stoichiometric_steve
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How do you distil a foaming liquid?
Today i tried to distil an amine which heavily foamed, there was no way to prevent this - neither magnetic stirring nor silicone oil helped a bit. How
can i cirvumvent this?
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chromium
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I distilled couple of years ago a lot of certain cleaning fluid wich consisted of 30% metanol, remarkable quantity of soaps and sufractants and of
water.
At first it seemed impossible to distill due to intense foaming but problem was successfully solved through pouring some cooking oil to boiling flask.
I am not sure if this (or something similar) can be used in your case.
[Edited on 22-3-2008 by chromium]
When all think alike, then no one is thinking. - Walter Lippmann
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evil_lurker
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Lecithin or however its spelled has been used as an anti-foam agent.
Might try going to a farm supply place and get some anti-foam used in herbicide rigs.
Also, a vigreux column would probably help as well.
Not all chemicals are bad. Without chemicals such as hydrogen and oxygen, for example, there would be no way to make water, a vital ingredient in
beer.
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leu
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This advice was posted long ago on the subject of bumping and foaming:
For reasons which are probably not fully understood it occasionally happens that the boiling of a liquid momentarily
ceases. In consequence a portion of the liquid becomes somewhat superheated and this unstable condition is then relieved by the sudden formation of a
correspondingly large volume of vapor, which causes the liquid to boil violently.
Ordinarily a liquid will boil without bumping, for the rising bubbles of vapor keep it moving and thus prevent superheating. Evidently we shall have
an infallible method for the avoidance of bumping if we can keep the liquid moving. For example, when the liquid is being boiled in an open beaker, it
is sufficient to make use of a mechanical stirrer. But when we are distilling, mechanical stirring is not conveniently provided. Here we can use the
method of distillation of by drawing through it a continuous current of dry air. This technique has become common practice for distillations
under reduced pressure, but it cannot be used when the objective is accurate measurement of the boiling point of the liquid, or when some of the
compounds are decomposed by air. Another good method employs uses the capillary tube. They are best made from glass tubing having a thin wall and a
bore of 2 - 3 mm. A suitable length of such tubing, supported at both ends and rotated, is heated at the in the edge of a flame until the walls melt
and and then quickly drawn apart. One end of the tube is then cut off. The over-all length of the tube is then tube should be such that, when the
lower end is resting on the bottom of the flask in which it is being used, the upper end will be supported by the neck of that vessel. The upper end
of the tube may be closed by a rubber tube and a pinchcock clamp. The liquid in the flask must be deep enough to keep the lower end of the tube
continously submerged. It is advisable to shield the equipment against the cooling effect of a possible current of air. And if several of these
tubes are used simultaneously, trouble that might arise from the failure of a single tube may he avoided. This technique also is limited by exposure
of the distilland to air. Other methods for the prevention of bumping succeed by providing conditions favoring the formation of bubbles of vapor at
one or more points where the liquid is in contact with the heated surface of the vessel containing it. Fragments of unglazed porcelain or a
small piece of pumice recently tone that has been weighted with platinum wire. Bumping can also be prevented by the introduction of 7-mm. discs of
Vapoglas (an air-conditioning fiberglass filter material) cut from 3/16-inch sheets with a cork borer. The discs were cleaned with hot dichromate
cleaning solution and rinsed with distilled water before being added to the hot digestion mixture. The porosity of the discs promotes a vigorous
boiling and, because of their lightness, they move about rapidly in the solution and prevent development of localized hot spots. The discs are easily
pulverized by pressure from a stirring rod thus assuring extraction of absorbed material. A small sliver or shred of polytetrafluoroethylene,
makes an excellent boiling chip for the prevention of bumping during evaporation operations in the laboratory. This material is chemically inert to
nearly known reagents (including fluorine and hydrogen fluoride) and is nonporous in nalure. Gas bubbles form very readily on its surface, and this
property does not disappear on continued use and reuse in aqueous solutions or organic solvents. The numerous devices proposed for the alleviation of
bumping during vacuum distillation may be classified generally as ebullators, packings, external capillary leaks, or mechanical agitators. Most
ebullators become ineffective with time or on momentary interruption of boiling. Their use also involves the risk of contamination. Packings
interfere with quantitative work. The use of an external capillary leak is generally quite satisfactory in prevent bumping. However, this results in a
false (low) boiling point and in seriously in the loss of material by entraininent. The loss has been found be appreciable over the period of time
required for a precision vacuum fractionation. Agitation is the most satisfactory method, except for the mechanical difficulties involved. The
external electrically or air driven magnet will spin the magnet in the flask over a distance of about an inch through an electric heating mantle, an
oil bath, or any other heating arrangement free of magnetic interference. The device has been used successfully in this laboratory for fractionations
and distillations of a number of high boiling, moderately viscous compounds in the range of I - 5 mm. The dimensions of the stirring
magnet(length and bulge) should be such as to provide fairly vigorous agitation.
