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The microstructure, cleanliness and chemical composition of the electrode surface determines how an electron-transfer reaction proceeds. Although a half-cell reaction of an oxidized species, Ox, being reduced to species R, looks simple

\[\mathrm{Ox + e^- = R}\]

the actual process is much more complex. Let us imagine that you can sit as an observer on the electrode surface and look out into the solution. What you would see, ignoring for the moment the composition of the electrode surface except to say that it is electrically conductive, is solvent molecules. If the solvent is water, its molecules will appear to be moving in and out, but most will seem to be staying at the surface with a preferred orientation. Besides water, cations or anions of the electrolyte are present and some may be adhering to the surface, competing with the water molecules. Assuming that you also have Ox species in the solution, Ox being less numerous due to its lower concentration may also be observed, albeit less frequently than other components, near the electrode surface. When species stick on the surface, we call it adsorption, and the extent with respect to its bulk concentration, is described by an adsorption isotherm. The amount adsorbed will go to a maximum at monolayer coverage or saturation. If you peer out at a distance beyond a few layers of water, species will be seen to be moving at random except when electrolysis consumes Ox and mass transfer of Ox from the bulk solution occurs to replace the depleted Ox species adjacent to the electrode with R being produced. It in turn diffuses out to the bulk solution where its concentration is zero. It is a very dynamic process.

What happens at atomic dimensions in the first layer or so of the solution next to the surface of the electrode depends on the value of the applied potential (with respect to the reference electrode) and on the chemical composition and structure of the surface. At some value of the applied potential, there is no net positive (+) nor negative (-) charge on the electrode surface. The applied potential at which this occurs is referred to as the potential of zero charge, E_pzc. Water molecules will orient themselves with the negative end of the dipole (oxygen) toward the electrode when the potential is positive of E_pzc and visa-versa when it is negative. Similarly, there will be an excess of anions to neutralize the charge when the surface has excess + charge and again, excess of cations when negative. This excess of cations or anions decreases very rapidly with distance as we move away from the electrode to the bulk solution. This region next to the electrode where excess charge may be contained is referred to as the double layer. We can determine the double layer charge as the applied potential is changed from one value to another by integrating the current that flows. For details, the reader is referred to references [Refs. 1-3] and the experiment titled, Chronoamperometry, in this eChem Manual. To address the issue of the chemical composition and structure of an electrode surface and how these can affect the electrode reaction will be illustrated next by discussing what happens at a Pt electrode.