CHARGE-TRANSFER KINETICS OF A SYSTEM WITH WEAK REACTANT ADSORPTION STUDIED BY THE COMBINATION OF DEMODULATION AND IMPEDANCE VOLTAMMETRY .1. THE TL+/TL(HG) REACTION IN 1-M HCLO4 AT THE DROPPING MERCURY-ELECTRODE

Mathematical expressions are derived and discussed with which the second-order demodulation method, combined with the first-order impedance method, can be used to unravel the effects due to diffusion, charge transfer and reactant adsorption simultaneously controlling an electrode reaction. By assuming that the adsorption is relatively weak, approximations are introduced in order to keep the mathematics tractable. The validity of the treatment is verified by a study of the Tl+/Tl(Hg) system in 1 M HClO4 supporting electrolyte. The Tl+ adsorption is described by the Langmuir adsorption isotherm, with the standard adsorption energy linearly dependent on the electrode potential. The charge-transfer process follows simple Butler-Volmer kinetics, with the transfer coefficient a close to 0.5 and the standard rate constant k(s) = 1.2 cm s-1 just above the detection limit of the impedance method. All parameters determined are independent of the Tl+ concentration, which confirms the internal consistency of the treatment.