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Author: Subject: Dehydrating KOH how to ?
metalresearcher
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[*] posted on 10-9-2017 at 12:29
Dehydrating KOH how to ?


I have KOH but it appears to contain some water, which has a detrimental effect on forming K metal on its reaction with Mg metal.
Is there a way to dry it ?
Heating up lets it just melt at a lower melting point lower than the designated 360 degrees, due to water content.




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[*] posted on 10-9-2017 at 18:14


Heat in nickel crucible to red heat. Can use iron too, if you can deal with iron contamination.



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[*] posted on 28-9-2017 at 08:07


Would refluxing the KOH with toluene work?
Or will the opposite happen (KOH absorbs more water from toluene)?




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[*] posted on 28-9-2017 at 08:27


It is impossible to prepare anhydrous KOH from KOH that is contaminated with water due to decomposition and production of K2O gases.

Real anhydrous KOH is prepared by reacting sodium hydroxide and potassium at high temperatures and purified through zone melting. It's used occasionally for research purposes but is expensive.

Edit: Note: It might be possible to dehydrate KOH at high temperatures in a stream of potassium vapor, but I'm not aware of anyone having ever tried this.

[Edited on 28-9-2017 by JJay]




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[*] posted on 28-9-2017 at 10:50


https://www.nist.gov/sites/default/files/documents/srd/jpcrd...

In this paper KOH is dehydrated to 0.01% water content by heating in a stream of nitrogen in a silver crucible, followed by heating in vacuum. K2CO3 sometimes appears when dehydrating KOH, likely because KHCO3 was present at the beginning; this is not likely to cause problems in most applications.
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[*] posted on 28-9-2017 at 11:26


That's interesting. I wonder how Sigma-Aldrich gets away with charging in excess of $100 for 5 grams. Also, wouldn't there be silver contamination?



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[*] posted on 28-9-2017 at 11:28


Quote: Originally posted by clearly_not_atara  
... likely because KHCO3 was present at the beginning...


Not very likely.
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[*] posted on 28-9-2017 at 11:35


Quote: Originally posted by JJay  
That's interesting. I wonder how Sigma-Aldrich gets away with charging in excess of $100 for 5 grams. Also, wouldn't there be silver contamination?

Packaging? Just a guess. Ensuring the K2CO3 content is <0.03% might be very hard.

If I were trying to do that, I might try something like making KOMe and heat this in a stream of water vapor. I have no idea if that would actually work, fwiw.

[Edited on 28-9-2017 by clearly_not_atara]
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[*] posted on 28-9-2017 at 11:43


Quote:
Not very likely.

The carbonate is usually there by dint of CO2 in absorbed moisture.




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[*] posted on 28-9-2017 at 11:59


Well, Mellor volume II didn't say much about dehydrating potassium hydroxide but did mention that it can be made in anhydrous form by reacting potassium carbonate and calcium hydroxide (I think there's a process involving potassium sulfate and calcium hydroxide as well). Also, he said the carbonate can be removed by dissolving it [the hydroxide] in ethanol. He also stated that potassium peroxide is a common impurity, but I'm not seeing a lot of information readily at hand suggesting that potassium hydroxide can't be dehydrated easily... which is weird because I am quite sure that I have read that it can't, and I certainly don't want to spread misinformation....

Mellor also mentioned that cesium hydroxide can be dehydrated in a stream of hydrogen at 500 C.

If anyone has the ACS monograph readily at hand, I'm sure it mentions something about water content.



[Edited on 28-9-2017 by JJay]




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