FireLion3
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Distilling to dryness under a vacuum - is there a safe limit?
Is there any safe limit in doing so? If, for instance the temperature of the flask never exceeds 50 or 60 degrees and I were to distill to the last
drop? Would the glass be subject to much harm at these temperatures? Or is there another reason not to distill to dryness?
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pichoro
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I don't know how truthful this is, and would be interested to find out...
I remember one of the very first things my Organic professor taught me years ago was to NEVER distill to dryness because of a chance of the mixture
having formed or already been containing peroxides, which would then present an explosion risk as the flask reached the point of dryness. We were
distilling ethanol from a previous week of fermentation, but he presented it as though it was a risk in all distillations.
Anyone?
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thesmug
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I think you should never distill to dryness under any condition. Of course that's just what I've learned.
Good eyes
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macckone
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Distillation to dryness is usually not recommended because you
distill heavier contaminants. IF the temperature is constant
and the compound doesn't form peroxides, then you can distill
as much as you can at the constant temperature.
A good example is distilling benzene on a water bath,
it is easy to regulate the temperature and benzene doesn't
form explosive compounds under such conditions.
Distilling alcohols to dryness can be dangerous,
not to mention you will carry over fusel oils and water
at higher temperatures with ethanol.
Ethers will form peroxides that are very explosive.
Other compounds will differ, but a rule of thumb is
don't distill to dryness unless you understand the pitfalls.
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BromicAcid
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Peroxides are not your only concern. You are concentrating all the muck from all your byproducts. It's amazing how unstable some pot residue
mixtures can be, especially since once you finish distilling, you lose that heat sink that distillation allows and your pot residue will begin to heat
very rapidly. Couple that with the fact that you are usually below your thermowell at that point so you don't know what's going on with the
temperature, then you can have a recipe for disaster.
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FireLion3
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So then how does a person go about isolating a target compound when the rest of the reaction mixture is maybe 5% gunk. Is it simply inevitable to be
forced to give up that last little bit that is in the mix? Assuming there is no higher boiling point compound that can be used to prevent dryness?
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macckone
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One solution is to intentionally add a much higher boiling point
solvent that does not form an azeotrope. Naphthalene is an
organic example, note that it is a solid at room temp.
For polar compounds, water with a salt to raise the boiling point
is used. Ether is one example where water is a good choice.
Although ether can be distilled over sodium metal to destroy
water and peroxides, it should not be distilled to dryness, ever.
Crown ethers are another good solvent.
You simply need something that won't react or form an
azeotrope with your desired compound.
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macckone
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Ethylene Glycol and glycerol are also useful solvents.
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BromicAcid
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Quote: Originally posted by macckone  | One solution is to intentionally add a much higher boiling point
solvent that does not form an azeotrope. Naphthalene is an
organic example, note that it is a solid at room temp. |
Napthalene? Surely you jest! Just because it has a high boiling point you are ignoring the vapor pressure. You would be seeing crystals hanging off
your thermowell and plugging up your condenser in no time. You're much better off with mineral oil. That is a good one to try and get the last
little bit out of your pot.
As for your post FireLion3, leaving behind the last little bit is the way of a distillation. Usually you look for cues that your distillation is
finishing up, your pot temp starts to climb, your head temp starts to drop or suddenly shoots up. Those sorts of things tell you you're done with
your distillation and you stop there. Pushing further risks impurities and unless you're willing to compromise quality for quantity that last 5%
isn't worth it.
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macckone
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Naphthalene works (from experience).
You are trying to extract a small amount of chemical,
you use a small amount of naphthalene which is likely
easier to separate (as crystals) than mineral oil which
is a complex mixture of unknown quality.
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