The Vespiary
The Hive => Chemistry Discourse => Topic started by: isop on June 11, 2002, 07:01:00 PM
-
I am somewhat familiar with the concept of maximum/minimum/other boiling point azeotropes but do not really know at all what happens at the other end of the liquid phase - freezing/melting point.
Are there any guidelines as to when would separation of miscible solvents work by simple freezing? What's the theory and practice here? Why don't I hear about that method being used very often, if at all?
Of interest are typical miscible mixtures containing EtOH, H2O, IPA, GBL, benzene, acetone and so on.
P.S. If you know of any references or links on this matter, I'd appreciate your help.
-
You don't hear about it very often because it usually doesn't work.
I'm not fat just horizontally disproportionate.
-
SWIM tried freezing out ETOH from vodka once and it did not work. Got the solution very cold and got some ice crystals on top, but could not get any more. Very impractical.
-
There's a reason why alcohols are used as anti-freeze.
I'm not fat just horizontally disproportionate.
-
EtOH and Acetone need a good source of liquid N2 to freeze. Benzene and H2O can be purified by freezing. An interesting experiment is freezing water out of a weak alcohol solution, the vikings called this 'winter ale'.
-
Is there any theory behind this or is it all empirical knowledge.. can I find some pre-WW1 papers on this or is it futile to attempt to purify anything by freezing?
Distilling anything boiling close to 200C is a bit too hard for SWIM (lousy self-made plastic/rubber parts and connectors leak or melt and there's too much heat loss too), so that's why I'm so interested about this method. ::)
-
Depending on how pickey [Pickford] you are about the language some solvents can be extracted by freezing. safrole [sag] is the solvent in sassafras [sat] oil, one could say that the 10% impurities are dissolved in the safrole [sag] making safrole [sag] the solvent and the impurities the solute. [solution] the (solvent) safrole [sag] can be extracted from the solutes [solution] (impurities) via crystalization [crystalline] in cold temps. [tempt]
This line of reasoning does not work always with what are commonly considered solvents, ie alcohols, hydrocarbons, etc, under normal conditions, ie 1 atm, RoomTemp, etc.
Remember the freezing pt, melting pt of solutions are different than the FP, MP of the individual substances. Look at alcoholic solutions. Antifreeze takes advantage of mixing the correct proportions of ethylene glycol and alcohols and water to get a desired freezing point/boiling point.
piece
-
Yes freezing to purify does work. High boiling point stuff usually gels rather than freezing to a crystaline form if a lot of impurities are present and sometime even when they aren't.