Standard Electrode Potentials: Units, Conventions, and Methods of Determination *

The standard electrode potential is a physical quantity and, like all physical quantities, it is the product of a numerical value and a unit (volt). It is, in fact, more precise to note that standard electrode potentials represent the Gibbs energy change in the cell reaction. In fact, the electrolyte near the Pt electrode is saturated with hydrogen and that near the AgCl electrode is saturated with AgCl. The direct reaction between these two solutions must be prevented and this is done by providing a long path between the two electrodes. It is not essential to use the hydrogen electrode as one electrode of the experimental cell provided that one of the electrodes is used has a standard electrode potential which is already known. It may be remarked that with some electrodes the experimental difficulties in obtaining reproducible results to errors which are greater than those resulting from the uncertainties in liquid junction potentials or activity coefficients.