Expert: Henry Boyter - 2/18/2004

Question
I study in University and this is my first year.
You can be sure that I've searched google and other search engines in many ways but i couldn't find an answer to this question. Maybe you can help about it. I am Turkish student and it makes it harder for me to find an answer as well.
My question is why should a stock potassium permanganate solution about 0.02 MOLARITY should be wrapped around an aluminum to store it, in other words why potassium permanganate solution should be kept in a dark place.
Thank you,
Mert Kumru

Answer


The following is taken from this given website.  Permanganate is very sensitive to several types of decomposition.


www.chem.ubc.ca/faculty/wassell/CHEM415MANUAL/Experiment5/415E5Ref3.htm


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.