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Author: Subject: molecular sieve capacity
soma
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[*] posted on 6-6-2014 at 13:58
molecular sieve capacity


I've read that molecular sieve can adsorb up to 22% it's weight in water. Would the number of molecules of ethanol adsorbed be the same as for water? Meaning that it would adsorb about 56% of it's weight in ethanol? I want to remove ethanol from ether using 4A sieve.

Thanks.
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Burner
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[*] posted on 6-6-2014 at 17:36


Don't have an answer for your question, but I do have a question. Why not distill it?
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EdMeese
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[*] posted on 6-6-2014 at 19:16


probably not ... ethanol is much larger than water so far fewer will fit inside the pores where all the action is. But I've never seen any data, either.
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sparkgap
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[*] posted on 6-6-2014 at 19:25


Also, you're assuming that ethanol is the only thing contaminating your ether; if it's also wet, then you'll need to add some more 4A just to fit in all the water, ethanol, and other possible gunk in your ether. Should you need to get the ethanol from the sieves afterward, you'll need to do a good amount of heating.

And thus, we go back to Burner's suggestion.

sparky (~_~)




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soma
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[*] posted on 7-6-2014 at 11:35


Actually, it's "anhydrous" ether with 2% ethanol stabilizer. I was going to add sieve to dry it more and then distill it from LAH to complete. I'd rather not have to distill it twice (once to remove ethanol, 2nd time from LAH).
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kristofvagyok
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[*] posted on 7-6-2014 at 13:50


Quote: Originally posted by soma  
Actually, it's "anhydrous" ether with 2% ethanol stabilizer. I was going to add sieve to dry it more and then distill it from LAH to complete. I'd rather not have to distill it twice (once to remove ethanol, 2nd time from LAH).

Why not add some sodium or activated magnesium, stir a few days and distill it from that? Much cheaper and easier to handle, than LAH.

P.S.: someone in our lab blown up a flask while distilling dioxane from LAH. Some peroxides formed during the past 20 years and when the distillate was concentrated enough on the LAH it blown up.




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Dan Vizine
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[*] posted on 9-6-2014 at 09:30


[/rquote]
Why not add some sodium or activated magnesium, stir a few days and distill it from that? Much cheaper and easier to handle, than LAH.

P.S.: someone in our lab blown up a flask while distilling dioxane from LAH. Some peroxides formed during the past 20 years and when the distillate was concentrated enough on the LAH it blown up.[/rquote]

C'mon, really? Distilling 20 yo dioxane w/o testing for peroxides and destroying them? It's hard to understand how someone gets to the point of even needing anhydrous dioxane without having learned that THF and p-dioxane (I assume) are quite prone to peroxide formation. I'm assuming he knew better but was in a hurry? Bad lapse of judgement.

Anyway, molecular sieves, zeolites, can remove whatever the cavities are optimized to hold to an astonishing degree. All sieves are not the same, they vary principally in the above mentioned feature. If you get a 3A sieve it will have 3 angstom holes. Choose accordingly, consult the manufacturers charts. Molecular sieves, for instance, can dry solvents to a much smaller residual water level than any perchlorate or pentoxide ever could. That being said....

Sieves are most effective in scavenging roles and not as good for bulk removals (I'd call 2% a bulk removal, it's certainly not considered trace).
Just as kristofvagyok said, it's easier to just "react" the ethanol out.
Once dry, molecular sieves will maintain it that way. Remember, peroxides are most easily formed in very dry, pure ether and especially in light-transmitting bottles.
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leu
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[*] posted on 9-6-2014 at 10:19


Some documents related to the safe handling of solvents prone to peroxide formation are in the attached archive :cool:

Attachment: steere+.zip (1.1MB)
This file has been downloaded 188 times





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soma
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[*] posted on 9-6-2014 at 12:05


I've wondered why someone would want to reflux (or distill) dioxane from LAH since dioxane boils at 101C and the flash point of LAH is 125. But, wouldn't the LAH get rid of the peroxides? Or maybe the explosion happened immediately upon adding the LAH?

[Edited on 9-6-2014 by soma]
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chemrox
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[*] posted on 9-6-2014 at 12:52


So far the best thing to come out of this was the link posted by Leu. Someone should also post the table of sieve sizes vs. cmpds captured. (I'm traveling and don't have it). I still think the best idea was to stir over sodium or magnesium, check for peroxides and distill. If peroxides are present you'd want to grab those before any distillation is run.



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ziqquratu
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[*] posted on 9-6-2014 at 15:18


Quote:

I've wondered why someone would want to reflux (or distill) dioxane from LAH since dioxane boils at 101C and the flash point of LAH is 125. But, wouldn't the LAH get rid of the peroxides? Or maybe the explosion happened immediately upon adding the LAH?


Remember that the flash point (lowest temperature at which the vapour pressure can sustain a flame) is NOT the same thing as the autoignition temperature (the lowest temperature at which a material will spontaneously ignite - without any external ignition source).
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