Sciencemadness Discussion Board

method for purification of LAH

soma - 20-3-2014 at 16:24

I've got a fair amount of old LAH. I was thinking of purifying it by dissolving it in ether and then vacuum filtering through a sintered funnel. I don't know if this type of funnel exists or would have to be custom made but I was thinking of a 1000 ml sintered funnel with a top that could fit a tapered fitting with a hose connection and connecting it to a drying tube.

Anyone know if this type of apparatus exists? Anyone tried purifying LAH this way?



[Edited on 21-3-2014 by soma]

[Edited on 21-3-2014 by soma]

Nicodem - 21-3-2014 at 08:14

Just don't do it. It is not worth risking your life for such reason.

If the LiAlH4 was stored absolutely air tight, it should be good enough for most uses even if it is ten years old, or more. Though it does very slowly decompose with time, the decomposition products do not interfere with most reactions. The problem is only its degradation due to oxygen and humidity exposure.

zed - 21-3-2014 at 16:55

I'm pretty sure there is a simple titration procedure which can tell you how much oomph remains in your hydride. If you are concerned, weigh out a very small amount, carefully hydrolyse it, and measure how much hydrogen is generated. Thereafter, if need be, you can adjust the amount of hydride you use for a reduction, to compensate for any decrease in activity.

Don't make up your own LiAlH4 titration procedure. Use one that is tried and true. That way, you will retain your hands, and you can continue to post here.

*FWOOSH* - 21-3-2014 at 22:11

Hands are overrated :P

But seriously, LAH is nothing to play with in large or even moderate quantities. I'd agree with zed and say titrate some if you're worried about quality, chances are it's still in decent shape.

soma - 21-3-2014 at 22:19

I once had really pure white LAH years ago. The reaction took 1/2 hour at room temperature. These days the best I've gotten has been off white. What I have now is grey. It takes 24 hours at 50C.

zed - 31-3-2014 at 17:25

Same reduction? I've never heard of such a problem.

Well, you could purify it by re-crystalization, but I surely would try to avoid filtering the solid.

Probably a decent procedure out there somewhere.

Never seen white LiAlH4 myself. Mostly just a grey putty/mud, stabilized with a paraffin oil.

Worked pretty well, the oil protected the LiAlH4 from the local atmosphere during transfer.

soma - 3-4-2014 at 23:27

Quote:
Quote: Originally posted by zed  
Same reduction? I've never heard of such a problem.


Yeah - the 1st time I ran this reaction was with the white LAH. It looked real sparkly and crystalline. At some point I ran the same reaction again but this time the LAH was grey and powdery. I got NO product. It took me quite awhile (months) to finally figure out what happened, thanks to a doctorate chemist friend who suggested raising the temp and going for 24 hours.

That solved the problem but I sure miss the pure LAH. (The reaction product is less pure also because of the long time at a higher temp)

In the description of the synthesis of LAH (J. Am. Chem. Soc. 69, 1199 (1947)) the authors wrote: Most of the reactions were carried out in three-necked flasks fitted with a mercury-sealed stirrer, a reflux condenser, and a dropping funnel, such as are used for Grignard reactions. Usually an atmosphere of dry, carbon dioxide-free nitrogen was employed both during the reaction and in the filtrations, but these precautions were found not to be absolutely essential.

[Edited on 4-4-2014 by soma]

Ephesian - 4-4-2014 at 05:28

I have around 500 g of LAH that needs to go, or its going my up on eBay if nobody wants it here .

Dr.Bob - 4-4-2014 at 05:38

Quote: Originally posted by zed  
Never seen white LiAlH4 myself. Mostly just a grey putty/mud, stabilized with a paraffin oil.

Worked pretty well, the oil protected the LiAlH4 from the local atmosphere during transfer.


Are you thinking of sodium hydride? That is normally sold in oil.

LAH is normally a powder and I have never seen it sold in oil. The color varies a bit from vendor to vendor and lot to lot, even new. If there is a known procedure to "purify" old LAH, that might be interesting to try carefully, but it is dangerous stuff. I have seen a good sized fire from someone handling it incorrectly. The best way to handle it I have seen is to weigh the solid LAH into a flask and then purge with nitrogen, then add any solvent. If you add LAH to solvent or solvent to LAH in air, enough heat can be generated to start a fire in SOME cases (not all). But I saw it happen, so I know it can. (Note, I had just told the person to do it the "proper" way and he ignored me and did it his way)

If you are looking for enclosed frit funnels, I have lots, designed for solid phase resin work, but they would work for that as well. Look at http://www.chemglass.com/search_category.asp?category=C27&am... for examples of them.

But I have also used fairly old LAH, and unless it was really stored poorly, it has always worked fine for me. But Fwoosh can likely hook you up if you need.

Tsjerk - 4-4-2014 at 09:02

Please be careful, I met a guy with third degree burns on his arm and second degree on his chest. Ten minutes after meeting him I saw the lab that burned down for 50%. The only reason it wasn't the whole building burning down, was that the fire department was located only 100 meters away.

Dan Vizine - 4-4-2014 at 12:50

According to:
Pohanish, R. P. (2008). Sittig's Handbook of Toxic and Hazardous Chemicals and Carcinogens (5th ed.). William Andrew Publishing. p. 1540. ISBN 978-0-8155-1553-1.

...Aged, air-exposed samples often appear white because they have absorbed enough moisture to generate a mixture of the white compounds lithium hydroxide and aluminium hydroxide.

I've done dozens of different LAH reductions and all reasonably well sealed cans were roughly comparable and that doesn't surprise me much. I've never detected the slightest degree of pressurization with old cans. The most important thing is to seal the inner bag well. You can recrystallize from diethyl ether but solubility isn't great so that means Soxhlet.
Generally (always?) it isn't worth the trouble.

zed - 7-4-2014 at 17:10

No, Dr. Bob, not NaH. LiAlH4 in oil used to be the clever way to go. The product used to be offered by the Ventron Corporation. Seems to me, Ventron was a division of Morton-Thiokol.

It is possible they still make the product, but they do not offer it for sale to the general public.

soma - 21-4-2014 at 02:53

Quote: Originally posted by Dr.Bob  

If you are looking for enclosed frit funnels, I have lots, designed for solid phase resin work, but they would work for that as well. Look at http://www.chemglass.com/search_category.asp?category=C27&am... for examples of them.


This one looks like it might work:



filter.jpg - 2kB

Are you selling these?