Voltammetry

File:Linear Potential Sweep.JPG
Linear potential sweep

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.[1][2]

Three electrodes system

File:Three electrode setup.png
Three-electrode setup: (1) working electrode; (2) auxiliary electrode; (3) reference electrode

Voltammetry experiments investigate the half cell reactivity of an analyte. Voltammetry is the study of current as a function of applied potential. These curves I = f(E) are called voltammograms. The potential is varied arbitrarily either step by step or continuously, and the actual current value is measured as the dependent variable. The opposite, i.e., amperometry, is also possible but not common. The shape of the curves depends on the speed of potential variation (nature of driving force) and on whether the solution is stirred or quiescent (mass transfer). Most experiments control the potential (volts) of an electrode in contact with the analyte while measuring the resulting current (amperes).[3]

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 charge 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 charge 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 roles of supplying electrons and providing a reference potential are 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 electrode's 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 overall 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 analyte solution and conducts well. The reference is the most complex of the three electrodes; there are a variety of standards used and it is worth investigating elsewhere. For non-aqueous work, IUPAC recommends the use of the ferrocene/ferrocenium couple as an internal standard.[4] In most voltammetry experiments, a bulk electrolyte (also known as a supporting electrolyte) is used to minimize solution resistance. It is possible to run an experiment without a bulk electrolyte, but the added resistance greatly reduces the accuracy of the results. With room temperature ionic liquids, the solvent can act as the electrolyte.

Theory

Data analysis requires the consideration of kinetics in addition to thermodynamics, due to the temporal component of voltammetry. Idealized theoretical electrochemical thermodynamic relationships such as the Nernst equation are modeled without a time component. While these models are insufficient alone to describe the dynamic aspects of voltammetry, models like the Tafel equation and Butler-Volmer equation lay the groundwork for the modified voltammetry relationships that relate theory to observed results.[5]

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. In 1942 Hickling built the first three electrodes potentiostat.[6] The 1960s and 1970s saw many advances in the theory, instrumentation, and the introduction of computer added and controlled systems. These advancements improved sensitivity and created new analytical methods. Industry responded with the production of cheaper potentiostat, electrodes, and cells that could be effectively used in routine analytical work.

Applications[7]

Voltammetric sensors A 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. These sensors can be employed for the analysis of various organic and inorganic analytes in various matrices.[8][9][10] [11] [12] [13] [14] [15] [16]

The oxygen electrode The 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.

References

1. Kissinger, Peter; William R. Heineman (1996-01-23). Laboratory Techniques in Electroanalytical Chemistry, Second Edition, Revised and Expanded (2 ed.). CRC. ISBN 0-8247-9445-1.
2. Zoski, Cynthia G. (2007-02-07). Handbook of Electrochemistry. Elsevier Science. ISBN 0-444-51958-0.
3. Bard, Allen J.; Larry R. Faulkner (2000-12-18). Electrochemical Methods: Fundamentals and Applications (2 ed.). Wiley. ISBN 0-471-04372-9.
4. Gritzner, G.; J. Kuta (1984). "Recommendations on reporting electrode potentials in nonaqueous solvents". Pure Appl. Chem. 56 (4): 461–466. doi:10.1351/pac198456040461. Retrieved 2009-04-17.
5. Nicholson, R. S.; Irving. Shain (1964-04-01). "Theory of Stationary Electrode Polarography. Single Scan and Cyclic Methods Applied to Reversible, Irreversible, and Kinetic Systems.". Analytical Chemistry 36 (4): 706–723. doi:10.1021/ac60210a007.
6. Hickling, A. (1942). "Studies in electrode polarisation. Part IV.-The automatic control of the potential of a working electrode". Transactions of the Faraday Society 38: 27–33. doi:10.1039/TF9423800027.
8. Sanghavi, Bankim; Srivastava, Ashwini (2010). "Simultaneous voltammetric determination of acetaminophen, aspirin and caffeine using an in situ surfactant-modified multiwalled carbon nanotube paste electrode". Electrochimica Acta 55: 8638–8648. doi:10.1016/j.electacta.2010.07.093.
9. Sanghavi, Bankim; Mobin, Shaikh; Mathur, Pradeep; Lahiri, Goutam; Srivastava, Ashwini (2013). "Biomimetic sensor for certain catecholamines employing copper(II) complex and silver nanoparticle modified glassy carbon paste electrode". Biosensors and Bioelectronics 39: 124–132. doi:10.1016/j.bios.2012.07.008.
10. Sanghavi, Bankim; Srivastava, Ashwini (2011). "Simultaneous voltammetric determination of acetaminophen and tramadol using Dowex50wx2 and gold nanoparticles modified glassy carbon paste electrode" 706. pp. 246–254. doi:10.1016/j.aca.2011.08.040.
11. Sanghavi, Bankim; Srivastava, Ashwini (2011). "Adsorptive stripping differential pulse voltammetric determination of venlafaxine and desvenlafaxine employing Nafion–carbon nanotube composite glassy carbon electrode". Electrochimica Acta 56: 4188–4196. doi:10.1016/j.electacta.2011.01.097.
12. Sanghavi, Bankim; Hirsch, Gary; Karna, Shashi; Srivastava, Ashwini (2012). "Potentiometric stripping analysis of methyl and ethyl parathion employing carbon nanoparticles and halloysite nanoclay modified carbon paste electrode". Analytica Chimica Acta 735: 37–45. doi:10.1016/j.aca.2012.05.029.
13. Mobin, Shaikh; Sanghavi, Bankim; Srivastava, Ashwini; Mathur, Pradeep; Lahiri, Goutam (2010). "Biomimetic Sensor for Certain Phenols Employing a Copper(II) Complex". Analytical Chemistry 82: 5983–5992. doi:10.1021/ac1004037.
14. Gadhari, Nayan; Sanghavi, Bankim; Srivastava, Ashwini (2011). "Potentiometric stripping analysis of antimony based on carbon paste electrode modified with hexathia crown ether and rice husk". Analytica Chimica Acta 703: 31–40. doi:10.1016/j.aca.2011.07.017.
15. Gadhari, Nayan; Sanghavi, Bankim; Karna, Shashi; Srivastava, Ashwini (2010). "Potentiometric stripping analysis of bismuth based on carbon paste electrode modified with cryptand 2.2.1 and multiwalled carbon nanotubes". Electrochimica Acta 56: 627–635. doi:10.1016/j.electacta.2010.09.100.
16. Sanghavi, Bankim; Srivastava, Ashwini (2013). "Adsorptive stripping voltammetric determination of imipramine, trimipramine and desipramine employing titanium dioxide nanoparticles and an Amberlite XAD-2 modified glassy carbon paste electrode". Analyst. doi:10.1039/C2AN36330E.