Page 374 - MODERN ELECTROCHEMISTRY

310 CHAPTER 3

It would, however, be advantageous if each electrolyte [e.g., and
were assigned a particular number that would reveal, without going through
the calculation of the extent to which the ions of that electrolyte associate in ion
pairs. The quantitative measure chosen to represent the tendency for ion-pair formation
was guided by historical considerations.
Arrhenius in 1887 had suggested that many properties of electrolytes could be
explained by a dissociation hypothesis: The neutral molecules AB of the electrolyte
dissociate to form ions and and this dissociation is governed by an equilibrium

Applying the law of mass action to this equilibrium, one can define a dissociation
constant

21
By analogy, one can define an association constant for ion-pair formation.
Thus, one can consider an equilibrium between free ions (the positive and the
negative and the associated ion pairs (symbolized IP)

21
The analogy must not be carried too far because it is only a formal analogy. Arrhenius’s hypothesis can
now be seen to be valid for ionogens (i.e., potential electrolytes), in which case the neutral ionogenic
molecules (e.g., acetic acid) consist of aggregates of atoms held together by covalent bonds. What is under
discussion here is ion association, or ion-pair formation, of ionophores (i.e., true electrolytes). In these
ion pairs, the positive and negative ions retain their identity as ions and are held together by electrostatic
attraction.