Anodic oxidation

In anodic oxidation , substances are oxidized , with an electric current flowing through an external circuit at the same time . Oxidation means that a substance gives off electrons . These are picked up by an electrode, the anode .

differences

There are different types of anodic oxidation:

• in galvanic cells and in batteries the reaction proceeds ' voluntarily ', i. H. under energy delivery. The substances that are oxidized give their electrons to the anode, which then becomes negatively charged. The anode is the negative pole of the galvanic cell or battery.
• In electrolysis cells the oxidation reaction is forced by an externally applied voltage. The withdrawal of electrons takes place at the positive pole , which receives the electrons: the anode is the positive pole of electrolysis cells. The secondary process that takes place at the anode during the electrolysis of acid and salt solutions, when the discharged “anion” is non-existent, leads to the release of gaseous oxygen through oxidation of hydroxide ions in the case of an unassailable anode.

The anodic oxidation in electrolysis can also oxidize very stable and inert substances, for example it is possible to oxidize chloride to chlorine and fluoride to fluorine . Because of the standard potential of fluorine, the latter is the only method for producing elemental fluorine from fluoride ions. Anodes connected to powerful DC power sources are the most powerful oxidizing agents . As they are stronger than all chemical oxidizing agents, anodic oxidation allows the production of even the strongest oxidizing agents such as peroxodisulfate , chlorine and fluorine in good yield.

Any oxidation can only take place if the electrons released are absorbed by other substances, which are reduced in the process. Oxidation and reduction always take place at the same time, a redox reaction takes place. In the case of anodic oxidation, the reduction takes place at the electrode that emits electrons, at the cathode . The anodic oxidation is the opposite of the cathodic reduction . Compared to the purely chemical redox reactions, the special thing about the electrochemical processes anodic oxidation and cathodic reduction is that they are spatially separated, run on different electrodes, and that a current flows through one at the same time outer circuit flows. In electrochemical processes, the two processes usually take place spatially separated, but only at the same time in a closed circuit. It is therefore impossible to carry out an anodic oxidation or a cathodic reduction alone, even if this would often be technically desirable.

One undesirable form of anodic oxidation is electrochemical corrosion .

Applications

Anodic oxidation is used in a variety of ways in chemistry , process engineering and environmental engineering . So she can z. B. to oxidize certain metals . This is used z. B. for the production of oxidic protective layers on metals, especially light metals (so-called anodizing ). The layers, which are created by converting the uppermost metal layers and can reach a thickness of up to 40 μm, are initially microporous with some metals and can therefore be easily colored with organic dyes. They still have to be subjected to redensification (hot water, steam, metal salt redensification processes) and then fulfill decorative and functional tasks. A special form of anodic oxidation is the anodizing of aluminum using electrolytic or plasma electrolytic anodic oxidation, but magnesium and zinc can also be anodized well. Valve metals such as titanium and tantalum form a dense oxide film which, with a thin layer (less than 1 μm), forms interference colors with the metallic background . In the case of titanium, this effect is used for decorative purposes e.g. B. used in the jewelry and watch industry. In the case of silver , silver (III) oxide can be produced from silver (I) compounds by anodic oxidation .

Anodic oxidation is also used to produce electrically insulating oxide layers that are used as dielectrics in electrolytic capacitors . It was also used in rectifiers in the past .

Anodic oxidation is also a useful method in the synthesis of organic substances. A variety of organic compounds, e.g. B. unsaturated hydrocarbons , aromatics , amides , amines , carboxylic acids ( Kolbe electrolysis ) can be anodically oxidized. Anodic oxidation can also be used to disinfect microbially contaminated water. This creates oxidants that are microcidal and disinfect the water.

Examples of anodic oxidations

Electrolysis process

In the case of chlor-alkali electrolysis , anodic oxidation of chloride takes place. Chlorine gas is formed: ${\displaystyle \mathrm {2\ Cl^{-}\longrightarrow \ Cl_{2}+2\ e^{-}} }$

Another important application of anodic oxidation is found in the production of 2- (4- tert- butylbenzyl) propionaldehyde by BASF . In the first step of the synthesis, a double anodic oxidation with methanol is carried out to obtain an acetal.

