Electrode Potentials and Energy Scales

Drops in electric potential between dissimilar phases are not experimentally measurable.29 This aspect was discussed at length in the literature between the early 1970s and 1990. The discussion was about what is actually measured as electrode potentials are measured. Now a general consensus seems to have been achieved. While readers are referred to the original literature,3 5 1517 29 31 the main conclusions are summarized here. 

The measurement of (0M - 0s) requires that the two terminals of the measuring instruments be connected to M and to S, respectively. While the former is a metal-metal contact, the latter implies immersion of the metal of the terminal (e.g., Cu) in solution. Thus a new interface (a new electrode) is created. Instead of (0M - 0s), the sum of three A0 is thus measured29  

Equation (11) shows that instead of (0M - 0s), or a relative value of (0M -0s), a difference in electronic energy (expressed in volts) is actually measured. This is perfectly reasonable since electrons move in an external circuit because their total energy (and not only the electrical part) is different in the two electrodes. 

A more general approach has been recently provided by Trasatti.32,33 Let us consider the cell illustrated in Fig. 1(a), whose potential difference is AE  

If M and R are in the same solvent S containing only an inert, surface-inactive supporting electrolyte, AE equals the difference in the potentials of zero charge between the two metals  

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