Potential, Standard

a physicochemical quantity that by convention is used to characterize the equilibrium potential difference between an electrode and a solution when the substances participating in the electrode reaction are in the standard state, that is, when the activities of the substances are equal to unity. Since there is no way to measure the actual potential difference between the electrode and the solution, quantities that characterize the potentials of different electrodes with respect to some reference electrode are used. A normal hydrogen electrode, whose potential is assumed to be zero at any temperature, is usually used as the reference electrode.

The potential of an electrode that is negatively charged with respect to the normal hydrogen electrode has a minus sign, while the potential of a positively charged electrode has a plus sign. An arrangement of the standard potentials of discharge-ionization reactions of metals and hydrogen, placed in ascending order, is called the electromotive-force series. One element will displace a second element from a solution containing the second element’s cations if the standard potential of the first element is less positive than that of the second. Standard potentials are calculated from the results of measurements of the electromotive force of voltaic cells and from the standard values of the change in the Gibbs free energy ΔG° during the reaction. The values of the standard potential can be used to calculate ΔG° and the equilibrium constant of chemical reactions. Such calculations are necessary for thermodynamic calculations and for assessing the possibility that a given chemical reaction will take place.