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ION–ION INTERACTIONS 229

Historically, however, the classification of electrolytes was made on the basis of
their behavior in one particular solvent, i.e., water. Weak electrolytes were those that
yielded relatively poorly conducting solutions when dissolved in water, and strong
electrolytes were those that gave highly conducting solutions when dissolved in water.
The disadvantage of this classification into strong and weak electrolytes lies in
the following fact: As soon as a different solvent (i.e., a nonaqueous solvent) is chosen,
what was a strong electrolyte in water may behave as a weak electrolyte in the
nonaqueous solvent. For example, sodium chloride behaves like a strong electrolyte
(i.e., yields highly conducting solutions) in water and acetic acid behaves like a weak
electrolyte. In liquid ammonia, however, the conductance behavior of acetic acid is
similar to that of sodium chloride in water, i.e., the solutions are highly conducting
(Table 3.1). This is an embarrassing situation. Can one say: Acetic acid is weak in
water and strong in liquid ammonia? What is wanted is a classification of electrolytes
that is independent of the solvent concerned. The classification into true and potential
electrolytes is such a classification. It does not depend on the solvent, but rather upon
the degree of ionicity of the substance constituting the solid lattice.

3.2.4. The Nature of the Electrolyte and the Relevance of Ion–Ion
Interactions
Solutions of most potential electrolytes in water generally contain only small
concentrations of ions, and therefore ion–ion interactions in these solutions are
negligible; the ions are on the average too far apart. The behavior of such solutions is
governed predominantly by the position of the equilibrium in the proton-transfer
reaction between the potential electrolyte and water.
In contrast, true electrolytes are completely dissociated into ions when the parent
salts are dissolved in water. The resulting solutions generally consist only of solvated
ions and solvent molecules. The dependence of many of their properties on concen-
tration (and therefore mean distance apart of the ions in the solution) is determined by
the interactions between ions. To understand these properties, one must understand
ion–ion interactions.