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Author: Subject: converting 85-90% KOH to anhydrous KOH?
EmmisonJ
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[*] posted on 18-5-2009 at 10:07
converting 85-90% KOH to anhydrous KOH?


it would definitely be nice to know how to drive off water from KOH, considering how hygroscopic it is. on the same token i'm curious to know if standard 85-90% KOH can realistically be made anhydrous?

Did a bit of searching without too much luck aside from US Patent # 5377398 describes that the KOH would need to be heated to temperatures in excess of 371+C (aka 700+F) to drive off the water. The patent states this as an approximate, which is strange because 371C is below KOH's melting point of 420C. I would think that you would need to heat it until it's a liquid in order to properly release the water from the KOH?

Quote:
Since commercial grade KOH contains about 10% water, in order to make it essentially anhydrous it should be heated to at least about 700° F. or higher (depending upon time at temperature) to drive off the water, thus achieving an essentially anhydrous salt bath.


http://www.patentstorm.us/patents/5377398/description.html

so it seems for all intents and purposes that unless you have a kiln, then it would not be worth the headache of figuring out how to make the KOH anhydrous... at least that how it seems to me.

[Edited on 18-5-2009 by EmmisonJ]
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[*] posted on 18-5-2009 at 11:28


Wouldn't heating KOH in air cause it to absorb CO2 from the air and form K2CO3 reducing the purity of the KOH?



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pHzero
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[*] posted on 18-5-2009 at 11:37


I expect so. He'd probably have to blow air over it constantly which has had its carbon dioxide removed by running it through KOH or NaOH. Maybe rig an air pump sort of thing up to a gas washing bottle containing NaOH sol'n?
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[*] posted on 18-5-2009 at 17:20


It's never truly anhydrous, since water is locked up in the lattice, and it soaks up water as soon as you open the bottle. But what would you need it to be anhydrous for anyway?
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len1
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[*] posted on 18-5-2009 at 21:16


The melting point of KOH is quoted at anywhere between 360 and 420 - depending of your source.

The reason for this as I could ascertain - is precisely the reason you are asking this question - the commercial product contains anything between 0-15% water. In the latter range the KOH melts as 360C. The completely anhydrous KOH melts at 420C. So this is the answer - if you need to drive water off it will melt around 370C.

At its melting point you will not drive all the water off, as the melting point will not rise to 420C.

To do the latter you either conduct electrolysis in the molten KOH, or simpler add Na to the molten KOH and stir.

[Edited on 19-5-2009 by len1]
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[*] posted on 18-5-2009 at 23:54


Another option is to add potassium methoxide (or ethoxide) and slightly heat. This drives off methanol (ethanol) and leaves behind anhydrous KOH. The same can be done with sodium methoxide and sodium hydroxide. This process is safer than adding potassium metal or sodium metal to the molten alkali. I'm not sure though how easy (or hard) it is to obtain methoxide or ethoxide. You can also make it from the alcohol and the alkali metal.



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[*] posted on 19-5-2009 at 00:06


The water can be driven out by strong heating. I've done this with commercial KOH before and it does release its water when heated in a silver crucible over the bunsen burner.
You really have to be careful with foaming though, the water release happens quite suddenly.

Silver is really the best metal to heat caustic melts to high temperatures in, there is zero corrosion because Ag2O is unstable at relatively low heat already.
I wonder whether silver can be used as a castner cell anode?
It may be easier to obtain than nickel for some (silver isn't that expensive).

[Edited on 19-5-2009 by garage chemist]




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DJF90
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[*] posted on 19-5-2009 at 01:24


You could possibly use a silver plate? This could be applied by electrolysis, or by reduction (like a large scale tollen's test ;) )
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[*] posted on 19-5-2009 at 17:46


I would be worried that the plateing may not be thick enough and imagine if there was the slightest flaw? Anyway lining with silver foil may be and easier and cheaper option although I'm sure a pretty high content silver dish could be had by careful shopping in a antiques shop or car boot.

