j_sum1
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Rookie question - drying potassium hydroxide
I have a decent amount of KOH and it is quite old.
I understand that it is quite hygroscopic (and also absorbs CO2, but that is immaterial for me at the moment.)
I wanted to dehydrate some for my next synth and my efforts thus far have been ... Uh... Problematic.
I dumped a pile of prills in a crystallising dish and put it in my drying oven at 120. No mass change over 90 minutes. So I cranked up the heat. And
after cooling I ended up with a very unmanageable solid cake and still no mass loss.
Is there anything straightforward I can do to eliminate excess water?
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Sulaiman
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I've not tried (because it is hazardous, and I can buy KOH cheaply)
but I believe that you will need to heat until you get molten potassium hydroxide (>410C) to remove water.
Molten KOH is scary - it eats glass,steel,flesh etc.
IF possible I recommend buying fresh KOH,
keeping your existing stock for wet chemistry use.
CAUTION : Hobby Chemist, not Professional or even Amateur
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Admagistr
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I would also remelt the KOH in a silver, nickel, or iron crucible, whichever I had available. I've done this before and it was fine from a safety
standpoint, I survived unscathed. I've even tried melting in borosilicate lab glass and craters formed in that vessel. This was during my childhood when I was not fully aware of the corrosiveness of
molten KOH to glass. The use of a vacuum pump may have helped to remove the water.
[Edited on 29-9-2024 by Admagistr]
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Precipitates
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I'm not sure if dissolving the KOH in pure methanol or ethanol would help - in theory you can remove any insoluble carbonate as well.
Take a certain amount of KOH, dissolve in a small amount of EtOH (very soluble) and then heat to dryness outside or in a fume hood.
For very pure KOH I'd perhaps try recrystallising from EtOH - although I'm not sure how successful this would be, as it may form an unfilterable
sludge rather than nice crystals.
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j_sum1
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Thanks, Precipitates. I will give that a try.
And hopfully I end up with a powder rather than a solid cake.
It gives me an excuse to grab some more methanol too.
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Precipitates
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Let us know how you get on.
I'm curious, although not curious enough/lazy to experiment myself!
Probably nearly out of methanol as well. I have loads of used methanol that I need to distill and dry - which I guess I'll do next time I need it.
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j_sum1
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Update.
A couple of things.
My KOH appears to be crap. It seems to be about 22% K2CO3.
I have a couple of kilos stuffed away in bulk storage but it is of a similar age and may well be as bad.
I have tried dissolving in both ethanol and methanol and then evaporating. Process is still underway, but it does not seem to be a particularly
efficient method and may not even be effective. My last weighing was still higher than the starting weight.
I will report back when i have definite results, but it is not looking good at present.
Next shopping list will include a large amount of Mg. I may as well do some magnesium thermite reactions and see if I can get some metallic potassium
- even if it is at low yield.
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Random
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I have left a solution of significantly hygroscopic ammonium nitrate. It was solution insid how we write it \\\H2O\\\.
In the Summer it was a solution.
In Winter it was dry crystalized NH4NO3
dsl
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Junk_Enginerd
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Putting it in a stainless steel pot and just heating it on the stove works pretty well imo. If you heat it strongly enough, so that you get bubbling,
you get a crusty result which is easily crushed. Of course you need to be careful not to heat it so you melt it, or you're back to problematic solid
cake.
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Keras
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There is nothing to be afraid of with KOH fusion. The compound is super tame, as long as you put it in a metal crucible. The problem is that after
cooling you risk ending up with a single mass of potash, which would be quite a pain to transform back into flakes or powder.
Also I’m not sure but KOH appears to attack the chromium part of stainless steel.
[Edited on 5-10-2024 by Keras]
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Random
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Quote: Originally posted by Sulaiman  | I've not tried (because it is hazardous, and I can buy KOH cheaply)
but I believe that you will need to heat until you get molten potassium hydroxide (>410C) to remove water.
Molten KOH is scary - it eats glass,steel,flesh etc.
IF possible I recommend buying fresh KOH,
keeping your existing stock for wet chemistry use. |
It really is.
I have produced NaOH by Ca(OH)2 rxn m\T Sodium BiCarbonate |||be careful here, first you need to get Sodium Carbonate|||
dsl
It was according to my personal memory inside 2010.
I am not sure how concentrated it was, but it \\\\ate\\\ Aluminium
dsl
Just a little \\\dot\\\ sprayed on my \\\according to my personal memory\\\ it was left arm.
...
According to my personal memory I felt it.
[Edited on 5-10-2024 by Random]
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clearly_not_atara
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I think KOH will deprotonate methanol. K2CO3 is known to have nontrivial solubility in MeOH and this can be used to prepare low concentrations of
KOMe. The presence of water should prevent the formation of ethoxide and the solubility of K2CO3 in ethanol should be much lower, so I think that
"recrystallizing" the KOH from ethanol might work.
But I would generally agree on using a crucible from a noble metal (Ni or better) at a very high temperature. It may be annoying to remove.
[Edited on 6-10-2024 by clearly_not_atara]
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Sulaiman
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A small note
Wear good eye protection when working with KOH or NaOH,
Tiny particles float in the air very easily when the (near) anhydrous material is handled,
one speck of the dust will make your eye sting and cry,
a significant quantity will damage your cornea, possibly permanently.
IF you have a choice when buying,
prills (little balls) absorb water and/or CO2 slower than flakes do,
and create less dust.
CAUTION : Hobby Chemist, not Professional or even Amateur
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Random
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| Quote: Originally posted by Sulaiman | Wear good eye protection when working with KOH or NaOH,
Tiny particles float in the air very easily when the (near) anhydrous material is handled,
one speck of the dust will make your eye sting and cry,
a significant quantity will damage your cornea, possibly permanently.
IF you have a choice when buying,
prills (little balls) absorb water and/or CO2 slower than flakes do,
and create less dust. |
I have experienced Ca(OH)2 in the eye. Before I was not sure if it got into my eye but when it definitely got into my eye I was \\\sure\\\ that it
did.
dsl
...
Also, be careful putting it into \\\water\\\.
Significantly egzothermic.
dsl
I put not that big amount of NaOH into water and it started boiling.
I said to myself, huh...
It was put into \\\glass\\\ should we call it \\\jar\\\\
...
It started boiling so I thought, what if glass breaks due to temperature, it will spray it all over.
dsl
I risked putting a cap on that, you know, just to prevent spreading of the vapor. I put something on it and took this boiling glass and I put it into
a plastic container and covered that, but not in the way that I would completely close it.
I was happy that the solution did not spray me.
It would be boiling solution of NaOH
dsl
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