Redox scale, zero point

The connection between this "absolute scale of redox potentials and the conventional scale is given by a linear shift of the zero points (shifted by the energy level of the electrons in the standard hydrogen electrode versus the vacuum level) and by an inversion of the sign of this scale. For photoelectrochemical reactions it is convenient to use the absolute scale even if one measures the redox potentials as usual in the conventional scale versus an arbitrarily chosen reference electrode. 

Redox Potential and Free Energy. The concept of redox potential, derived from the above experimental setup, has been an invaluable aid in chemistry. The concept is intimately associated with the free energy of an oxidation-reduction reaction, because the reaction in a galvanic cell is reversible and electric energy is made available for useful work. Thus the redox potential becomes a direct measure of the free energy (cf. Chapt. V-2), except that it is expressed in different units. It must always be remembered, however, that the redox potential invariably refers to the reaction with gaseous hydrogen. That is the zero point of the redox scale. 

Zero Point on the Biochemical Redox Scale. Since hydrogen ions participate in most biochemical redox reactions, their concentration should also be set at 1 M in order to measure the standard potentials. Of course, this is not feasible because the enzymes are not active at such a low pH. Furthermore, it seems more reasonable to choose physiological conditions. In biochemistry, therefore, it is customary to use standard potentials Eo referring to pH of 7. At that pH, the hydrogen electrode has a potential difference of —0.42 volts with respect to the hydrogen electrode of pH 0. 

---