Practical Problems in Voltammetry 3: Reference Electrodes for Voltammetry

the reactions of interest occur at the surface of the working electrode. Therefore, we are interested in controlling the potential drop across the interface between the surface of the working electrode and the solution (i.e., the interfacial potential). However, it is impossible to control or measure this interfacial potential without placing another electrode in the solution. Thus, two interfacial potentials must be considered, neither of which can be measured independently. Hence, one requirement for this counter electrode is that its interfacial potential remains constant, so that any changes in the cell potential produce identical changes in the working electrode interfacial potential. An electrode whose potential does not vary with the current is referred to as an ideal non-polarizable electrode, and is characterized by a vertical region on a current vs. potential plot (F1). However, there is no electrode that behaves in this way (although some approach ideal non-polarizable behavior at low currents). Consequently, the interfacial potential of the counter electrode in the twoelectrode system discussed above varies as the current is passed through the cell. This problem is overcome by using a three-electrode system, in which the functions of the counter electrode are divided between the reference and auxiliary electrodes. This ensures that the potential between the working and reference electrodes is controlled and the current passes between the working and auxiliary electrodes. The current passing through the reference electrode is further diminished by using a highinput-impedance operational amplifier for the reference electrode input. The requirements for the counter electrode of the two-electrode system include a high exchange current (fast electron transfer kinetics), a very large surface area (to lower the current density) and a high concentration of the species involved in the redox reaction, such that the concentrations are not significantly changed by the passage of a current. One previously widely used reference electrode that fulfills these criteria is the saturated calomel electrode (with a large surface area mercury pool). However, since the current passing through the reference electrode in the three-electrode system is many orders of magnitude lower than the current that passes through the twoelectrode system, the requirements for the reference electrode are less demanding; hence, smaller, more polarizable electrodes can be used. One aspect that is often overlooked is the variation of the reference electrode potential with temperature. Ideally, the potential should be temperature independent; however, it typically changes by 0.5 1 mV per degree Celsius. Consequently, precise potential measurements require the use of a constant temperature apparatus. In addition, the temperature at which the measurements were made should always be reported. The absence of any temperature control limits the accuracy of the measurements to about 5 10 mV (although this level of Practical Problems in Voltammetry 3: Reference Electrodes for Voltammetry