I would have to ask why would you want anhydrous KOH? I assume 85% min purity then adjust the quantity I use to take this into account. Can always standardise the solutions if it is really important to know.

Heating in a dish with a blanket of dry N2 would remove CO2 from the air around the melt and displace H2O as it is driven off, then allow it to cool in a vacuum dessicator over P2O5 for example. But why bother?

I like the idea of Ag as an electrode for Castner Cell, but I think silver may be corroded by sodium producing an alloy... solubility of Na in Ag is supposedly 1% according to a quick google and springlink journal abstract.





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DJF90
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[*] posted on 19-5-2009 at 18:45


But surely for the sodium to be soluble in the silver, the silver has to be molten. Now off the top of my head silver melts at ~920C... Castner operates about 600C with the CaCl2 added to the NaCl to reduce the mp. from about 850C. So you've got a decent gap to operate in. And I dont do "flawed" :P
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[*] posted on 19-5-2009 at 19:05


So then does gold have to be molten to dissolve in mercury? Sugar molten to dissolve in water?

Obviously extreme examples, but I would not be surprised if the the sodium produced started to dissolve the silver metal.

Molten aluminum does the same thing with an iron crucible.

[Edited on 20-5-2009 by Fleaker]




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[*] posted on 19-5-2009 at 19:35


Quote: Originally posted by Fleaker  
So then does gold have to be molten to dissolve in mercury? Sugar molten to dissolve in water?

Obviously extreme examples, but I would not be surprised if the the sodium produced started to dissolve the silver metal.

Molten aluminum does the same thing with an iron crucible.

[Edited on 20-5-2009 by Fleaker]


Does aluminum dissolve iron? I thought it didn't but I know zinc sure will.

Off-topic anyway.

[Edited on 5-20-09 by UnintentionalChaos]




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[*] posted on 19-5-2009 at 20:33


That would be the solubility of silver in sodium, not vice versa. Although I'm not sure if this is the same thing or not

[Edited on 20-5-2009 by DJF90]
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[*] posted on 19-5-2009 at 21:00


I think there will be alteration at the silver surface, and after then, the inner mass too.
This might be alteration of physical properties like hardness, melting point, etc,which I don't really know.

Isn't this analogous to amalgamation? at certain temperature, mercury is a liquid metal which will amalamate, dispersing itself into other solid metal forming alloys.

molten sodium (and potassium), at certail temperature, is a liquid metal which will disperse itself to reach the equilibrium of 1% (Na 1:99Ag ratio), to silver, which is other solid metal

Is my assumption above correct?

[Edited on 20-5-2009 by Pomzazed]




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[*] posted on 20-5-2009 at 03:33


Pomzazed your assumption is correct. Solubility of metals is simple diffusion, and a process which is accelerated by heat. I think woelen has a nice experiment where he shows zinc plating a copper coin, then the result after the "silver" penny has stood for a while, the zinc diffuses into the copper to produce a bronze. My believe would be that silver would dissolve to some extent in the sodium by solvation and the sodium would dissolve in the solid silver to some extent by diffusion. I don't know to what extent this would occur though. And likely, for amateur purposes the % contamination would not be critically important, most LR grade sodium metal is only something like 98-99% anyway.



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len1
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[*] posted on 20-5-2009 at 03:44


According to my corrosion handbook, and stuff I have read about Castner, molten sodium metal, and sodium dissolved in NaOH is extremely corrosive. It diffuses into interstitial sites in the host metal, weakening its bonding, and aiding dissolution in the NaOH. Nickel withstands molten alkali very well, however place it in the molten NaOH / dissolved Na / O2 / H2O environment of a Castner cell and it also starts corroding.
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[*] posted on 20-5-2009 at 05:28


panziandi, even so, the silver contamination is bad for many reactions! On organic synthesis silver will depriotonate terminal alkynes (acetylide) thus forming unwanted carbanion. Or else silver itself will make a redox reaction at functional groups, or being a destabilizer for solvent and products. Even if it doesn't interfere at reaction step but it might contaminate the product with insoluble salt.:(






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