Voltammetry

Voltammetry is a category of electroanalytical methods used in analytical chemistry and various industrial processes. In voltammetry, information about an analyte is obtained by measuring the current as the potential is varied.

Three electrode system

Voltammetry experiments investigate the half cell reactivity of an analyte. Most experiments control the potential (volts) of an electrode in contact with the anaylte while measuring the resulting current (amps). [Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications. New York: John Wiley & Sons, 2nd Edition, 2000.]

To conduct such an experiment requires at least two electrodes. The working electrode, which makes contact with the analyte, must apply the desired potential in a controlled way and facilitate the transfer of electrons to and from the analyte. A second electrode acts as the other half of the cell. This second electrode must have a known potential with which to gauge the potential of the working electrode, furthermore it must balance the electrons added or removed by the working electrode. While this is a viable setup, it has a number of shortcomings. Most significantly, it is extremely difficult for an electrode to maintain a constant potential while passing current to counter redox events at the working electrode.

To solve this problem, the role of supplying electrons and referencing potential has been divided between two separate electrodes. The reference electrode is a half cell with a known reduction potential. Its only role is to act as reference in measuring and controlling the working electrodes potential and at no point does it pass any current. The auxiliary electrode passes all the current needed to balance the current observed at the working electrode. To achieve this current, the auxiliary will often swing to extreme potentials at the edges of the solvent window, where it oxidizes or reduces the solvent or supporting electrolyte. These electrodes, the working, reference, and auxiliary make up the modern three electrode system.

There are many systems which have more electrodes, but their design principles are generally the same as the three electrode system. For example, the rotating ring-disk electrode has two distinct and separate working electrodes, a disk and a ring, which can be used to scan or hold potentials independently of each other. Both of these electrodes are balanced by a single reference and auxiliary combination for an over all four electrode design. More complicated experiments may add working electrodes as required and at times reference or auxiliary electrodes.

In practice it can be very important to have a working electrode with known dimensions and surface characteristics. As a result, it is common to clean and polish working electrodes regularly. The auxiliary electrode can be almost anything as long as it doesn't react with the bulk of the anaylte solution and conducts well. The reference is the most complex of the three electrodes, there are a variety of standards used and its worth investigating elseware. For non-aqueous work, IUPAC recommends the use of the ferrocene/ferrocenium couple as an internal standard. In most voltammetry experiments a bulk electrolyte (also known as supporting electrolyte) is used to minimize solution resistance. It can be possible to run an experiment without an bulk electrolyte but the added resistance greatly reduces accuracy of the results. In the case of room temperature ionic liquids the solvent can act as the electrolyte.

Theory

The Nernst equation is fundamental to voltammetry and can be used for a reversible reaction. In this equation the R represents the reduced species and O the oxidized.

$E\; =\; E^0\; -\; cfrac\{RT\}\{nF\}\; lncfrac\{c\_R^0\}\{c\_O^0\}$

R = Molar gas constant
T = temperature in K
n = number of electrons transferred (as determined by the stoichiometry of the cell reaction)
F = Faraday constant
E = applied potential
E⁰ = standard reduction potential

Another useful equation in voltammetry is the Butler-Volmer equation. This equation represents the relationship between concentration, current, and potential.

$cfrac\{i\}\{nFA\}\; =\; k^0lbrace\; c\_O^0\; exp\; [-alpha\; heta]\; -c\_R^0\; exp\; [(1-alpha)\; heta]\; brace$

$heta\; =\; nF(E-E^0)/RT$
$k^0$ = heterogeneous rate constant
$alpha$ = transfer coefficient
A = area of the electrode
i = current
n = number of electrons transferred

Types of voltammetry

* Linear sweep voltammetry
* Staircase voltammetry
* Squarewave voltammetry
* Cyclic voltammetry - A voltammetric method that can be used to determine diffusion coefficients and half cell reduction potentials.
* Anodic stripping voltammetry - A quantitative, analytical method for trace analysis.
* Cathodic stripping voltammetry - A quantitative, analytical method for trace analysis.
* Adsorptive stripping voltammetry - A quantitative, analytical method for trace analysis.
* Alternating current voltammetry
* Polarography - a subclass of voltammetry where the working electrode is a dropping mercury electrode (DME), useful for its wide cathodic range and renewable surface.
* Rotated electrode voltammetry - A hydrodynamic technique in which the working electrode, usually a rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE), is rotated at a very high rate. This technique is useful for studying the kinetics and electrochemical reaction mechanism for a half reaction.
* Normal pulse voltammetry
* Differential pulse voltammetry
* Chronoamperometry

History

The beginning of voltammetry was facilitated by the discovery of polarography in 1922 by the Nobel Prize winning chemist Jaroslav Heyrovský. Early voltammetric techniques had many problems, limiting their viability for everyday use in analytical chemistry. The 1960s and 1970s saw many advances in the theory and instrumentation of all voltammetric methods. These advancements improved sensitivity and created new analytical methods. Industry responded with the production of cheaper instruments that could be effectively used in routine analytical work.

Applications

Voltametric SensorsA number of voltammetric systems are produced commercially for the determination of specific species that are of interest in industry and research. These devices are sometimes called electrodes but are, in fact, complete voltammetric cells and are better referred to as sensors.

The Oxygen ElectrodeThe determination of dissolved oxygen in a variety of aqueous environments, such as sea water, blood, sewage, effluents from chemical plants, and soils is of tremendous importance to industry, biomedical and environmental research, and clinical medicine. One of the most common and convenient methods for making such measurements is with the Clark oxygen sensor, which was patented by L.C. Clark Jr., in 1956.

ee also

Electroanalytical Methods

References

1. http://www.drhuang.com/science/chemistry/electrochemistry/polar.doc.htm

2. http://www.autolab-instruments.com/download/content/Appl021.pdf

3. http://www.amelchem.com/download/items/voltammetry/manuals/eng/manual_eng.pdf

4. http://new.ametek.com/content-manager/files/PAR/App%20Note%20E-4%20-%20Electrochemical%20Analysis%20Techniques1.pdf

5. http://www.prenhall.com/settle/chapters/ch37.pdf