ORIGINAL PAPER Double layer effects in the electroreduction of transition metal ions W. Ronald Fawcett & Magdalena Hromadová Received: 15 December 2006 / Revised: 15 May 2007 / Accepted: 4 June 2007 / Published online: 4 December 2007 # Springer-Verlag 2007 Abstract Experimental data for the reduction of transition metal complex ions obtained recently at mercury and single-crystal gold electrodes are reviewed. It is shown that the effective charge on the reactant is generally not the same as the nominal charge, but that it may be found on a basis of an analysis of the kinetic data together with the appropriate double layer data. In addition, the effective charge on the product is usually smaller than the nominal value. As a result, the analysis of the double layer effect involves the construction of corrected Tafel plots, which differ significantly from those described in the early work of Frumkin. Introduction Oleg Petrii is best known for his work on platinum electrochemistry and electrocatalysis. In fact, his early work with Frumkin involved the study of double-layer effects in electrode reactions at mercury [1, 2]. They examined in detail the effect of reactant location in the double layer and of ion pairing on the kinetics of electro- reduction of anions. This work made use of extensive double-layer data to construct corrected Tafel plots (cTps). Anion electroreduction is especially interesting at potentials negative of the point of zero charge (pzc) because the reactant is strongly repelled from the double layer and large double-layer effects are observed. The electroreduction of transition metal ions is also a group of reactions in which double-layer effects are large and depend greatly on the position of the standard potential for the electrode process with respect to the pzc of the polarizable electrode. Extensive studies of these systems were made by Weaver and Anson [3–5]. Initially, the reduction of eight Cr(III) complexes of the general structure [Cr(OH 2 ) 5 X] z+ were examined at a mercury electrode in concentrated perchlorate solutions at low pH (∼2) [3]. The response of the system to iodide was determined in an experiment in which perchlorate ion was replaced by a small amount of iodide ion while keeping the ionic strength constant. This allowed Weaver and Anson to distinguish between inner sphere and outer sphere mechanisms for the reduction reaction. In the inner sphere mechanism, the anion X p- in the complex is adsorbed on the mercury electrode and interacts repulsively with adsorbed iodide ions when they are introduced. In the outer sphere mechanism, the anion X p- does not specifically adsorb and remains in the diffuse layer. Under these circumstances, the addition of iodide accelerates the reaction because it makes the electrical field at the outer Helmholtz plane (oHp) more negative. Three systems followed the outer sphere pathway, namely, [Cr(OH 2 ) 6 ] 3+ , [Cr(OH 2 ) 5 F] 2+ , and [Cr(OH 2 ) 5 SO 4 ] + . These reactants have different charges and therefore different double-layer effects. The initial study was extended later to Cr(III) ammine complexes [4] and Co(III) ammine complexes [5]. Thus, a rich variety of octahedral complex ions are available to study the outer sphere electron transfer mechanism. However, the J Solid State Electrochem (2008) 12:347–351 DOI 10.1007/s10008-007-0404-3 Dedicated to Professor Oleg A Petrii on the occasion of his 70th birthday and in honor of his significant contributions to the field of electrochemistry. W. R. Fawcett (*) Department of Chemistry, University of California, Davis, CA 95616, USA e-mail: wrfawcett@ucdavis.edu M. Hromadová J. Heyrovsky Institute of Physical Chemistry, Dolejskova 3, Prague 8, Czech Republic