Two electroneutrality and constant-field solutions of the diffusion–migration problem under steady-state conditions on micro-electrodes were compared; where both approximations permitted analytical solution. Analytical equations for the potential drop across the diffusion layer, in terms of RT/F and the migration factor, were obtained for 3- and 4-component systems which contained 2 and 3 types of electro-active ion, respectively, and one type of ion that took no part in the reaction. Both methods yielded virtually identical potential drops. The migration coefficients at large absolute values of potential drop differed noticeably. The migration factor versus potential drop dependences in the 2 approaches differed. The migration-factor values yielded by these methods were close only for absolute potential drops below unity.

Diffusion–Migration Currents on Microelectrodes - Comparing Two Approximate Approaches. Z.A.Rotenberg, B.B.Damaskin: Russian Journal of Electrochemistry, 2004, 40[8], 837-42