Reference electrodes and the Gibbs phase rule

Experimental work reported in the electrochemical literature often involves the use of different reference systems, and it is often difficult to trans late between them. Reference electrodes used in solid state electrochemical systems are generally based upon the potentials of electrically neutral ch emical species, and cell voltages can be calculated by the use of normal ch emical thermodynamics. The identity and properties of the electrolyte play no role. In aqueous electrochemistry it is common to use reference electrod es that involve neutral species/ion equilibria at the electrochemical inter face. The pH of the electrolyte is an important consideration in some cases , but not in others. These differences can be understood in terms of the Gi bbs phase rule and the difference between zero-degree-of-freedom (ZDF) elec trodes and those in which an additional intensive parameter, such as the el ectrolyte pH, must also be specified. The interrelationship between these t wo fundamentally different types can be readily seen using potential-pH plo ts, or Pourbaix diagrams. (C) 2000 Elsevier Science B.V. All rights reserve d.