starman - 24-7-2008 at 15:32
Hi guys,a basic(ha ha) question regarding KOH solutions.In Australia the only KOH I have been able to find is 40% aq solution(pH up for hydroponics).
Evaporating this to dryness(required prolonged high temp and etched my pyrex evap dish)Simply would not dissolve in my dried(MgSO4) denatured
ethanol,even at vigorous boil and stir?What have I done incorrectly?Can I add the aq KOH then dry.
ScienceSquirrel - 24-7-2008 at 15:41
I think that you are not getting the potassium hydroxide dry enough.
I have 80% potassium hydroxide pellets and it dissolves easily in methanol and 96% ethanol to form highly caustic solutions.
Klute - 24-7-2008 at 15:57
Yes, if there is enough water around, you will obtain two layers as the KOH "salts" out the water from the alcohol. Drying KOH to a solid isn't
trivial, and will definitively eat through pyrex glass...
You cannot dry the biphasic KOH/H2O/EtOH mixture as such by adding a dessicant. Maybe adding some toluene and removing the teriray azeotrope, but
that's going to be evry long and wastefull..
On a side note, MgSO4 will not dry EtOH azeotrope, it needs to be refluxed and distilled from CaO (with a little NaOH/KOH during reflux to condense
most of the denaturants).
Do you absolutely need water-free ethanolic KOH solutions? There always is a little water in there as KOH is generally sold as 85% pellets containing
up to 10% H2O, and it's so hygroscopic that water is absorbed by the solution if not handled under anhydrous conditons. Or is it just to obtain a
single phase solution?
Depending on what you want to do with it, the biphasic mixture can suffice, or maybe you might try another option (like K2CO3 if doing a williamson
for example).
starman - 24-7-2008 at 16:22
Thanks Klute,very informative.Doing a couple of things with the solutiions.One of which has an an alternative of of ethanolic NaCO3 which is
presumably ethanolic aqueous NaCO3 so water content obviously not that important.The other is allyl/ propenyl rearrangement not sure if this needs to
be anhydrous but it reads that way.As for CaO,haven't found a source yet.(Short of decomposing my Ca(NO2)2.
Klute - 24-7-2008 at 16:31
Calcium oxide, or quicklime, is usually sold in gardening stores in pretty large quantities, to decompose dead animals and things like that. It's also
used to desinfect humid basements.
I'm not too sure on the rearrangements, but I guess too much water might cause some hydration of the double bond... Maybe solid NaOh? (more common
than KOH pellets in hardware shops I think)
chemrox - 24-7-2008 at 16:41
I wonder if these guys would help you: http://www.tigerchem.com/indus.html
Otherwise why not reach one of the many US firms that offer biodiesel supplies?
not_important - 24-7-2008 at 17:11
Note that the KOH will suck the CO2 right out of the air, giving K2CO3. If you're slow enough in evaporating it you'll get a lot of carbonate
formation, which will not dissolve in alcohol.
A stainless steel asparagus pot similar to this one http://www.williams-sonoma.com/products/sku4547964/index.cfm... is handy for working with strong alkali, so long as halides are kept away. I've
picked up several second hand ones for just that purpose, for a good deal less than the new price. You'll want an all metal lid rather than the
see-through ones that are popular now.
As Klute suggested, you can dry it using azeotropic distillation. Use pentanes/hexanes if you don't want the hydrocarbon left in the alcoholic mix
after dying, otherwise toluene or xylene works fine. Distill the hydrocarbon first, fractionating as well as you can and leaving about 1/4 in the
still pot, this removes heavier compounds that might cause difficulties later.
You can make CaO from CaCO3 - chalk, limestone - or Ca(OH)2 - slaked lime - by heating to ~900-1000 C; a pottery kiln does the job.
ScienceSquirrel - 24-7-2008 at 17:12
Generally potassium hydroxide is sold as a material that contains less water than sodium hydroxide.
And it is more soluble than sodium hydroxide in the lower alcohols.
Potassium hydroxide in IMS was used in the classic 'base bath' for cleaning lab glassware.
The solution would go from a pale yellow to deep brown with age and it dissolved almost everything.
[Edited on 25-7-2008 by ScienceSquirrel]
starman - 24-7-2008 at 18:48
Thanks for all the replies guys.Yep Not important was a slow evaporation and wouldn't dissolve at all so the carbonate formation would be my answer.I
have a stainless pressure cooker,so might try to rig that for vacuum evaporation.
Klute - 24-7-2008 at 19:37
A bit OT, but I use NaOh for my base bath at home and at work, beacsue it's cheaper, and I've found that even under 10% solution works just aswell as
the classical ratios.
Using denaturated EtOh, the solution goes dark brown in less than 6H and then gradaully worsens 
Once I started cleaning my glassware with a base bath, I'ev never stoppe dusing them... I have never had any etched glassware (some dissolved
Rodaviss, but that was after standing over 2 weeks in there stoppe dleaving them
on the glassware now) and I'm convinced this is actually less damaging for the glassware than scrubbing. The glassware always looks as new after a
carefull rince. I've been advising the use of base baths for years on this foum, but I gues alot are reluctant which is understandable considering the
potential hazard, but it is well worth it IMHO if you use your glassware more than occasionaly; adequate base-resisatnt thick gloves, full facial
protection and common sense is all that is needed for a safe use.
Ok I'll stop here. ScienceSquirrel go me started on a subject i like