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Why potassio]um permanagnate is kept in dark bottles?

Why potassio]um permanagnate is kept in dark bottles?

Grade:12

3 Answers

SAGAR SINGH - IIT DELHI
879 Points
10 years ago

Dear student,

otassium permanganate is not a primary standard. It is difficult to obtain the substance perfectly pure and completely free from manganese dioxide. Moreover, ordinary distilled water is likely to contain reducing substances (traces of organic matter, etc.) which will react with the potassium permanganate to form manganese dioxide. The presence of the latter is very objectionable because it catalyses the auto-decomposition of the permanganate solution on standing. The decomposition:



4MnO4- + 2H2O  =  4MnO2+3O2+4OH-



is catalysed by solid manganese dioxide. Permanganate is inherently unstable in the presence of manganese(II) ions:



2MnO4-+3Mn2+ +2H2O = 5MnO2 + 4H+;



this reaction is slow in acid solution, but is very rapid in neutral solution. For these reasons, potassium permanganate solution is rarely made up by dissolving weighed amounts of the highly purified (e.g., A.R.) solid in water; it is more usual to heat a freshly prepared solution to boiling and keep it on the steam bath for an hour or so, and then filter the solution through a non-reducing filtering medium, such as purified glass wool or a sintered glass filtering crucible (porosity No. 4).

PRIYANKA TATA ASKIITIAN
78 Points
10 years ago

KMnO4 is stored in dark bottles so that it is not exposed to light....when it is exposed to light it will start decomposing slowly.even pure solutions will get slowly decomposed.

priyanka tata

Rishi Sharma
askIITians Faculty 646 Points
10 months ago
Dear Student,
Please find below the solution to your problem.

potassium permanganate is not a primary standard. It is difficult to obtain the substance perfectly pure and completely free from manganese dioxide. Moreover, ordinary distilled water is likely to contain reducing substances (traces of organic matter, etc.) which will react with the potassium permanganate to form manganese dioxide. The presence of the latter is very objectionable because it catalyses the auto-decomposition of the permanganate solution on standing. The decomposition:
4MnO4- + 2H2O = 4MnO2+3O2+4OH-
is catalysed by solid manganese dioxide.
Permanganate is inherently unstable in the presence of manganese(II) ions:
2MnO4-+3Mn2+ +2H2O = 5MnO2 + 4H+;
this reaction is slow in acid solution, but is very rapid in neutral solution.
For these reasons, potassium permanganate solution is rarely made up by dissolving weighed amounts of the highly purified (e.g., A.R.) solid in water; it is more usual to heat a freshly prepared solution to boiling and keep it on the steam bath for an hour or so, and then filter the solution through a non-reducing filtering medium, such as purified glass wool or a sintered glass filtering crucible (porosity No. 4).

Thanks and Regards

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