Bedlasky - 16-12-2019 at 07:26
Hi.
I recently done few experiments with potassium permanganate.
Reduction by oxalic acid:
I added in to five test tubes aproximately the same amount of oxalic acid, 1ml of 38% H2SO4 and 5ml of distilled water.
In first test tube was only this mix.
In second test tube was hot mix.
In third test tube was mix with additon of MnSO4
In fourth test tube was hot mix with addition of MnSO4
In fifth test tube was mix with addition of CoCl2
In to each test tube I added drop of potassium permanganate solution.
Here are observations:
1. Very long time before reaction visibly began, after that solution quickly turned in to slightly yellow colour. After long time it finally turned in
to colourless.
2. Solution turned in to slightly yellow much quicker, but yellow colour resisted for long time.
3. Solution immediately turned in to yellow-brown and after some time it turned in to slightly yellow which again resisted for long time.
4. The same as 3. but much quicker, without slightly yellow colour in the end.
5. After half an minute solution turned in to very slightly pinkish (due to Co2+ ions)
I also made mix of oxalic acid, sulfuric acid and water of the same concentration in the crystallization dish, add three drops of permanganate and
place it under UV light. Decolorization was slower than in hot solution, but still quicker than in cold solution. Decolorization was complete, none
signs of slightly yellow colour.
Questions: Why was sligthly yellow colour so stable? It seems that colour was caused by very small amount of MnO2. But my experience from lab are
different - reaction between oxalic acid and permanganate led in to colourless solution without yellow intermediate everytime.
Why Mn-catalyzed reaction led firstly in to formation of MnO2? This never happend to me in lab. In lab reaction always led in to decolorization of
solution (but not immediate decolorization).
Reduction by sodium chloride:
I prepared another set of test tubes, but instead of oxalic acid I used sodium chloride.
1. Cold solution
2. Hot solution
3. Cold solution with ammonium ferric sulfate
4. Hot solution with ammonium ferric sulfate
5. Cold solution with MnSO4
Here are results:
1. Very slow decolorization of solution
2. Nothing happened
3. Nothing happend
4. Quick decolorization of solution
5. Nearly immediate formation of MnO2
I also tried this reaction under UV light. Decolorization was much quicker.
Questions: Why hot chloride solution was stay purple and cold not? It doesn't make sense.
If cold NaCl solution was slowly oxidized by permanganate, why Fe-catalyzed solution wasn't? Fe catalyzator work in hot solution, so what's the
problem?
Why Mn-catalyzed reaction stops on MnO2 and doesn't continue in to Mn2+?
woelen - 16-12-2019 at 07:59
Are you sure that experiments 1 and 2 of the second series with NaCl are OK? Could experiment 1 have had some mildly reducing contaminant in the test
tube? Could experiment 2 have been performed without NaCl added or with another non-oxidizeable compound added?
Bedlasky - 16-12-2019 at 12:17
I'll try this experiments again, but I probably won't have time for it this week. I'll write if results would be the same or different.