Finite controlled potential experiments

Constant-current experiments, such as that described in the preceding subsection, are not as popular as experiments in which the potential is the controlled variable. In this section, we shall address controlled-potential experiments in cells with finite planar geometry, even though such experiments do not lend themselves readily to kinetic studies. We shall assume, as in previous sections, that the anode reaction is the converse of the cathode reaction. 

For reasons already mentioned in Sect. 3.1, cells with finite planar geometry are usually thin cells (i.e. L is small, usually a fraction of a millimetre) and there are only two electrodes. A controlled-potential experiment thus usually involves fixing the potential between the two electrodes, though this does not necessarily mean fixing the potential across either electrode. That is, the way in which the applied potential divides itself between the anode and the cathode will, in general, change with time, even if the total applied potential remains constant. For this reason, the simplification that normally attends experiments carried out at constant applied potential is not achieved with finite planar cells. 

An atom or molecule that approaches the surface of a solid always experiences a net attractive potential ). As a result there is a finite probability that it is trapped on the surface and the phenomenon that we call adsorption occurs. Under the usual environmental conditions (about one atmosphere and 300 K and in the presence of oxygen, nitrogen, water vapor and assorted hydrocarbons) all solid surfaces are covered with a monolayer of adsorbate and the build-up of multiple adsorbate layers is often detectable. The constant presence of the adsorbate layer influences all the chemical, mechanical and electronic surface properties. Adhesion, lubrication, the onset of chemical corrosion or photoconductivity are just a few of the many macroscopic surface processes that are controlled by the various properties of a monolayer of adsorbates. 

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