Furch
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Determination of mol sieves pore size?
I have gotten my hands on a bulk amount of molecular sieves. I'm very happy about this, but my problem is that I don't know the pore size, which is
crucial when it comes to drying solvents of low molecular weight.
Is there anyone who has any idea as to how one can determine the approximate pore size of these sieves?
Any suggestions or info would be appreciated!
[Edited on 18-3-2007 by Furch]
\"Those who say do not know, those who know do not say.\" -Lao Tzu
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matei
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Furch,
The adsorption of molecules onto molecular sieves is exothermic and also depends on the "critical diameter" of the respective molecule. I guess a
simple method of determining the pore diameter would be to take a sample of activated sieves, put it in a test tube with a thermometer and than add a
little methanol. Methanol is adsorbed onto all sieve types except 3A, so the temperature should rise if your sieve is not 3A. Than take another sample
and add some n-butanol - this is not adsorbed onto 3A and 4A, because its critical diameter is larger than 4 angstroms. Finally try with benzene or
toluene. These are adsorbed only onto the 13X sieve which has a pore diameter of 10 angstroms.
Theoretically this should work. I'll try to do this in the lab tomorrow to see if it really works.
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Furch
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Thanks a lot, Matei! That's a totally brilliant idea! I'll get right on it, and then report if the method worked.
Thanks again 
\"Those who say do not know, those who know do not say.\" -Lao Tzu
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Furch
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Methanol was adsorbed onto my molecular sieves, so was anhydrous IPA, and so was anhydrous toluene.
Since toluene has a molecule diameter of 6.7 Ångström (http://www.sigmaaldrich.com/Brands/Aldrich/Tech_Bulletins/AL...), this must mean that the sieves are of the type 13X.
This is too bad, since I want to use the sieves to dry lower alcohols and other low molecular weight solvents etc. I guess I'll have to order some
instead.
Thanks again, matei, your advice has been of most use!
\"Those who say do not know, those who know do not say.\" -Lao Tzu
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Pyridinium
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Matei, thanks also for that info. I have just picked up some 3A on ebay so I can do some experiments on drying MeOH and EtOH. Probably a lot
cheaper to buy the sieve in bulk, but I only needed a little.
If I figure correctly, 1 lb (454 g) of 3A sieve can treat about 2000 g of 95% EtOH, which is about 1600 ml (based on a density of about 0.81 g/cm3 for
neutral grain spirits.)
(EDIT: Thanks to Furch for pointing this out. 2000g of 95% EtOH is about 2470 mL. I have a tendency to invert fractions when doing math in my head.
I msut eb lysdexic.)
Neutral grain spirits USP are not a cheap source of ethanol, but buying it and drying it out w/ sieves is still cheaper than buying 1 L of reagent
grade EtOH. (not sure of dissolved solid contaminants, but one can always redistill).
[Edited on 19-3-2007 by Pyridinium]
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bio2
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You have to specify Absolute Alcohol when buying "dry"
ethanol reagent grade, otherwise you get the 96%.
USP ethanol is generally only used when in a
product intended for human consumption. That
is why the alcohol tax is applied. USP is available
in 96 or 99% without denaturants
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Pyridinium
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| Quote: | Originally posted by bio2
You have to specify Absolute Alcohol when buying "dry"
ethanol reagent grade, otherwise you get the 96%.
USP ethanol is generally only used when in a
product intended for human consumption. That
is why the alcohol tax is applied. USP is available
in 96 or 99% without denaturants |
I do realize this, I am just saying USP is still cheaper than reagent grade (whether absolute or the more typical 95.5-96%). When you don't need EtOH
of utmost purity, there's no reason you can't use molecular sieves to dry out the cheap stuff.
Another nice thing about USP is its low degree of heavy metal impurity.
Even after paying the alcohol tax, a 1 L bottle of neutral grain spirits costs less than 1 L of reagent EtOH.
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Furch
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Good news for you, Pyridinium: 2000 g of 95% ethanol is almost 2500 ml! Also based on the density of 0.81 g/ml 
I wish I had those sieves of yours :'(
\"Those who say do not know, those who know do not say.\" -Lao Tzu
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Pyridinium
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| Quote: | Originally posted by Furch
Good news for you, Pyridinium: 2000 g of 95% ethanol is almost 2500 ml! Also based on the density of 0.81 g/ml
I wish I had those sieves of yours :'( |
D'oh!!! thanks for the math correction. I should not try math in my head. Thanks for not being too rough on me. 
I will repeat on a chalkboard:
ethanol is less dense than water.
ethanol is less dense than water.
ethanol is less dense than water.
The bright side is, you can pick up 1.5 lbs of the sieves on ebay currently for ~$10 plus shipping. The seller has more. Many good experiments
ahead!
[Edited on 19-3-2007 by Pyridinium]
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