J Electi-oanal. Chem., 201 (1986) 33-45 Elsevier Sequoia S.A., Lausanne - Printed m The Netherlands 33 zyxwvutsrqp ON THE CATALYTIC EFFECT OF THIOUREA ON THE ELECTROCHEMICAL REDUCTION OF CADMIUM(I1) IONS AT THE DME FROM AQUEOUS 1 M KF SOLUTIONS R.M. SOUTO l , M. SLUYTERS-REHBACH and J.H. SLUYTERS Van ‘t Hoff Laboratory State Umuersrty, Padualaan 8, 3584 CH Utrecht (The Netherlands) (Received 10th September 1985; in revised form 19th November 1985) ABSTRACT From Impedance measurements at varied dc potential it is found that by the presence of throurea the rate constant ki is increased in such a way that the effect is mathematrcally expressed by k: = zyxwvutsrqponm k:‘(l + BF,,), where B is a proportionality constant and Tro stands for the surface excess of thtourea. Because of the stepwise reaction mechamsm an explanation in terms of the brtdging or the surface reaction model fads. A possible alternative interpretation of the effect is given. (I) INTRODUCTION The electrochemical reduction of cadmium(I1) ions at a mercury electrode has frequently served as a model reaction in the study of electrode kinetics. Quite often the purpose was to determine the kinetic parameters k,, (the standard heteroge- neous rate constant) and (Y (the transfer coefficient) in a certain supporting electrolyte, in order to test a new method or apparatus [l-8]. In other cases the influence of e.g. the supporting electrolyte, the solvent or an added inhibitor or catalyst on these parameters was investigated [9-131. As a result, at present a large number of k,, and (Y values, obtained by different methods, and in a variety of electrolyte compositions, is available [14], all based on the validity of the Butler-Volmer equation, i.e. on the assumption that the two electrons are trans- ferred simultaneously in one reaction. Only recently solid evidence was presented [15-171 that this assumption does not hold, and that the reduction reaction proceeds via a so-called CEE mechanism, consisting of the following reaction steps: Rc : Cd(I1) 2 Cd@)* Rl : Cd(II)* + e- = Cd(I) R2 : Cd(I) + e- + Cd(Hg) * On leave from the Department of Physical Chemistry, University of La Laguna, Canary Islands, Spain