STEADY-STATE LIMITING CURRENTS DETERMINED BY COUPLED DIFFUSION, MIGRATION, AND CHEMICAL-EQUILIBRIUM

A simple theoretical model is presented for the reduction of a singly charged cation under conditions where migration is important and the c ation is coupled to a neutral species through a chemical equilibrium, AB = A(+) + B-. Only the steady-state transport-limited current, I-l, is considered. Simple algebraic equations describe the ratio of I-l to the diffusion-limited current, l(d), as it depends on the degree of d issociation, determined by the ratio of equilibrium constant to formal concentration, K-AB/C(AB). The ratio I-l/I-d is found to depend on t he ratio of electrolyte to equilibrium concentration of A(+) in bulk s olution just as for the well-known result for the case without the equ ilibrium (i.e., K-AB-->infinity). The results are in accord with publi shed experimental data for weak acids. Agreement and disagreement with other theoretical treatments of this problem are discussed, The main results are for 1:1 supporting electrolytes; extensions are made to 2: 1, 1:2, and 2:2 supporting electrolytes.