chemrox
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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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not_important
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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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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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not_important
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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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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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contrived
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The partition coefficients are based on n-octanol if that helps jog your memory.
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not_important
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| 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..
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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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