Voltammetry is a category of
electroanalytical methodsused in analytical chemistryand 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 electrodeis 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 electrodepasses 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 electrodehas 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,
IUPACrecommends the use of the ferrocene/ ferroceniumcouple 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.
Nernst equationis fundamental to voltammetry and can be used for a reversible reaction. In this equation the R represents the reduced species and O the oxidized.
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.
= heterogeneous rate constant
= transfer coefficient
A = area of the electrode
i = current
n = number of electrons transferred
Types of voltammetry
Linear sweep 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 techniquein 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 kineticsand electrochemical reaction mechanismfor a half reaction.
Normal pulse voltammetry
Differential pulse voltammetry
The beginning of voltammetry was facilitated by the discovery of
polarographyin 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.
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
electrodesbut 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.
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voltammetry — voltamperometrija statusas T sritis Standartizacija ir metrologija apibrėžtis Elektros srovės stiprio priklausomybės nuo įtampos nustatymo metodas. atitikmenys: angl. voltammetry vok. Voltammetrie, f rus. вольтамперометрия, f pranc. voltammétrie … Penkiakalbis aiškinamasis metrologijos terminų žodynas
voltammetry — voltamperometrija statusas T sritis chemija apibrėžtis Elektros srovės stiprio priklausomybės nuo įtampos nustatymo metodas. atitikmenys: angl. voltammetry rus. вольтамперометрия … Chemijos terminų aiškinamasis žodynas
voltammetry — noun Any of several electrochemical analytical technique in which a current is measured as a voltage is varied … Wiktionary
voltammetry — vol·tam·met·ry .vōl tam ə trē n, pl ries the detection of minute quantities of chemicals (as metals) by measuring the currents generated in electrolytic solutions when known voltages are applied vol·tam·met·ric .vōlt ə me trik adj … Medical dictionary
voltammetry — vol·tam·met·ry … English syllables
voltammetry — … Useful english dictionary
Cyclic voltammetry — Typical cyclic voltammogram where ipc and ipa show the peak cathodic and anodic current respectively for a reversible reaction. Cyclic voltammetry or CV is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment… … Wikipedia
Differential pulse voltammetry — (AKA Differential Pulse Polarography or DPP) is often used to make electrochemical measurements. It can be considered as a derivative of linear sweep voltammetry or staircase voltammetry, with a series of regular voltage pulses superimposed on… … Wikipedia
Anodic stripping voltammetry — is a voltammetric method for quantitative determination of specific ionic species. The analyte of interest is electroplated on the working electrode during a deposition step, and oxidized from the electrode during the stripping step. The current… … Wikipedia
Squarewave voltammetry — is a further improvement of staircase voltammetry which is itself a derivative of linear sweep voltammetry. In linear sweep voltammetry the current at a working electrode is measured while the potential between the working electrode and a… … Wikipedia