STEADY-STATE MULTICOMPONENT DIFFUSION AND MIGRATION TO A PLANAR ELECTRODE .2. MULTIPLE ELECTRODE-REACTIONS

A general solution has been obtained for steady-state one-dimensional diffusion and migration to a planar electrode in a multicomponent syst em involving any number of electrode reactions and non-reacting suppor ting species. The solution includes kinetic effects, making it valid o ver the entire potential range, and is independent of the particular f orm of the kinetic rate expressions. As an example, it has been applie d for a comprehensive study of a system (CuSO4 + H2SO4 + Na2SO4) havin g two competing cathodic reactions (CU2+/CuO and H-/H-2 reduction), fo r which kinetic and transport data are available from the literature. Analysis of the effect of electrolyte composition on the electrode res ponses of both reactions show that there is a significant interaction between the two processes which depends on the presence or absence of supporting electrolyte. Of particular interest are effects that occur when the applied potential is lowered sufficiently that the more elect ropositive reactant (H+ in this case) begins to react and no longer be haves as a supporting ion. One of these effects - enhancement in the l imiting current of the first reaction (Cu2+ reduction) - has been stud ied and simple formulae for this phenomenon have been derived. A secon d effect. which has not been reported previously, is the occurrence of a minimum in the concentration profile of the more electropositive sp ecies. Both phenomena are universal and are not restricted to the part icular system under consideration.