Potential due to electron transfer reaction

So far, we have discussed the electrical potential due to ionized molecules or the electrical polarization within a molecule. It is known that there is another type of potential due to electron transfer reactions, a part of which can be measured as an electrical potential by conventional electrodes. 

Oxidation-reduction reactions (also called redox reactions) are those in which there is a transfer of electrons from an electron donor (the reducing agent) to an electron acceptor (the oxidizing agent). 

Oxidizing and reducing agents function as conjugate redox pairs, corresponding to the conjugate nature of acid-base pairs  

For biological systems this tendency is expressed by the standard reduction potential, Eq, defined as the electromotive force (emf) in volts given by a half-cell in which the reductant and oxidant species are both present at 1.0 M concentration unit activity, at 25°C and pH 7.0 in equilibrium with an electrode which can reversibly accept electrons from the reductant species, according to the equation  

The standard reduction potentials of a number of biologically important redox couples are given in Table 12. Systems having a more negative standard reduction potential than the H2-2H couple have a greater tendency to lose electrons than hydrogen those with a more positive potential have a lesser tendency to lose them. 

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