Electrode Potential, E, and the Rate Equations for Electron Transfer Reactions

Electrode Potential, E, and the Rate Equations for Electron Transfer Reactions 

A single metal-solution potential difference cannot, in principle, be measured so that E must be expressed with respect to the potential of some reference electrode, preferably in the same solution. In certain cases, it is possible to employ a reference electrode at which the same electrode process is occurring (but reversibly) as that being studied [for example, in studies of the hydrogen evolution reaction at a cathode, the reference electrode is simply a hydrogen electrode (H2/platinized platinum) on open circuit with no current passing]. For this case, the quantity E - Ereh which can be measured directly, is called the polarizationt or overpotential (17 

10 sec) than that for electron redistribution (ca. 10 sec). The effects are analogous to those described by the Franck-Condon principle in photochemical excitation of molecules. All electron transfer reactions at electrodes in solution are radiationless photoemission that is sometimes observed is a chemiluminescence (e.g., in oxidation/reduction of rubrene) due to follow-up reactions. 

electron transfer to or from the electrode always takes place to or from electronic energy levels in the metal at or near the so-called Fermi level, defined as the energy state having a probability of occupancy by electrons of 

5 at finite temperatures. The electron transfer is always between states in the organic molecule which have the same energy as those in the electrode, 

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