ANALYTICAL SOLUTION FOR THE STEADY-STATE DIFFUSION AND MIGRATION INVOLVING MULTIPLE REACTING IONS - APPLICATION TO THE IDENTIFICATION OF BUTLER-VOLMER KINETIC-PARAMETERS FOR THE FERRI- FERROCYANIDE REDOX COUPLE/

An analytical solution for the one-dimensional steady-state transport of ions in an electrolyte towards a planar electrode is obtained. This electrolyte contains more than one electroactive species and any numb er of non-reacting species. The mass and charge transport equations gi ve rise to an implicit form of a set of non-linear algebraic equations which must be solved numerically. The solution is generally applicabl e and can deal with any kind of overpotential relation at the electrod e. The analytical solution is used to determine the mass and charge tr ansport parameters for the Fe(CN)(6)(3-/4-) redox couple in a KCl solu tion for two different electrolyte concentrations (0.03 M Fe(CN)(6)(3- /4-)+1.0 M KCl and 0.005 M Fe(CN)(6)(3-/4-)+0.2M KCl). The agreement b etween the experimental and analytical current densities is perfect fo r both electrolyte solutions under investigation. It is shown that, al though an excess of supporting electrolyte is added, neglecting migrat ion results in an under/overestimation of the diffusion coefficient of ferri-/ferrocyanide of 3 to 5% for the Fe(CN)(6)(3-/4-)/KCl ratios in vestigated. Furthermore, a mathematical background is given for the wi de range of values found in the literature for the charge transfer coe fficient and the rate constants of the ferri-/ferrocyanide redox syste m. The same approach can also be useful for other systems, as to param eter identification procedures. (C) 1997 Elsevier Science S.A.