Corrosion: Fundamentals, Testing, and Protection P2

A good reference electrode must reach its potential quickly, be reproducible, and remain stable with time. It must have a practically nonpolarizable metal-solution interface; that is, its potential must not depart significantly from the equilibrium value on the passage of a small current across the interface. The potential of the junction between the electrolytes of the reference and test electrodes must be minimized. These criteria are detailed subsequently. | Reference electrode Power supply Reference electrode compartment Luggin capillary Test electrode Auxiliary electrode Fig. 1 Three-electrode device. V voltmeter E. Protopopoff and P. Marcus Potential Measurements with Reference Electrodes Corrosion Fundamentals Testing and Protection Vol 13A ASM Handbook ASM International 2003 p 13 16 Potential Measurements with Reference Electrodes E. Protopopoff Laboratoire de Physico-Chimie des Surfaces CNRS and P. Marcus Ecole Nationale Supérieure de Chimie de Paris Université Pierre et Marie Curie Electrode Selection Characteristics A good reference electrode must reach its potential quickly be reproducible and remain stable with time. It must have a practically nonpolarizable metal-solution interface that is its potential must not depart significantly from the equilibrium value on the passage of a small current across the interface. The potential of the junction between the electrolytes of the reference and test electrodes must be minimized. These criteria are detailed subsequently. Stable and Reproducible Potential. Electrodes used as references should rapidly achieve a stable and reproducible potential that is free of significant fluctuations. To obtain these characteristics it is advantageous whenever possible to use reversible electrodes which can easily be made. The reference electrode arbitrarily chosen to establish a universal potential scale is the standard hydrogen electrode SHE . It consists of a platinized or black platinum wire or sheet immersed in an aqueous solution of unit activity of protons saturated with hydrogen gas at a fugacity of one bar psia . The half-cell reaction is H aq e H2 g . Any non-standard reversible hydrogen electrode with well-controlled H activity and H2 fugacity can also be used as a reference. The equilibrium potential Eeq of a nonstandard reversible hydrogen electrode versus the SHE is from the Nernst equation Eq 25 of the article Electrode Potentials in this Section of the volume Eq

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