25: Voltammetry
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- 333378
In voltammetry we apply a time-dependent potential to an electrochemical cell and measure the resulting current as a function of that potential. We call the resulting plot of current versus applied potential a voltammogram, and it is the electrochemical equivalent of a spectrum in spectroscopy, providing quantitative and qualitative information about the species involved in the oxidation or reduction reaction [Maloy, J. T. J. Chem. Educ. 1983, 60, 285–289]. The earliest voltammetric technique is polarography, developed by Jaroslav Heyrovsky in the early 1920s—an achievement for which he was awarded the Nobel Prize in Chemistry in 1959. Since then, many different forms of voltammetry have been developed. Before we examine some these techniques and their applications in more detail, we must first consider the basic experimental design for voltammetry and the factors influencing the shape of the resulting voltammogram.
- 25.1: Potential Excitation Signals and Currents in Voltammetry
- In voltammetry we apply a time-dependent potential to an electrochemical cell and measure the resulting current as a function of that potential.
- 25.2: Voltammetric Instrumentation
- Although early voltammetric methods used only two electrodes, a modern voltammeter makes use of a three-electrode potentiostat. The potential of the working electrode is measured relative to a constant-potential reference electrode that is connected to the working electrode through a high-impedance potentiometer. The auxiliary electrode generally is a platinum wire and the reference electrode usually is a SCE or a Ag/AgCl electrode.
- 25.3: Linear Sweep Voltammetry
- In the simplest voltammetric experiment we apply a linear potential ramp as an excitation signal and record the current that flows in response to the change in potential. Among the experimental variables under our control are the initial potential, the final potential, the scan rate, and whether we choose to stir the solution or leave it unstirred. We call this linear sweep voltammetry.
- 25.4: Cyclic Voltammetry
- In linear sweep voltammetry we scan the potential in one direction, either to more positive potentials or to more negative potentials. In cyclic voltammetry we complete a scan in both directions.
- 25.5: Polarography
- The first important voltammetric technique to be developed—polarography—uses the dropping mercury (DME) electrode as the working electrode. In polarography, as in linear sweep voltammetry, we vary the potential and measure the current. The change in potential can be in the form of a linear ramp, as was the case for linear sweep voltammetry, or it can involve a series of pulses.
- 25.6: Stripping Methods
- Another important voltammetric technique is stripping voltammetry, which consists of three related techniques: anodic stripping voltammetry, cathodic stripping voltammetry, and adsorptive stripping voltammetry. Because anodic stripping voltammetry is the more widely used of these techniques, we will consider it in greatest detail.
- 25.7: Applications of Voltammetry
- Voltammetry finds use for both quantitative analyses and characterization analyses. Examples of each are highlighted in this section.