Sciencemadness Discussion Board

How to purify 90% pure KOH?

BStone - 17-4-2006 at 19:36

Swim has 90% pure KOH grains. There is no konwledge of what might the 10% impurities consist of. 10% of impurities is quite a lot and propably affects undesirable way to any reactions in which that (not so pure) KOH is used. How can swim purify that KOH to be as pure KOH as possible? Despite the extensive web search, no answer has come up.

Thanks for any advise.

DeAdFX - 17-4-2006 at 19:37

Prehaps the KOH is more soluble in water than the other stuff...

Forgot another obvious one. If the seller has a msds for KOH check that. The msds might mention what the other 10% is.

[Edited on 18-4-2006 by DeAdFX]

BromicAcid - 17-4-2006 at 19:41

KOH is fairly soluble in organics, you can dissolve quite a bit in alcohols for instance. But the problem is in knowing weather your impurities are of the organic or inorganic variatey, the first of which would surely dissolve in alcohols with your KOH whereas most of the latter would remain as insoluble stuff. If most of your stuff is indeed inorganic you can likely make a saturated solution of KOH in ethanol and filter out the crap then distill off the ethanol to give your solid KOH noticeably purer.

BStone - 17-4-2006 at 21:11

Quote:
Originally posted by DeAdFX
Prehaps the KOH is more soluble in water than the other stuff...
If the seller has a msds for KOH check that.

Yes, the problem is really that these impurities are unknown. Swim has even cheked with the manufacturer for possible consisteny of that unknown 10%, but no relevant information was obtained.

Would it be safe to assume that most of the impurities of the industrially made KOH are inorganic?

Initially swim was thinking of making solution of boiling water and KOH (solubility 178g/ml), filter it and then cool the solution down (solubility 97g/ml) to produce purer KOH.
Quote:
Originally posted by BromicAcid
If most of your stuff is indeed inorganic you can likely make a saturated solution of KOH in ethanol and filter out the crap then distill off the ethanol to give your solid KOH noticeably purer.

This method seems very appropriate. My guess is that above mentioned method produces sufficiently pure KOH.

Thanks for a tip. How about that possible organic impurity? Any way of getting rid of that, if need arises?

garage chemist - 18-4-2006 at 04:31

All those suggestions don't work, since nobody here knows what the impurity is.

At Merck, I read a FAQ some time ago, covering the exact topic here.
Given as how analytical grade KOH from Merck is still only 86- 90% pure (they say so explicitly), the impurities cannot possibly be something that will interfere with any analytical method.

In fact, the 10% "impurity" in your KOH is just plain water.
The problem here is that KOH is REALLY DIFFICULT to dehydrate, so difficult that it is easier to sell it with 10% water content and hope that most people don't complain. And the strategy works.

Your KOH is perfectly suitable for all purposes where it has to be dissolved. You'll see, if you stored it properly it will dissolve in alcohol without any rersidue.
You just have to remember:
1 mol of KOH is 66g.
It is NOT 56g.

Your KOH just can't be used for any purpose where anhydrous conditions have to be employed.
For such purposes, it will have to be dehydrated by prolonged heating to red heat (over 800°C) in a nickel crucible.

BStone - 18-4-2006 at 08:35

Quote:
Originally posted by garage chemist
Given as how analytical grade KOH from Merck is still only 86- 90% pure (they say so explicitly), the impurities cannot possibly be something that will interfere with any analytical method.

In fact, the 10% "impurity" in your KOH is just plain water.

You just have to remember:1 mol of KOH is 66g. It is NOT 56g.

Your KOH just can't be used for any purpose where anhydrous conditions have to be employed.

You appear to be correct.

After further research, swim has also found a strong evidence that all analytical grade KOH pellets actually consists of 85% KOH + 15% H20 with no other significant impurities present, which confirms 1 mole of KOH pellets to be approximately 66 grams.

No purification was needed after all, just correct value for mole calculations. Thank you all for your help, I think this problem is now solved.