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. 61, 2821 (1983).The kinetics of electroreduction of the hexamminocobalt(l1I) cation have been studied at a Hg electrode in aqueous solutions of NaNO, + NaF and KNO, + KF with an ionic strength of 0.2 M. The rate of reaction was observed to increase with increasing nitrate ion concentration but was independent of the cation nature. Analysis of the double layer effect for this system indicates that ion pairs formed between the reacting cation and anions of the base electrolyte also participate in the electrode reaction, and that attractive interactions between the reactant and adsorbed anions have a significant influence on the interfacial kinetics.W. RONALD FAWCETT et KVETA MARKUSOVA. Can. J. Chem. 61, 2821 (1983).On a CtudiC la cinCtique de 1'ClectrorCduction du cation hexamminocobalt(III) utilisant une electrode au Hg dans des solutions aqueuses de NaN03 + NaF et de KNO, + KF avec une force ionique de 0,2 M. On a note que la vitesse de la rCaction augmente avec la concentration en ions nitrates, mais elle ne depend pas de la nature du cation. L'analyse de I'effet de couche double dans ce systkme, indique que les paires d'ions formees entre le cation rCagissant et les anions de I'electrolyte de base participent egalement a la reaction d'Clectrode et que les interactions attractives entre le reactif et les anions absorb& ont une influence significative sur la cinetique I'interface.[Traduit par le journal] Introduction In reviewing the literature concerned with discreteness-ofcharge effects in interfacial electrochemistry, one finds considerable interest in the effects of specifically adsorbed ions on the kinetics of simple electrode reactions (1 -4). A large fraction of the experimental work has been directed to studies of the effects of specifically adsorbed non-reacting anions on the electroreduction of a second anion; an example of such a system, which has been studied in some detail (5-7), is the electroreduction of tetrathionate anion at mercury in the presence of specifically adsorbed iodide anions. In this case, significant deceleration of the reaction rate is observed due to the increase in the electric field at the interface in the negative direction with increasing anion adsorption. The opposite occurs in the case of the electroreduction of a cation; the reaction rate increases with increasing anion adsorption. However, because of the attractive interactions between the reacting cation and adsorbed anion, local ion pairing effects involving the adsorbed species are also possible in the latter system (8-10).Double layer effects on the electroreduction of a variety of Co(II1) ammine complexes were recently studied by Satterberg and Weaver (1 I). They showed that the increase in the logarithm of the apparent rate constant at constant potential is a linear function of adsorbed anionic charge density for a variety of anions including C1-, Br-, C10,-, NO3-, and N3-. They concluded, on the basis of the magnitude of the double layer effects, that the reaction site is in the inner layer. They also discussed di...

W. RONALD FAWCETT and KVETA MARK'USOVA. Can. J. Chem. 60,2038Chem. 60, (1982.The kinetics of electroreduction of tetrathionate anion have been studied at a Hg electrode in aqueous solutions of NaI + NaF and KI + K F with an ionic strength of 0.25 M. The rate of reaction was observed to decrease as the iodide ion concentration was increased, and when the cation was changed from K+ to Na+ at constant electrode potential. Analysis of the double layer effects on the basis of the Frumkin model results in an overestimation of the repulsive effect of the adsorbed iodide anions. This result is interpreted on the basis of the discreteness-of-charge effect and the possible non-coincidence of the reaction plane and outer Helmholtz plane. Chem. 60,2038Chem. 60, (1982. On a Ctudie la cinitique de I'ClectrorCduction de l'anion thathionate au niveau d'une Clectrode au Hg dans des solutions aqueuses de NaI + NaF et de KI + KF avec une force ionique de 0,25 M. On a observe que la vitesse de reduction dicroit lorsqu'on augmente laconcentration en ion iodure et lorsqu'on passe du cation K+ au cation Na+ a potentiel d'tlectrode constant. L'analyse des effets de la couche double en fonction du modele de Frumkin conduit a une surevaluation tres marquee de I'effet rkpulsif des anions iodures absorbts. On interprete ce resultat en fonction du caractere discret de I'effet de charge et de la non-coincidence possible du plan de la reaction et du plan extkrieur de Helmholtz.[Traduit par le journal] Introduction Discreteness-of-charge effects in electrode kinetics have been discussed in detail in recent literature (1-14). One essential feature of the theory for these effects is that the rate of a simple redox process at an inert electrode is changed less than one would predict on the basis of the classical Frumkin theory of double layer effects (15,24) in the presence of specifically adsorbed non-reacting ions. For example, the rate of electroreduction of an anion such as tetrathionate at a mercury electrode is slower in the presence of specifically adsorbed iodide anions than in their absence (16,17), but the extent of deceleration is less than that estimated on the basis of the corresponding shift in the potential drop across the diffuse layer in the negative direction.The electroreduction of tetrathionate was first studied by ieiula (18)(19)(20) who showed that a minimum occurs on the current-potential curve at potentials negative of the point of zero charge (pzc) similar to that found in the electroreduction of other anions (21). The presence of a large double layer effect over a wide potential range for this system makes it a good candidate for more detailed examination. Foresti and Guidelli (16) studied the effects of adsorbed ions on the electroreduction of tetrathionate; however, these experiments were

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