Transport Control at the Interface

The simplest aspect of transport control is that in which the control arises from the diffusion of ions in solution to the electrode. Let us consider a cathodic reaction, in which electrons are emitted from the electrode to the solution. It is clear that we must here supply ions to the interface. 

In the conditions which are assumed to exist in this chapter so far, the supply of ions is plentiful, i.e., the claims of the electrode for electron transfer to ions are always easily met by diffusion of ions from the solution. When one part of a consecutive series of reactions is easy it is generally said to be in quasiequilibrium. For example, one might think of 100 ions arriving at a small patch on the electrode, 99 % of them going back again, 

At the other end of the spectrum of events, when the demand by electrons is greater than can be easily supplied by diffusion, so that each ion that reaches the electrode is at once used up by the waiting electrons, the control of events will pass to transport ions in the solution and this will be no longer governed by the Butler-Volmer equation. 

The reason why so much attention is devoted to the Butler-Volmer equation and to the exponential dependence of current on potential is interesting to note. First of all, many electrochemical experiments, particularly the fundamental ones, are carried out in the region where the influence of transport control from the solution side is purposely avoided. One simply calculates what the limiting current will be (see below) and then makes one s experiments in the situation when the currents examined are much less than the limiting current, so that the Butler-Volmer equation is applicable. 

Sometimes this is not practical, particularly in industrial practices, where the highest possible rate of production of material is needed, and therefore industrial plants tend to work near the limiting current region. Conversely, in biological situations, it may be that, as mentioned, charge transport within the insulator protein will control the rate of the reaction. 

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