Analytical Voltammetry and Polarography

Gunter Henze, Institut fiir Anorganische und Analytische Chemie der Technischen Universitat Claus-thal. Clausthal-Zellerfeld, Germany 

The term voltammetry is used to classify that group of electoanalytical techniques in which the current (ampere) that flows through an electrochemical cell is measured as the potential (volt) applied to the electrodes in the cell is varied. The term is derived from the units of the electrical parameters measured — volt-am (pere)- ue/ry. 

The essential difference between voltammetric and other potentiodynamic techniques, such as constant current coulometry, is that in voltammetry an electrode with a small surface area ( 10mm ) is used to monitor the current produced by the species in solution reacting at this electrode in response to the potential applied. Because the electrode used in voltammetry is so small, the amount of material reacting at the electrode can be ignored. This is in contrast to the case in coulometry where large area electrodes are used so that all of a species in the cell may be oxidized or reduced. 

When mercury is used as the electrode in the form of small drops falling slowly from a fine capillary tube in the test solution, the technique has the special name polarography. This name is derived from the fact that the electrode can be polarized. An electrode is said to be polarized when no direct current flows across its interface 

The voltammetric cell is a multi-phase system, in the simplest form designed with two electronic conductors called electrodes immersed in an electrolytic conductor (ionic cell solution). The electrolytic conductor consists of the sample solution with the electrochemically active analyte and an excess of an inert supporting electrolyte. The analyte is an inorganic or organic species and can be present as a cation, an anion or a molecule. 

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Electrochemical methods continue to be important as well, including inverse voltammetry, coulometry. amperometry, and potentiometry (- Analytical Voltammetry and Polarography) indeed, their overall role has actually been expanded with the development of such chemical techniques as ion chromatography and chelate HPLC. 

The signal current is the result of an electrochemical conversion of the analyte by oxidation or reduction at the working electrode (Fig. 17). The currents are in the pA-pA range and are measured by the three-electrode technique (- Analytical Voltammetry and Polarography). As a result extremely low detection limits at the O.l-pmol level can be achieved. 

For the fast-scan technique in HPLC, in which the potential is varied rapidly across its full range, the method of square-wave voltammetry (- Analytical Voltammetry and Polarography) has provided the basis for the development of a rapid-scan square-wave voltammetric detector ... 

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