Galvanic cells

In alkaline manganese cells, the anodic oxidation is carried out by zinc: . When lithium-ion batteries are discharged, lithium dissolves at the negative pole and the anodic oxidation reaction takes place. In lithium batteries , lithium ions released from a sheet of lithium metal: . ${\displaystyle \mathrm {Zn\longrightarrow \ Zn^{2+}} }$ ${\displaystyle \mathrm {LiC_{6}\longrightarrow \ C_{6}+\ Li^{+}+\ e^{-}} }$ ${\displaystyle \mathrm {Li\longrightarrow \ Li^{+}+\ e^{-}} }$

Individual evidence

1. ↑ ^{a b c d} Entry on anodic oxidation. In: Römpp Online . Georg Thieme Verlag, accessed on May 11, 2016.
2. ↑ ^{a b c d} Sidney D. Ross, Manuel Finkelstein, Eric J. Rudd: Anodic Oxidation Organic Chemistry: A Series of Monographs . Elsevier, 2013, ISBN 978-1-4832-1986-8 , pp. 211 ( limited preview in Google Book search).
3. ↑ ^{a b} Ralf Steudel: Chemistry of Non-Metals Syntheses - Structures - Binding - Use . Walter de Gruyter, 2014, ISBN 978-3-11-030797-9 , pp. 538 ( limited preview in Google Book search).
4. ^ Naumann: Fluorine and fluorine compounds . Springer-Verlag, 2013, ISBN 978-3-642-72344-5 , p. 3 ( limited preview in Google Book Search).
5. ↑ Martin Bertau, Armin Müller, Peter Fröhlich, Michael Katzberg, Karl Heinz Büchel, Hans-Heinrich Moretto, Dietmar Werner: Industrial Inorganic Chemistry . John Wiley & Sons, 2013, ISBN 978-3-527-64958-7 ( limited preview in Google Book Search).
6. ↑ Arno Behr, David W. Agar, Jakob Jörissen: Introduction to Technical Chemistry . Springer-Verlag, 2009, ISBN 978-3-8274-2195-1 , pp. 228 ( limited preview in Google Book search).
7. ↑ Horst Briehl: Chemistry of materials . Springer-Verlag, 2014, ISBN 978-3-658-06225-5 , pp. 108 ( limited preview in Google Book search).
8. ↑ Engelbert Westkämper, Hans-Jürgen Warnecke: Introduction to manufacturing technology . Springer-Verlag, 2011, ISBN 978-3-8348-9798-5 , pp. 196 ( limited preview in Google Book search).
9. ↑ Günter Spur: Handbook Heat Treatment and Coating . Carl Hanser Verlag GmbH Co KG, 2015, ISBN 978-3-446-43003-7 , pp. 21 ( limited preview in Google Book search).
10. ↑ Hans P. Latscha, Helmut Klein: Inorganic Chemistry Chemistry Basics I . Springer-Verlag, 2013, ISBN 978-3-662-05762-9 , pp. 416 ( limited preview in Google Book search).
11. ↑ Houben-Weyl Methods of Organic Chemistry Vol. IV / 1b, 4th Edition Metallic and Organic Oxidation Agents, Antioxidants . Georg Thieme Verlag, 2014, ISBN 3-13-179684-7 , p. 1019 ( limited preview in Google Book search).
12. ↑ Cian Kingston, Maximilian D. Palkowitz, Yusuke Takahira, Julien C. Vantourout, Byron K. Peters: A Survival Guide for the “Electro-curious” . In: Accounts of Chemical Research . tape 53 , no. 1 , January 21, 2020, ISSN  0001-4842 , p. 72-83 , doi : 10.1021 / acs.accounts.9b00539 .