The relation between the Gibbs phase rule and reference electrodes

Reference electrodes play an important role in the study of many aspects of electrochemical systems. Experimental work in the literature often involves the use of different reference systems, and it is sometimes difficult to translate between measurements made with one from those made using another. This situation is made even worse by the fact that reference electrodes that are used in solid state electrochemical systems are based upon the potentials of electrically neutral chemical species. On the other hand, the general practice in aqueous electrochemistry is to use reference electrodes that involve the properties of ions. Examples are discussed of the translation between reference states based upon neutral species on the basis of normal chemical thermodynamics. The identity and properties of the electrolyte plays no role. In aqueous systems the pH of the electrolyte becomes important in some cases, but not in others. These matters are discussed in terms of the Gibbs Phase Rule, showing the difference between zero-degree-of-freedom (ZDF) electrodes, and those in which an additional parameter, such as the electrolyte pH, must be specified. The interrelationship between these two types is illustrated using potential-pH plots, or Pourbaix diagrams. This provides a simple understanding of the glass electrode systems that are used to measure the pH of electrolytes. In electrodes with a mixed-conducting matrix and an internal ZDF reaction the potential is determined by the internal chemical reaction, rather than the external electrochemical reaction. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998