Discreteness-of-charge effects in electrode kinetics. I. The electroreduction of tetrathionate anion in the presence of specifically adsorbed iodide anions

Abstract

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

Discreteness-of-charge effects in electrode kinetics. III. The electroreduction of aquopentamminochromium(III) cation in the presence of specifically adsorbed iodide anions

“…At the most negative potentials the influence is greatest, and it decreases steadily as +" -+d becomes more positive. A similar trend was found for the effect of adsorbed I-on the corrected rate of reduction of tetrathionate anion (12). In assessing the variation in y with adsorbed charge density, u, it should be remembered that according to the Frumkin model of double layer effects, y should be independent of solution composition, and therefore, u,, when the effect of the latter is considered in estimating +d.…”

“…[4] is zero. In most cases, g is not exactly one, and in the case of iodide adsorption for the conditions of the present experiment, it is less than one (6,12). In fact, since (a--z) is negative, the first term leads to an increase in the corrected rate of reaction with increase in the extent of iodide adsorption.…”

Discreteness-of-charge effects in electrode kinetics. II. The electroreduction of hexamminocobalt(III) cation in the presence of specifically adsorbed nitrate anions

“…scribed previously (7). The drop time of the mercury electrode was controlled at 5 s and the flow rate was 0.26 mg s-I.…”

“…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)(6)(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 Kinetics of Heterogeneous Electron Transfer Reactions in Polar Solvents

“…This reactant is more hydrophobic and bears a smaller charge. These results can also be partially attributed to discreteness-of- anions, the decelerating effect of the adsorbed charge is overestimated when discreteness-ofcharge effects are ignored, and the value ofy increases with increase in gs [ (130]. Further analysis of these data [130][131][132][133] led to a more detailed description of the double layer effect including the location of the reaction site with respect to adsorbed ions and counter ions.…”