Standard Potential and Fermi Level of Redox Systems

4 Standard Potential and Fermi Level of Redox Systems 

Sections 3.2.1 and 3.2.2 dealt with electrochemical reactions at metal electrodes where metal ions were transferred across the metal-electrolyte interface. Redox couples are characterized by molecules or ions in a solution which can be reduced and oxidized by a pure electron transfer. The corresponding reaction is given by 

First we consider only the redox couple itself, dissolved in an electrolyte without any electrode. We again use reaction (3.41) but assume that the electron is still located in the solution. The electrochemical potential of this electron is then given by 

The question arises concerning what is a reasonable reference level. This problem becomes clear by means of another thought experiment . Let us assume that an electron from the vacuum level far outside the electrolyte is captured by a redox ion ML+i)(iiq) (oxidized species) in the electrolyte, which leads to the formation of M (liq) (reduced species). We have then the reaction 

Assuming that the electrochemical potential of the electron in the vacuum is zero, one obtains essentially the same equation as Eq. (3.43), namely 

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