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Author: Subject: Crystallization of potassium permanganate
kaviaari
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[*] posted on 4-2-2007 at 23:41
Crystallization of potassium permanganate


I made potassium permanganate by heating 5g of MnO2 and about 7g of KOH in beaker. When the greenish mixture had cooled down to room temperature 70ml of 10% acetic acid was added. A very deep purple solution of potassium permanganate formed. Remaining MnO2 was filtrated and now solution contains potassium acetate and potassium permanganate.

How would it be most reasonable to get potassium permanganate crystals out of it. I thought extraction with some kind of passive solvent but that would be quite impractical. Then I thought that water could be slowly evaporated from the solution until permanganate starts to crystalize out. Then the upper solution layer (containing KAc) could be carefully separate with capillary pipette. This could be repeated few times to get most of potassium permanganate out.

Is there a easier route to crystalize pure potassium permangante out of this solution?

[Edited on 5-2-2007 by kaviaari]

kmno4_filtrated.jpg - 33kB




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[*] posted on 5-2-2007 at 00:16


The solubility of potassium permanaganate in water at 20C is 6.3g/100ml.

So there is about 5 grams of KMnO4. Try boil off some water and chilling solution in freezer. I think that potassium acetate is pretty soluble, so it won't precipitate.
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[*] posted on 5-2-2007 at 01:04


Was only air used as the oxidiser? If yes, a lot of your MnO2 remained unconverted. You have to add KNO3 or KClO3 (less recommended) to the melt to serve as the oxidiser. The process with air as oxidiser takes hours of good stirring to achieve reasonable MnO2 conversion IIRC.

KMnO4 is isolated by boiling the solution down to a small volume (e.g. 20ml or even less), filtering again (hot!) if necessary and then cooling to 0°C to crystallize most of the KMnO4.

BTW, did you heat the solution in a beaker made of glass? Thats a bad idea, the KOH will attack the glass and give rise to impurities in the product (silica gel, colloidal SiO2).
A metal crucible must be used. Fortunately, an old soup can (plastic lining on the inside previously burned off) will do fine.
A crucible of nickel or even better silver would be ideal.




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12AX7
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[*] posted on 5-2-2007 at 08:16


Even just recrystallizing commercial KMnO4, I noticed a lot of brown crud. Permanganate is a really unhappy anion. Yields will be low, and that's if there's much of the stuff in your beaker to begin with!

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[*] posted on 5-2-2007 at 10:11


You would probably get better results trying to make sodium permanganate first, then crystallize out potassium permanganate by adding a hot saturated solution of KCl. Avoid all organics.
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[*] posted on 5-2-2007 at 12:59


Oh yes, permanganate ion in aqueous solution is quite unstable. I noticed both in alkaline and in acidic solution a fairly quick decomposition, leading to formation of MnO2.

Probably the best method to make KMnO4 is by heating an oxidizer/alkali/MnO2 mix and dissolving this in water, giving a solution of manganate. Avoid the use of potassium salts. Bubble Cl2 through this very alkaline mix and the manganate is converted to permanganate, without formation of MnO2. Once you have an alkaline solution of permangante, with some excess hypochlorite, then add warm solution of KCl to this. This will lead to a good yield of KMnO4, crystallizing from solution, but unfortunately, it will be somewhat contaminiated with KClO3, due to the disproportionation of the hypochlorite.
A recrystallization of the raw permanganate can be done to get rid of that KClO3, but the disadvantage is the loss of KMnO4 and formation of MnO2. Permanganate is not that difficult to make, but isolating it in a reasonably pure state certainly is not easy.




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[*] posted on 6-2-2007 at 11:03


Use acetone. I did it in the thread "KMnO4 synthesis," but denaturants in it caused decomposition of KMnO4.
KMnO4 is soluble in acetone, but Potassium Acetate is not soluble. If you evaporate the water from your solution and dissolve it in pure acetone, only the KMnO4 will dissolve.

Evaporating the acetone will leave KMnO4. (Hopefully)

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[*] posted on 6-2-2007 at 13:11


Why doesn't it react?

Ohhh...that reminds me, I once did that, and I think I recall getting a purple solution with the acetone. So I lit the vapors and it burned down. Eventually the acetone boiled and the permanganate decomposed, burning intensely with some sparking and ozone (nearby KMnO4 decomposing from the heat, I suppose).

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[*] posted on 6-2-2007 at 17:04


Quote:
Originally posted by 12AX7
Why doesn't it react?

Ohhh...that reminds me, I once did that, and I think I recall getting a purple solution with the acetone. So I lit the vapors and it burned down. Eventually the acetone boiled and the permanganate decomposed, burning intensely with some sparking and ozone (nearby KMnO4 decomposing from the heat, I suppose).

Tim


Its not possible to oxidize the carbonyl of the ketone because it has not hydrogen on it. It is possible under harsher conditions to oxidize the other carbons, but at room temp, it is safe to say it is unreactive.




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[*] posted on 6-2-2007 at 19:27


Isn't there something about permanganate splitting ketones, or is that ozone?

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[*] posted on 6-2-2007 at 23:40


The route through acetone is not really suitable. KMnO4 is soluble in acetone, but only sparingly. A solution may look quite impressive, due to the deep color, but when looking at percentages, I think it will be well below 1%.



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[*] posted on 7-2-2007 at 06:13
Why used acetic acid


Why used acetic acid ?

Its oxidizing agent or not /?
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[*] posted on 7-2-2007 at 09:19


Potassium Manganate
Place a fragment of solid caustic potash KHO, with about the same quantity of potassium chlorate KClO3 and of black oxide of manganese MnO2 on a piece of platinum foil (1 inch by 2 inches and it may be reused...). Hold the foil by a small pair of forceps in a blowpipe flame for a few minutes untill the fused mix had become dark green.

Potassium Permanganate
Place the foil with its mass in water and boil for a short time.

The procedure gives only mention of the solution, I suspect that vacuum drying would give crystals. Perhaps doing it with this platinum foil allows for a more pure product.

A piece of platinum foil this size would prolly run about the same as a pair of nikes..

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[*] posted on 28-3-2007 at 13:16


Quote:
Originally posted by mericad193724
Use acetone. I did it in the thread "KMnO4 synthesis," but denaturants in it caused decomposition of KMnO4.
KMnO4 is soluble in acetone, but Potassium Acetate is not soluble. If you evaporate the water from your solution and dissolve it in pure acetone, only the KMnO4 will dissolve.


What was your acetone source? Klean-Strip's acetone doesn't state its denatured; however, its MSDS says its 100%-95% aectone, leaving me to question their quality control more than some attempt to render it less useable...

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[*] posted on 28-3-2007 at 16:02


I was experimenting with this a year or two ago, but I put the whole project aside because I ran into the same problems you did. Forget boiling down the solution- decomposes the KMnO4. Forget organic solvents, even acetone decomposes it (though I didn't try ultrapure reagent grade). And as woelen said, solubility is poor.

I didn't get around to trying vacuum evaporation, nor did I try the sodium manganate / KCl route.

I need a vac pump that can boil water at room temp, then maybe I'll try it. I haven't had time to take up this project lately, but it's been something I've been wanting to do, just to try growing those beautiful violet crystals. And just because it's a challenge.
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