β Scribed by D.J. Bieman; W.R. Fawcett
No coin nor oath required. For personal study only.
The qualitative effects of ion pairing have been discussed with respect to electrode reactions for which double layer effects are important ~. In the case of reduction reactions, three effects have been considered, namely, ion pairing between a reacting anion and cations in the bulk of the solution 2'3, ion pairing between a reacting anion and cations at the reaction site in the double layer 4, and ion pairing between ions of the base electrolyte 5. The first two effects have been cited as possible causes of the anomalous dependence of the kinetics of anion electroreduction on the nature and charge of the base electrolyte cation. For these systems the charge on the species transported through the double layer to the reaction site must be determined in order to analyse the kinetic data. The usual method of charge determination 6 involves comparison of kinetic data obtained at varying base electrolyte concentration, that is, at varying concentration of the cation which may be involved in ion pairing. Frumkin and co-workers 6'7 pointed out that, when electron transfer is the rate controlling step, the charge calculated is that on the parent anion even if ion pairs participate in the reaction mechanism. However, they did not consider the effects of participation of more than one anionic species on the shape of the resultant corrected Tafel plots.
It was first pointed out by Levich 8 that the concentration of reacting anions may be reduced below the equilibrium value at the reaction site if migration through the diffuse layer is important in the mass transfer process. This effect increases in importance with decrease in ionic strength and increase in the charge on the reacting anion. Levich 9 emphasised the importance of the migration effect with respect to the question of participation of ion pairs in the reaction mechanism, and pointed out that one could consider two limiting cases : if the equilibrium between ion pairs and the parent anion in the bulk is not disturbed significantly by the electrode reaction, then the migration effect results in the ion pair being the predominant reacting species at the electrode; on the other hand, if the bulk equilibrium is sufficiently disturbed by the electrode reaction, the concentration of anions at the reaction site is determined by mass transfer of the parent anion. In the latter case, the possibility arises that the rate determining step becomes a prior chemical reaction involving formation of the ion pair in the double layer.
π SIMILAR VOLUMES
In the rather polar organic solvents dimethoxyethane, tetrahydrofuran and 3-methyltetrahydrofuran, the behaviour of contact ion pairs, solvent-separated ion pairs and free carbanions of polystyryl sodium has been investigated by kinetic and conductivity measurementsβ’ Both the equilibrium between the