Differential nonlinearity: Difference between revisions

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[[File:Linear Potential Sweep.JPG|right|400px|thumb|'''Linear potential sweep''']]
'''Linear sweep voltammetry''' is a [[Voltammetry|voltammetric method]] where the current at a [[working electrode]] is measured while the potential between the working electrode and a [[reference electrode]] is swept linearly in time.  Oxidation or reduction of species is registered as a peak or trough in the current signal at the potential at which the species begins to be oxidized or reduced.


== Experimental method ==
The experimental setup for linear sweep voltammetry utilizes a potentiostat and a three-electrode setup to deliver a potential to a solution and monitor its change in current. The three-electrode setup consists of a working electrode, an auxiliary electrode, and a reference electrode. The potentiostat delivers the potentials through the three-electrode setup. A potential, E, is delivered through the working electrode. The slope of the potential vs. time graph is called the scan rate and can range from mV/s to 1,000,000&nbsp;V/s.<ref>{{cite web|last=Tissue|first=Brian M.|title=Linear Sweep Voltammetry|website=CHP|url=http://www.files.chem.vt.edu/chem-ed/echem/linsweep.html}}</ref> At higher scan rates the current is found to increase which improves the signal to noise ratio. Therefore higher scan rates lead to better signal to noise ratios.


The working electrode is where the oxidation/reduction reactions occur. The equation below gives an example of an oxidation occurring at the surface of the working electrode. ES is the standard reduction potential of A. As E approaches ES the current on the surface increases and when E=ES then the concentration of [A] = [A-] at the surface.<ref>{{cite web|last=|first=|title=Voltammetry|website=CHP|url=http://mail.chiangmai.ac.th/~scijjkmn/voltammetry.htm}}</ref> As the molecules on the surface of the working electrode or oxidized/reduced they move away from the surface and new molecules come into contact with the surface of the working electrode. This flow of molecules to and from the working electrode causes the current.
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<math>A+e^-=A^-</math>   E_s=0.00V
 
Oxidation of molecule A at the surface of the working electrode
 
The auxiliary and reference electrode work in unison to balance out the charge added or removed by the working electrode. The auxiliary electrode balances the working electrode, but in order to know how much potential it has to add or remove it relies on the reference electrode. The reference electrode has a known reduction potential. The auxiliary electrode tries to keep the reference electrode at a certain reduction potential and to do this it has to balance the working electrode.<ref>{{cite book|last=Kounaves|first=Samuel P.|title=Voltammetric Techniques. Handbook of Instrumental Techniques for Analytical Chemistry|pages=709–725}}</ref>
 
== Characterization ==
Linear sweep voltammetry can identify unknown species and determine the concentration of solutions. E1/2 can be used to identify the unknown species while the height of the limiting current can determine the concentration. The sensitivity of current changes vs. voltage can be increased by increasing the scan rate. Higher potentials per second result in more oxidation/reduction of a species at the surface of the working electrode.
 
== Variations ==
For reversible reactions cyclic voltammetry can be used to find information about the forward reaction and the reverse reaction. Like linear sweep voltammetry, cyclic voltammetry applies a linear potential over time and at a certain potential the potentiostat will reverse the potential applied and sweep back to the beginning point. Cyclic voltammetry provides information about the oxidation and reduction reactions.
 
== Applications ==
While cyclic voltammetry is applicable to most cases where linear sweep voltammetry is used, there are some instances where linear sweep voltammetry is more useful. In cases where the reaction is irreversible cyclic voltammetry will not give any additional data that linear sweep voltammetry would give us.<ref>{{cite web|title=Instrumentation, Pine Research. Linear Sweep Voltammetry|year=2008|website=CHP|url=http://www.voltammetry.net/pine/aftermath/echem/linear_sweep_voltammetry.}}</ref>
In one example,<ref>{{cite paper|last=Cheng|first=Shaoan|last=Xing|first=Defeng|last=Call|first=Douglas F|last=Logan|first=Bruce E.|title=Direct Biological Conversion of Electrical Current into Methane by Electromethanogenesis|year=2009|journal=Environ. Sci. Technol.|pages=3953–3958.}}</ref> linear voltammetry was used to examine direct methane production via a biocathode. Since the production of methane from CO2 is an irreversible reaction, cyclic voltammetry did not present any distinct advantage over linear sweep voltammetry. This group found that the biocathode produced higher current densities than a plain carbon cathode and that methane can be produced from a direct electrical current without the need of hydrogen gas.
 
==See also==
* [[Cyclic voltammetry]]
* [[Electroanalytical Methods]]
* Linear Sweep Voltammetry/Cyclic Voltammetry. [Online] http://www.basinc.com/mans/EC_epsilon/Techniques/CycVolt/cv.html.
* webmaster@ceb.cam.ac.uk. Linear Sweep and Cyclic Voltametry: The Principles. Department of Chemical Engineering and Biotechnology, University of Cambridge. [Online] http://www.ceb.cam.ac.uk/pages/linear-sweep-and-cyclic-voltametry-the-principles.html.
 
==References==
{{Reflist}}
 
{{Electroanalytical}}
 
[[Category:Electroanalytical methods]]

Latest revision as of 09:43, 22 September 2014


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