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Author: Subject: drying the water out of hygroscopic crystals
chemrox
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[*] posted on 11-10-2007 at 20:00
drying the water out of hygroscopic crystals


I have a product. In this case it happens to be a small amount of Me-am.HCl. It's very hygroscopic and has picked up so much water it won't stay crystalline but reverts to a concentrated, very cat-pee smelly, aqueous solution when the heat is removed. I collected it in a RB and was planning to run the water out with toluene by adding the Ph-me and boiling the azeotrope off leaving dry Me-am.HCl behind. Is there any reason this shouldn't work? I couldn't find a thread with this exact discussion so if there is one I'm sorry for redundancy. Thanks, CRX
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[*] posted on 11-10-2007 at 23:40


I've had better luck when drying amine salts by first heating them with a lower alcohol, usually getting them fully into solution, then adding toluene/xylene/alkane to azeotrope off water+alcohol+nonpolar. Using just toluene should work, but stir well during the process and don't let it go to dryness.
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chemrox
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[*] posted on 12-10-2007 at 19:00


what alkane(s) could I use that wouldn't flash off before the toluene/water azeotrop?
I have some kerosene ... no high boiling ligroine though ..
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[*] posted on 13-10-2007 at 00:54


Check azeotrope tables : n-hexane is 91% hexane, 9% water at about 61-62 C n-heptane is 87% heptane, 13% water at about 79 C, octane/water 75/25 @ 90C All those from memory, but should be close.

Automobile fuel works; wash with a little water to remove polar stuff, distill through a column to get a cut boiling in the range you want. The actual alkane(s) aren't too important, it's the boiling point range you want to pick.
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JohnWW
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[*] posted on 13-10-2007 at 03:59


Are you sure, Not_Important? I doubt that paraffin hydrocarbons could possibly be that miscible with water near their boiling-points, to produce azeotropes with such compositions, in view of the data in Perry's Chemical Engineers' Handbook. In fact, they form separate layers, when attempted to be mixed. I think you may have confused those paraffins with the corresponding 1-alcohols, or ternary mixtures of these with the paraffins and water..
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[*] posted on 13-10-2007 at 11:13


The partition coefficients are based on n-octanol if that helps jog your memory.
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[*] posted on 13-10-2007 at 20:49


Quote:
Originally posted by JohnWW
Are you sure, Not_Important? I doubt that paraffin hydrocarbons could possibly be that miscible with water near their boiling-points, to produce azeotropes with such compositions, in view of the data in Perry's Chemical Engineers' Handbook. In fact, they form separate layers, when attempted to be mixed. I think you may have confused those paraffins with the corresponding 1-alcohols, or ternary mixtures of these with the paraffins and water..


That's right, they form separate layers, which makes it easy to return the hydrocarbon to the still pot. This also makes it easy to clean up the aqueous layer, because most of the heptane is in the lighter organic layer, distilling the aqueous layer will result in all the heptane distilling overs with a small percentage of the total water.

Azeotropes do not have much to do with the liquids being miscible. Check the CRC azeotrope tables, or some of the others around. The heptane/water azeotrope is the first listed in the 'H' section at http://www.chemeng.ed.ac.uk/people/jack/azeotrope/hetero.htm...
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