The Rise of Voltammetry1: From Polarography to the Scanning Electrochemical Microscope

An excellent discussion of the work of Jaroslav Heyrovsky and the development of polarography is given by Zuman (1). Of critical importance in the invention of the polarographic technique was the fact that the electrochemical experiments were carried out using the dropping mercury electrode (DME). Mercury is unique among metal electrodes for several reasons. Because it is a liquid, its interface with the solution is atomically smooth and of well-defined area. Moreover it has a high overpotential for hydrogen evolution (i.e., the proton and water reduction reactions at mercury are slow compared, for example, to these reactions on platinum), so that it has a large range in the negative potential region in water where many reduction reactions can be studied. Finally, in the DME configuration, the surface is continually renewed, preventing contamination of the surface from solution impurities. Such contamination, even from low levels of impurities, can plague measurements with solid electrodes and lead to time-dependent and irreproducible behavior. To understand why Heyrovsky began working with the DME in 1922, we have to go back a number of years (see the time line in Figure 1). In 1873, Gabriel Lippmann described experiments in which he measured the rise of mercury in a vertical capillary tube in contact with an aqueous solution (Figure 2A). The position of the mercury in the tube is determined by the surface tension of the interface. Lippmann showed that this position depended upon the potential applied across the interface, since the surface tension of mercury depends upon the charge at its surface. The surface tension is highest at the potential of zero charge, when there is no excess charge at the interface. When it is charged either positively or negatively from this position, the surface tension decreases, because the charged mercury atoms repel one another. The device, known as a Lippmann electrometer, could be used to measure small potential differences, but the main interest was in studies of the mercury–aqueous electrolyte interface and, in particular, for obtaining electrocapillary curves, which showed the change in surface tension as a function of potential. These experiments were followed by experiments in which the mercury was allowed to drop out of a pulled capThe Rise of Voltammetry1: From Polarography to the Scanning Electrochemical Microscope