Measurement of Current and Potential

Control of Potential and Measurement of Current. With the formulation of the laws of electrolysis by Michael Faraday in 1834, the basis for relating electrolysis currents to chemical quantities was established. Although the concept of electrolysis was known prior to then, its utility in terms of chemical analysis depended on a quantitative relationship between current and equivalents of substance. Because an electrolysis current always necessitates mass transfer to or away from the electrode, the formulation of equations for diffusion by Fick was an important event in developing quantitative relationships.1 With the laws of electrolysis and diffusion established, Heyrovsky combined these in a preferred form to provide a practical analytical method, namely, polarography.2 His real contribution beyond combining the important concepts of Faraday and Fick was to realize that a reproducible and continuously renewed 

Although solid-electrode voltammetry had serious limitations because of the changing character of solid-electrode surfaces, it provides the ability to go to much more positive potentials than are possible with the mercury electrode. Contemporary solid electrodes, particularly glassy carbon, allow one to obtain reproducible results (see Chapter 5). 

Another limitation of solid electrodes has been their complex diffusion-current response relative to time with slow-sweep voltammetry. The development of a capillary hanging-mercury-drop electrode (HMDE) by Kemula and Kublik,4,5 together with modem electronic instrumentation, allowed the principles of voltage-sweep voltammetry and cyclic voltammetry to be established. The success has been such that this has become one of the most important research tools for electrochemists concerned with the kinetics and mechanisms of electrochemical processes. These important contributions by Nicholson and Shain6 7 rely, as have all electrochemical kinetic developments, on the pioneering work by Eyring et al.8 

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We are now in a position to see the major difference between measurement of potential and measurement of current. We recall that we want a zero current when measuring the potential. We see from the argument above that a zero current implies that no compositional changes occur inside an electrochemical cell. Conversely, compositional changes do occur during measurement of current, precisely because charge is transferred. 

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