Reference Electrodes

Abstract

In most electrochemical measurements it is necessary to keep one of the electrodes in an electrochemical cell at a constant potential. This so-called reference electrode allows control of the potential of a working electrode (e. g. in voltammetry) or the measurement of an indicator electrode (e. g. in potentiometry, see Chap. II.9). The standard hydrogen electrode plays the role of a basic reference element in electrochemical devices; however, in practice, it is difficult to handle. Therefore, secondary reference electrodes are preferred in most experiments. A secondary reference electrode must fulfill the following criteria: (i) it should be chemically and electrochemically reversible, i. e. its potential is governed by the Nernst equation and does not change in time; (ii) the potential must remain almost constant when a small current passes through the electrode and reverse to its original value after such a small current flow (i. e. a non-polarisable electrode); and (iii) the thermal coefficient of potential should be small. Whereas there is no reference electrode that offers all these properties to the same extent, some electrodes are very close to this ideal behaviour. Here, the most important and most widespread reference electrodes will be described. In general, secondary reference electrodes are electrodes of the second kind, i.e. metal electrodes coupled to a solubility equilibrium of a salt of this metal and an electrolyte solution containing a fixed concentration of the anion of the sparingly soluble metal salt.