Why pottasium permanganate solution kept for few days?

Answer 1

The permanganate solution has to cool and stand overnight before it can be filtered to remove MnO2 (manganese dioxide)

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).

Alternatively, the solution may be allowed to stand for 2-3 days at room temperature before filtration. The glass-stoppered bottle or flask should bc carefully freed from grease and prior deposits of manganese dioxide: this may be done by rinsing with dichromate-sulphuric acid cleaning mixture and then thoroughly with distilled water. Acidic and alkaline solutions are less stable than neutral ones. Solutions of permanganate should be protected from unnecessary exposure to light; a dark-coloured bottle is recommended. Diffuse daylight causes no appreciable decomposition, but bright sunlight slowly decomposes even pure solutions.