soma
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drying alcohol stabilized ether with molecular sieve
I recently got alcohol stabilized anhydrous ether. I tried removing the alcohol using 4A molecular sieve. After leaving it sit over the sieve for
about 2 days, I tried testing a small amount in a test tube using LAH. The LAH bubbled quite a bit and was pretty much completely used up.
This made me think that the sieve didn't do too well. The sieve is active - (when I put a drop of water on a couple beads it gets pretty hot).
Does this sound right?
I was thinking of trying calcium hydride instead.
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peach
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Did you try testing the sieve after you took it out of the ether, to see if it still had any capacity?
I'm only wondering if perhaps there wasn't enough in there.
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soma
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I didn't try testing the sieve afterwards, but when I first added it, it didn't noticeably bubble, so I thought the ether must have been pretty dry to
start with. (It's still sitting in the ether and I'd have to filter it out to get to it.)
[Edited on 22-7-2011 by soma]
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peach
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Maybe scoop out a test tube of the ether, add more sieve and retest it.
Is there any possibility you could regenerate the sieve to make sure it's entirely empty?
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soma
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Quote: Originally posted by peach  | Did you try testing the sieve after you took it out of the ether, to see if it still had any capacity?
I'm only wondering if perhaps there wasn't enough in there. |
Has anyone ever tried removing alcohol with the 4A sieve from alcohol stabilized ether? I've got some pure (unstabilized) thf and am going to try
drying it with the 4A sieve and see if I get the same results. About 6 months ago I used this same sieve and it dried the thf successfully. I'm
wondering if this isn't 4A sieve, or if it isn't too efficient at removing alcohol.
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peach
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It's one of the recommended methods for ether;
Diethyl ether
CaCl2; Na; Pb/Na; LiAlH4; Na wire/benzophenone; Molecular sieve 0.4nm
Residual Water mg H2O/L
0.001
g H2O/g Desiccant
0.18
Regeneration
250C
Adsorbs
Molecules of diameter <4 angstroms, Ethanol, H2S, CO2, SO2, C2H4, C3H4, and strong acids
Calcium chloride, on the other hand, leaves a water residue of 0.14-0.25 mg H2O/L, so the sieve is about one or two hundred times more thorough in
terms of dryness.
If it's not in an original supplier bottle with a specific pore size written on it, as you say it's possible you've got 3A, or a blend of different
sizes, and it's simply not absorbing the ethanol.
Perhaps you could weigh the sieve before and after to check the mass change? Trying it in some already dry ethanol should reveal whether or not it's
actually capable of absorbing ethanol.
[Edited on 26-7-2011 by peach]
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soma
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I think I found the problem. -- I don't know if this is usual but this ether has 2% ethanol as stabilizer. I had tried using P205 with celite. It
didn't fizz or bubble when I added it (either in the test tube or the flask), which I would have thought would have happened if it was a large amount
of ethanol. But, when the temperature went up to around 60C towards the end of the distillation, I went and looked at the bottle and saw it was 2%
ethanol. I had been assuming that it only would have trace amounts, but 2% comes out to about 150ml ethanol (in 2 gallons). That seems like alot to
me. That's about 118 gms. I don't know if it's the same ratio as for water, but that comes out to about 650 gms of molecular sieve (if it was water),
which is a pretty large amount of sieve.
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soma
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Further questions:
In the last distillation I stopped distilling at around 60C. (2 gallons of ether was being distilled from a 12L flask with a thermometer at the
distillation head.)
I then added more molecular sieve. A test tube test with LAH produced no detectable bubbling.
I then added more P2O5/celite and redistilled (from the same 12L flask. The temperature went over 40C at the head (and it started distilling at 30C).
I'm thinking that the ether gas can get overheated in the large extra space of the flask?
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soma
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After searching through a few lab manuals under "distillation" I finally found one that said to distill slowly (1 drop per second) or the vapor might
get "superheated".
However, I don't want to wait around for the many hours it would take to distill off 2 gallons. When I lowered the rate down to about 1 drop per
second, it was distilling at around 35C.
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