Electroanalysis

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Electroanalysis can be defined as the intersection of analytical chemistry and electrochemistry.

The electrochemical analysis , including electro-analytical chemistry or electrochemical analysis called, is a part of the field of analytical chemistry and electrochemistry . Electrochemical methods are used, especially the measurement of currents and / or voltages . This is to achieve an analytical goal, e.g. For example, statements can be made about the type of components in a solution (qualitative analysis) or a concentration can be determined (quantitative analysis). Some electroanalytical methods are of great practical importance and are used routinely worldwide, for example for blood tests in clinical laboratories (e.g. for determining Ca 2+ ). Many other electroanalytical methods are used in electrochemical research.

The multitude of electroanalytical methods is divided into different categories, depending on which physical variable of the electrochemical cell is controlled and which is measured. The main categories are potentiometry , in which a difference in the potentials of two electrodes is measured as a voltage, amperometry , in which a current is measured, coulometry , in which a charge is determined, and voltammetry , in which a current is measured, while changing the potential of an electrode. In addition, conductometry and electrogravimetry are also among the electroanalytical methods.

Potentiometry

Potentiometry with ion-selective electrodes

The measurement of the pH value with the help of a glass electrode, which is used in many laboratories, is one of the best-known measuring methods in which a concentration (more precisely: an activity ) is determined using an electrical voltage. The voltage of the glass electrode is measured against a reference electrode; this is often built into the glass electrode ( combination electrode ). Due to the voltage measurement, it is a potentiometric method. Analogously, all measurements with ion-selective electrodes are potentiometric determinations of concentrations or activities. With the help of ion-selective electrodes, the concentrations of medically important ions ( chloride , H + and cations of sodium , potassium and calcium ) in the blood and urine are determined. Therefore, potentiometry is also important for clinical analysis, but also for water and environmental analysis, e.g. B. with the fluoride-selective electrode .

In these cases, the potential measurement takes place practically without current, so that the examined solution is hardly changed. Suitable measuring devices can also output the pH value or the calculated concentration instead of the measured voltage.

Chronopotentiometry

Chronopotentiometry consists in measuring the electrode potential as a function of time, with a constant current flowing to the electrode from the beginning of the measurement. In this way, diffusion parameters of a component of the solution reacting at the electrode can be determined, as was suggested by Heinrich Friedrich Weber in 1879 . The evaluation of chronopotentiometric measurements is usually carried out using the Sand equation .

Coulometry

In coulometry, a species to be determined is transferred as completely as possible from one oxidation state to another with the aid of a current. The total amount can be calculated by determining the total load. To do this, Faraday's laws are used to convert the charge into an amount or mass of substance. If a constant current is used (galvanostatic coulometry), the charge is simply the product of current and time. A constant potential can also be used (potentiostatic coulometry) to ensure that only the species to be determined reacts. The current is then integrated over time to maintain the charge.

One of the most important and frequently used coulometric determinations is the coulometric variant of the Karl Fischer method , which is always used when small water contents, e.g. B. in solvents or drugs, must be precisely determined.

Amperometry

Amperometry includes the procedures in which a current (unit: ampere ) is measured. Once the electrode potential has been determined, it is a question of potentiostatic amperometry. In the simplest case, the current is then proportional to the concentration of the analyte. If the time course of the current is evaluated, the procedure is called chronoamperometry .

The potentiostatic amperometry is of great practical importance, for example it is used in Germany in every public swimming pool to regulate the chlorine content of the swimming pool water. The oxygen content of water, e.g. B. in the biological treatment of wastewater , is often monitored with the help of amperometric oxygen sensors, z. B. with the so-called Clark electrode .

Voltammetry

In voltammetry - usually in a three-electrode cell and with the help of a potentiostat - the potential of an electrode is changed and the resulting current is measured. For each oxidation or reduction there is then a maximum or a minimum of the current. The type of analyte can be deduced from the peak position, while the peak area depends on its concentration. So in simple cases, for. B. several different metal ions can be identified and determined next to each other. If the electrode potential is changed quickly enough, the amount of substance converted at a small electrode remains small, so that the composition of the solution can remain practically unchanged. The cyclic voltammetry is a voltammetric measurement method, which is rarely used in technical analysis, but that counts in the electrochemical basic research to the main routine process. Since a current is measured, voltammetry can also be counted among the amperometric methods.

Polarography

Polarography is a method of voltammetry that uses a mercury drop electrode as a working electrode . Due to the high overvoltage of hydrogen in relation to mercury, many metal ions can be determined without disruptive hydrogen development.

Historical

As early as 1801, immediately after the discovery of electrolysis, William Cruickshank suggested that electrolysis could also be used to detect metal ions, e.g. B. of copper, can be used. Thereafter, electrolysis was used as an aid in qualitative chemical analysis. If a separation of certain metal ions was helpful for the analysis, these metal ions were cathodically reduced. The resulting metal was then dissolved in nitric acid and still detected by wet chemical means. From 1860 Oliver Wolcott Gibbs introduced electrogravimetry as the first quantitative electroanalytical method. In 1890 Edgar Fahs Smith published a first textbook on electrical analysis; this was reprinted several times (1894, 1902, 1907). From 1921, Jaroslav Heyrovský developed polarography. For this he received the Nobel Prize in Chemistry in 1959 . Since August 1959 the Journal of Electroanalytical Chemistry has been published by Elsevier , and since January 1989 the journal Electroanalysis by John Wiley & Sons .

literature

  • Joseph Wang: Analytical electrochemistry . 3. Edition. John Wiley & Sons, Chichester 2006, ISBN 978-0-471-67879-3 .
  • Hubert H. Girault: Analytical and Physical Electrochemistry . EPFL Press / Marcel Dekker, Inc, Lausanne 2004, ISBN 2-940222-03-7 ( limited preview in Google book search).
  • Kenneth I. Ozomwna (Ed.): Recent Advances in Analytical Electrochemistry 2007 . 1st edition. Transworld Research Network, 2007, ISBN 978-81-7895-274-1 .
  • Edouard Albert Marie Fernand Dahmen: Electroanalysis . Theory and Applications in Aqueous and Non-Aqueous Media and in Automated Chemical Control (=  Techniques and Instrumentation in Analytical Chemistry . Volume 7 ). Elsevier Science Publishing, Amsterdam / Oxford / New York / Tokyo 1986, ISBN 978-0-444-42534-8 ( limited preview in Google book search).

Web links

Individual evidence

  1. ↑ a b Branko N. Popov: Chronopotentiometry . In: Advanced Topics in Electrochemical Science & Corrosion Engineering ECHE 789b, Theory and Applications, Fall 2003 . April 25, 2002, Chapter 7 ( che.sc.edu [PDF; accessed December 31, 2015]).
  2. ^ Heinrich Friedrich Weber : Investigations on the elementary law of hydrodiffusion . on Nov. 25, 1878. In: Gustav Heinrich Wiedemann (Ed.): Annals of Physics and Chemistry . tape 243 , no. 7 . Publisher by Johann Ambrosius Barth, Leipzig 1879, p. 469–487 , doi : 10.1002 / andp.18792430708 ( zs.thulb.uni-jena.de [PDF] online on the pages of Gallica - Bibliothèque nationale de France ).
  3. ^ A b c Edgar Fahs Smith: Electro-Analysis . P. Blakiston's Son & Co., Philadelphia 1907, 4. Historical, p. 19–32 ( online on the Internet Archive website [accessed January 2, 2016]).