Manganometry

The Manganometry or permanganometry is a method of chemical quantitative analysis , in which the special properties of potassium permanganate , a redox titration allow no other indicator.

reaction

The permanganate ion MnO ₄^- is a strong oxidizing agent , which itself oxidizes the chloride from the hydrochloric acid to chlorine. Upon contact with oxidizable substances, it is reduced to the Mn ²⁺ ion in an acidic solution :

{\ displaystyle \ mathrm {MnO_ {4} ^ {-} \ + \ 8 \ H_ {3} O ^ {+} + \ 5 \ e ^ {-} \ longrightarrow \ Mn ^ {2 +} \ + \ 12 \ H_ {2} O}}

The strong pH dependence of the reaction results from the number of protons required.

A special feature of manganometry is that it does not require the addition of an indicator , since solutions containing permanganate are colored intensely red-violet, while Mn ²⁺ ions are colorless.

Usually potassium permanganate is used as a standard solution for titration. The permanganate is reduced and decolorized until the titration end point is reached. If further MnO ₄^- ions are then added, these can no longer be reduced and the solution is then permanently red-violet in color due to the permanganate ions it contains.

Customized solution

A diluted potassium permanganate solution is used as a standard solution. The permanganate content of this solution is normally determined using oxalic acid or sodium oxalate as the base substance . Since the reaction is catalytically accelerated by Mn ²⁺ ions, a small amount of it can be added. Otherwise the reaction is slow at the beginning and only becomes faster through autocatalysis due to the Mn ²⁺ formed . Alternatively, the solution can be heated and the standard solution added to the still hot solution. So the addition of a catalyst is not necessary.

Identifiable substances

The substances that can be determined with the help of manganometry include iron salts , in which iron (II) ions are oxidized to iron (III) ions, hydrogen peroxide , from which oxygen is produced in the reaction with permanganate , and solutions containing manganese (II) where the divalent manganese is oxidized to manganese (IV) oxide , as well as oxalates. The reaction of permanganate and oxalate can also be used to determine calcium by first precipitating calcium as poorly soluble calcium oxalate and, after dissolving it with sulfuric acid, determining the oxalate content manganometrically.

In water analysis , manganometry is used to determine oxidizable inorganic and organic substances present in water. This sum determination is used to estimate the degree of pollution. The water sample is boiled with a potassium permanganate solution and the consumption of KMnO _{4 is determined} either by back titration with an oxalic acid solution or photometrically . In clean waters, the KMnO ₄ consumption is 1 to 8 mg / L and in polluted waters it is 20 to 150 mg / L or even higher ( waste water ).

Disruptions

All substances that can also be oxidized (such as chloride or organic compounds) interfere with the reaction.

literature

J. Strähle, E. Schweda: Jander · Blasius - Introduction to the inorganic-chemical practical course. 14th edition. Hirzel, Stuttgart 1995, ISBN 978-3-77-760672-9 , pp. 375-379.

Individual evidence

^ Karl Höll: Wasser, Walter de Gruyter Berlin, 6th edition 1979, p. 47ff, ISBN 3-11-007560-1 .

Web links

Wikibooks: Practical Inorganic Chemistry / Manganometry - Learning and Teaching Materials

Manganometry on chempage.de

[1] Karl Höll: Wasser, Walter de Gruyter Berlin, 6th edition 1979, p. 47ff, ISBN 3-11-007560-1 .