Sciencemadness Discussion Board

Crystallization of KMnO4

AlexTheChemist - 22-4-2016 at 16:58

If I have a solution of KMnO4 and KNO3, KCl , which I can serve solvent to separate and to crystallize the KMnO4 ?

Daffodile - 22-4-2016 at 22:19

Acetone

solitanze - 24-4-2016 at 12:04

I wouldn't recommend acetone, or any other organic solvent, which could be oxidized. And it's only sparingly soluble in acetone anyway.

Boiling down aqueous permanganate solutions give a lot of brown mud (MnO2). It's just not very stable.

How did you obtain this mixture?

morsagh - 24-4-2016 at 12:49

Just let it stand to evaporate most of water and filter crystallized KMnO4 as the less soluble salt. This is best you can do. Maybe try adding a little bit of your solution to saturated solution of KNO3 but it is all affected by that if most solubility of KMnO4 is from dissociation in water but i don´t think so.

chemplayer... - 25-4-2016 at 19:49

We've tried this a few times (we also did a video on preparing it a while back) but we've never got any substantial crystals from it.

The best you can do really is boil down / evaporate the solution and chill for an extended period at close to 0C. The permanganate will crystallise out slowly but when we've tried it we got some crystals but they were clearly impure with other salts co-crystallising.

As others have commented, best not to add anything organic or oxidisable (obviously alcohols, but also I wouldn't put ketones in either) to try to crash it out.

Normally though permanganate is used in solution for reactions anyway, so you may be able to get away with using the solution as it is. Perhaps titrate and get an estimate of the concentration though first as permanganate is generally not very forgiving on organic matter in large quantities!

Daffodile - 25-4-2016 at 20:51

It might be a better idea to generate the Permanganate from Manganate in Acetone by adding Hydrochloric Acid, since its less likely to be oxidized than dust and crap that gets into the beaker.

morsagh - 27-4-2016 at 11:49

I think permanganate will oxidise HCl to chlorine... So not best idea...

aga - 27-4-2016 at 11:53

Quote: Originally posted by Daffodile  
dust and crap that gets into the beaker.

There is special chemistry apparatus specifically designed to prevent gravity-influenced foreign objects interfering with experiments.

It's called a bit of paper, towel, filter or whatever placed on top.

Daffodile - 27-4-2016 at 12:24

Quote: Originally posted by aga  
Quote: Originally posted by Daffodile  
dust and crap that gets into the beaker.

There is special chemistry apparatus specifically designed to prevent gravity-influenced foreign objects interfering with experiments.

It's called a bit of paper, towel, filter or whatever placed on top.


Yeah but since I don't clean my beakers with Nitric, there are usually contaminants that reduce the Permanganate within a few days.

clearly_not_atara - 27-4-2016 at 16:05

Add excess K2CO3 and cool to around -5 C.

To reduce the solubility of MnO4-, your best bet is to increase the concentration of K+, since the solubility product [MnO4-][K+] is the relevant physical constant. Potassium carbonate is the most soluble potassium salt, so it wins. The precipitate may contain some KNO3 and KCl; this should not affect it too much, unless there's way too much KNO3 in your solution.

Daffodile - 27-4-2016 at 17:06

Quote: Originally posted by clearly_not_atara  
Add excess K2CO3 and cool to around -5 C.

To reduce the solubility of MnO4-, your best bet is to increase the concentration of K+, since the solubility product [MnO4-][K+] is the relevant physical constant. Potassium carbonate is the most soluble potassium salt, so it wins. The precipitate may contain some KNO3 and KCl; this should not affect it too much, unless there's way too much KNO3 in your solution.


So what about dissolving the Manganate in water, boiling down to minimal volume, bubbling CO2 through the mixture, adding Potassium Carbonate, filtering, cooling, and allowing to crystallize? Sounds like an alright plan.

clearly_not_atara - 28-4-2016 at 12:25

I would not boil it at standard pressure; I'm concerned that the permanganate will degrade to manganate upon boiling. Similarly, introducing carbon dioxide (an acid) could cause permanganate to decompose. Permanganate is very unstable.

https://en.wikipedia.org/wiki/Potassium_manganate#Synthesis
Quote:
In the laboratory, K2MnO4 can be synthesized by heating a solution of KMnO4 in concentrated KOH solution followed by cooling to give green crystals


Boiling the solution down under vacuum, no hotter than 50ºC (I made this number up), should work okay, but should only be necessary if the concentration of permanganate is already very low. The instability and reactivity of KMnO4 makes it hard to isolate.

http://pubs.acs.org/doi/abs/10.1021/ja01424a003?journalCode=...

This paper discusses the precipitation of KMnO4 from mixed potassium salt solutions with an eye to thermal effects. If someone can download it, the procedures given will certainly be better than the one I'm making up as I go along.

Using Wikipedia numbers, the solubility product [K+][MnO4-] = 0.164 (mol^2 / L^2) at 20 C, or 2.5 at 65 C, according to Wikipedia; the solubility of K2CO3 is about eight moles per litre at 20 C, or sixteen moles of K+ per litre, so if the solution is saturated with K2CO3, the solubility of MnO4 should drop to (0.164 / 16.2 = 0.0101) moles per litre, causing most of it to precipitate. Because, in real solutions, inter-ionic interactions are nonzero, this estimate is going to be a little off, but the gist of the idea makes sense.

You should be able to get crystals by adding K2CO3 to the solution when it is warm and then cooling; otherwise, precipitation may be a bit of a mess.