size of the ions and may vary with concentration. If this were so, the ion
size parameter a, calculated back from experiment, would indeed have to vary with
concentration. The problem therefore is: What factors, forces, and interactions were
neglected in the Debye–Hückel theory of ionic clouds?
