Electroanalytical method

Electroanalytical methods are a class of techniques in analytical chemistry which study an analyte by measuring the potential (volts) and/or current (amperes) in an electrochemical cell containing the analyte.[1][2][3][4] These methods can be broken down into several categories depending on which aspects of the cell are controlled and which are measured. The three main categories are potentiometry (the difference in electrode potentials is measured), coulometry (the cell's current is measured over time), and voltammetry (the cell's current is measured while actively altering the cell's potential).

Potentiometry

Potentiometry passively measures the potential of a solution between two electrodes, affecting the solution very little in the process. The potential is then related to the concentration of one or more analytes. The cell structure used is often referred to as an electrode even though it actually contains two electrodes: an indicator electrode and a reference electrode (distinct from the reference electrode used in the three electrode system). Potentiometry usually uses electrodes made selectively sensitive to the ion of interest, such as a fluoride-selective electrode. The most common potentiometric electrode is the glass-membrane electrode used in a pH meter.

A variant of potentiometry is chronopotentiometry which consists in using a constant current and measurement of potential as a function of time. It has been iniatiated by Weber[5]

Coulometry

Main article: Coulometry

Coulometry uses applied current or potential to completely convert an analyte from one oxidation state to another. In these experiments, the total current passed is measured directly or indirectly to determine the number of electrons passed. Knowing the number of electrons passed can indicate the concentration of the analyte or, when the concentration is known, the number of electrons transferred in the redox reaction. Common forms of coulometry include bulk electrolysis, also known as Potentiostatic coulometry or controlled potential coulometry, as well as a variety of coulometric titrations.

Voltammetry

Main article: Voltammetry

Voltammetry applies a constant and/or varying potential at an electrode's surface and measures the resulting current with a three electrode system. This method can reveal the reduction potential of an analyte and its electrochemical reactivity. This method in practical terms is nondestructive since only a very small amount of the analyte is consumed at the two-dimensional surface of the working and auxiliary electrodes. In practice the analyte solutions is usually disposed of since it is difficult to separate the analyte from the bulk electrolyte and the experiment requires a small amount of analyte. A normal experiment may involve 1–10 mL solution with an analyte concentration between 1 and 10 mmol/L. Chemically modified electrodes are employed for high sensitive electrochemical determination of organic molecules as well as metal ions.[6][7][8] [9] [10] [11] [12] [13] [14]

Polarography

Polarography is a subclass of voltammetry that uses a dropping mercury electrode as the working electrode.

Amperometry

Amperometry is the term indicating the whole of electrochemical techniques in which a current is measured as a function of an independent variable that is, typically, time or electrode potential. Chronoamperometry is the technique in which the current is measured, at a fixed potential, at different times since the start of polarisation. Chronoamperometry is typically carried out in unstirred solution and at fixed electrode, i.e., under experimental conditions avoiding convection as the mass transfer to the electrode. On the other hand, voltammetry is a subclass of amperometry, in which the current is measured at varying the potential applied to the electrode. According to the waveform that describes the way how the potential is varied as a function of time, the different voltammetric techniques are defined. Confusion arose recently about the correct use of many terms proper of electrochemistry/electroanalysis, often owing to the diffusion of electroanalytical techniques in fields where they constitute an instrument to use, not being the 'core business' of the study. Though electrochemists are pleased about this, they invite to use the terms properly, in order to avoid fatal misunderstandings.

References

  1. ↑ Skoog, Douglas A.; Donald M. West; F. James Holler (1995-08-25). Fundamentals of Analytical Chemistry (7th ed.). Harcourt Brace College Publishers. ISBN 0-03-005938-0.
  2. ↑ 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.
  3. ↑ Bard, Allen J.; Larry R. Faulkner (2000-12-18). Electrochemical Methods: Fundamentals and Applications (2 ed.). Wiley. ISBN 0-471-04372-9.
  4. ↑ Zoski, Cynthia G. (2007-02-07). Handbook of Electrochemistry. Elsevier Science. ISBN 0-444-51958-0.
  5. ↑ H. F. Weber, Wied. Ann., 7, 536, 1879
  6. ↑ 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.
  7. ↑ 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.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ 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.
  11. ↑ 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.
  12. ↑ 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.
  13. ↑ 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.
  14. ↑ 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.

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