Redox titration

In chemistry, redox titration is a concentration determination method by titration . It can be used to determine the content of substances that can be oxidized or reduced .

Basics

In oxidimetry, the substance to be determined is allowed to react with an oxidizing or reducing standard solution ( redox reaction ). As soon as the substance to be determined is completely oxidized or reduced by gradually adding dropwise the standard solution, the next drop of the standard solution no longer finds any reactants. The disappearance of the substance to be determined or the standard solution that is now in excess in the sample must be recognized as the end point of the reaction in order to be able to determine the amount of standard solution used up to that point.

Frequently used methods are bromatometry , in which bromate ions are used as oxidizing agents :

${\displaystyle {\ce {BrO3- + 5Br- + 6H3O+ -> 3Br2 + 9H2O}}}$

Synproportionation of bromate ions (oxidation state: +5) and bromide ions (Oxs .: −1) to bromine (Oxs .: 0)

the manganometry , which uses permanganate ions as oxidizing agents:

${\displaystyle {\ce {MnO4- + 5Fe^{2}+ + 8H3O+ -> Mn^{2}+ + 5Fe^{3}+ + 12 H2O}}}$

Redox titration of permanganate ions (Oxs .: +7) with iron (II) ions.

as well as cerimetry , in which tetravalent cerium ions are used for oxidation:

${\displaystyle {\ce {Ce^{4}+ + Fe^{2}+ -> Ce^{3}+ + Fe^{3}+}}}$

Oxidation of iron (II) ions with cerium (IV) ions.

The potassium dichromate method is now used less frequently , with dichromate ions acting as an oxidizing agent:

${\displaystyle {\ce {Cr2O7^{2}- + 6 Fe^{2}+ + 14 H3O+ -> 2 Cr^{3}+ + 6 Fe^{3}+ + 21 H2O}}}$

Here z. B. Diphenylamine can be used as a redox indicator .
Likewise, can be combined with iron (II) sulfate titrated to give but the Mohr salt (ammonium iron (II) sulfate) was used for the standard solution.

Another example is iodometry , in which either iodine is reduced to iodide, e.g. B. in the determination of tin (II) salts, or iodide is oxidized to iodine, such as. B. the determination of hydrogen peroxide .

${\displaystyle {\ce {Sn^{2}+ + I2 -> Sn^{4}+ + 2I-}}}$

${\displaystyle {\ce {H2O2 + 2 I- + 2 H3O+ -> I2 + 4 H2O}}}$

Detection of the reaction endpoint

By self-indication

The self-indication is mainly used in manganometry (KMnO ₄ ) and iodometry (but by adding starch, since iodine forms a colored complex with starch ), because potassium permanganate and iodine solutions are colored solutions.

During the direct titration, the added standard solution is continuously decolorized by the reaction with the sample. Once the reaction has ended, i. H. the sample substance has reacted, the standard solution is no longer converted and therefore colors the titration solution.

With the help of redox indicators

Redox indicators are substances that are oxidized or reduced themselves and therefore show a change in color. z. B. Ferroin . Even non-redox-active substances can act as indicators, e.g. B. shows in bromatometry the discoloration by the destruction of a dye to the end point of the reaction.

Potentiometric

During a redox titration, the voltage between the sample solution and an electrode changes . With the potentiometric method, this measured voltage is applied as a function of the added volume of the standard solution, so that a titration curve is obtained that can be evaluated graphically or by calculation.

Historical

The first redox titration was carried out by François Antoine Henri Descroizilles in 1791 after attending a failed demonstration of the use of sodium hypochlorite in Rouen . He discovered that the concentration of the lye was decisive for the success of chlorine bleaching , and he developed a titration method based on the decolouration of an indigo solution by chlorine, and a device for it, a still very simple burette. This did not yet have a tap, but was closed and opened with the finger.

literature

• G.-O. Müller: Quantitative-inorganic internship . 7th edition, Verlag Harri Deutsch, Frankfurt / Main 1992 , ISBN 3-8171-1211-4 .

Individual evidence

1. ↑ Entry on oxidation – reduction (redox) titration . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.O04364 Version: 2.1.5.
2. ↑ Jander / year / Knoll: volumetric analysis, collection Göschen band 221 / 221a, Walter de Gruyter Berlin 1966th
3. ^ ^{A b} Axel Johansson: The development of the titration methods: Some historical annotations . In: Analytica Chimica Acta . tape 206 . Elsevier BV, 1988, p. 97-109 , doi : 10.1016 / s0003-2670 (00) 80834-x .
4. ^ Clément Duval: François Descroizilles, the Inventor of Volumetric Analysis . In: American Chemical Society ACS (Ed.): Journal of Chemical Education . tape 28 , no. 10 . ACS Publications, October 1951, ISSN  0021-9584 , p. 508-519 , doi : 10.1021 / ed028p508 .