Electrolytic Dissociation

the separation of a compound into ions upon entering solution. Electrolytic dissociation occurs as a result of the interaction of the solute and the solvent. Spectroscopic results indicate that this interaction is largely chemical in nature (seeSOLVATION). In addition to the solvating capacity of the solvent molecules, the dielectric constant of the solvent, a macroscopic property, also plays a certain role in electrolytic dissociation.

The classical theory of electrolytic dissociation, developed by S. Arrhenius and W. Ostwald in the 1880’s, is based on the assumption of the incomplete dissociation of the solute, characterized by the degree of dissociation α, that is, the fraction of the electrolyte molecules that dissociate. The dynamic equilibrium between the undissociated molecules and the ions is described by the law of mass action. For example, the electrolytic dissociation of a binary electrolyte CA is described by an equation of the type CA ⇄ C⁺ + A^–. The dissociation constant K_d is determined by the activities of the cations a_c+, anions a_A, and undissociated molecules a_CA as follows:

The value of K_d depends on the nature of the solute and solvent, as well as on the temperature, and may be determined by several experimental methods. The degree of dissociation α may be calculated for any concentration c of the electrolyte using the equation

where f^± is the mean activity coefficient of the electrolyte (see alsoOSTWALD DILUTION LAW).

The classical theory of electrolytic dissociation is applicable only to dilute solutions of weak electrolytes. Strong electrolytes in dilute solutions are virtually completely dissociated, and consequently the idea of an equilibrium between ions and undissociated molecules is meaningless. According to concepts advanced by V. K. Semenchenko (USSR), N. Bjerrum (Denmark), and R. M. Fuoss (USA) in the 1920’s and 1930’s, ion pairs and more complex aggregates are formed in solutions of strong electrolytes at medium and high concentrations. Modern spectroscopic data indicate that the ion pair consists of two ions of opposite charge in contact (contact ion pair) or separated by one or several solvent molecules (solvent-separated ion pair). Ion pairs are electrically neutral and do not participate in the transmission of electricity. In relatively dilute solutions of strong electrolytes, the equilibrium between individually solvated ions and ion pairs may be described approximately, in a manner similar to the classical theory of electrolytic dissociation, by the dissociation constant (or the inverse term, the association constant). This makes possible the use of equation (2) for calculating the corresponding degree of dissociation on the basis of experimental data.

In the simplest cases (large monoatomic, singly charged ions), the approximate dissociation constants in dilute solutions of strong electrolytes may be calculated theoretically on the basis of a purely electrostatic interaction between the ions in a continuous medium—the solvent.