A Note on the Application of the Glass Electrode to the Determination of Oxidation-Reduction Potentials

The electromotive force of the chain consisting of a glass electrode and a bare noble metal is influenced by the presence of oxidation-reduction systems in the solution.It has been frequently stated that such an electrode system may be used to determine oxidation-reduction potentials independently of simultaneous effects of pH, but this statement must bo extensively qualified, as will be seen from the following discussion.Ignoring the small asymmetry potential existing between the two glass surfaces, there are four sources of electrical activity in the chain.They are the half cell inside the glass electrode, the liquid-glass interface potential on the inner surface of the glass, the glass-liquid interface potential on the outer surface of the glass, and the oxidation-reduction potenr tial imparted to the platinum.Any solution containing an oxidation-reduction system in which the glass electrode and the platinum electrode are immersed will affect the electromotive force of the chain in two ways.The oxidation-reduction system will impart a potential to the platinum, and the hydrogen ions present will modify the glass-liquid interface potential on the outer surface of the glass.If the pH of the solution remains constant, then only changes in the oxidation-reduction potential will modify the E.M.F. of the chain.The true Eh of the oxidation-reduction system may be calculated by applying a calibration correction obtained by observing the E.M.F. of the chain when immersed in some known system at the same pH as that of the unknown