Impedance of multistep mechanisms: equivalent circuits at equilibrium

The form of the electrochemical impedance of multistep mechanisms at the eq uilibrium potential is investigated for mechanisms constructed from (i) ads orbed species obeying the Langmuir isotherm, static species (soluble specie s with fast mass transport) and electrons or (ii) diffusing species, static species and electrons. Except for some esoteric cases, the number of bends in a Bode plot and the number of circuit elements in the equivalent circui t is a maximum at equilibrium. These numbers depend only on the number of i ndependent reactions and on whether or not a chemical reaction can be const ructed from static species and electrons. The equivalent circuits at equili brium are shown to be RC circuits for case (i), and resistance-Warburg circ uits for case (ii). Inductive behaviour is not possible at the equilibrium potential. (C) 1999 Elsevier Science Ltd. All rights reserved.