If magnetic stirring didn't reduce the bumping adequately then the agitation wasn't enough to prevent superheating, you may need a more powerful
magnetic stirrer and/or magnetic stir bar  In the case of air sensitive materials
one can connect a tank of argon or perhaps nitrogen to the capillary tube 
Chemistry is our Covalent Bond
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MagicJigPipe
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I heard that simethicone (dimethicone and silica; GasX anti-gas liquid and gelcaps) is used to reduce foaming in many industrial applications. Just
as it is used to reduce foaming your stomach (it actually doesn't decrease the amount of gas but decreases the amount of foaming, slowing the rate at
which gas is realeased in the stomach).
It's definitely worth a try. Like I said, I've heard it's used extensively in industry and it is readily available OTC.
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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Klute
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Octan-1-ol is a very efficient antifoaming agent. A drop or two is enough.
Try using a excessively large flask for the first half of the distn, and then use a more adapted one when what is left is clearly too small for the
flask and gives irregular take-off. I have noticed that foaming often occurs only during the first 1/4-1/2 of the distn, when using solvants that foam
under vacuum, such as toluene.
Was the distn at atm pressure?
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stoichiometric_steve
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wow, thanks for all your replies. as stated, i used silicone oil as an antifoaming agent which has helped me a lot in former distillations, but not
this time. i suspect there's a lot of toluene left in the compound to be distilled, but it seems just impossible to get it out (except for distilling
it out of there).
leu: the stir bar was 1/3 the size of the diameter of the distillation flask, so i guess it was adequately sized and the stirring speed was on max
(~1200rpm).
klute: the distn pressure was varied from ~20mmHg to 120mmHg, with no change in the tendency to foam.
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Klute
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Hum, if you really can't get by with distilling, and you think it's the toluene responsible, you could always perform a A/B extraction and recover
your amine with DCM or ether...
Wait, you have a rotavap right? Then you could use it by putting the vacuum on/off as soon as the foaming comes up to high, with your finger on
vacuum release. The disadvantage is that if your amine in a liitle sensitive or basic enough to form carbonates, it could suffer a little from the air
current. IIRC, distilling will remove any co2 trapped. Surely the foaming will stop when enough of the toluene has been removed.
Have you already distilled much? Or does the foamming really prevent any take off?
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bio2
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Sometimes stirring is not enough to control foaming.
A few drops of Castor oil breaks the foam and works best
when there is some aqueous content in the flask.
Castor oil available at the pharmacy works better than cooking oil as it's much higher boiling.
The vacuum release method usually works in the
beginning to break the foam until boiling is steady.
The vacuum needs to be reduced momentarily to a low value. Try a finger on an open bleed valve and
pulsate the vacuum using the finger every time the foam rises.
Otherwise resorting to the Castor oil or high boiling
alcohols is necessary.
Maximum headroom helps a lot, as mentioned, so try filling
the vessel maybe a third full.
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Panache
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My trick
If i have problems with foams i don't ever bother with anything else but this, it takes some setting up but never ever fails.
Double back a glass rod on itself that can be inserted into your vessel opening, just above the top of your liquid level attach a stainless cable tie
and trim it to the radius of your flask. This stirrer, for me, has never ever failed in destroying foams as they are created just above the level of
the liquid. for an effective simple seal for the stirrer, simply run the stirrer on lowish speed, and wrap teflon tape quite firmly around until it is
about the size of your opening. Stop the stirring and drop the stirrer into fitting, to remove just cut the teflon tape plug off.
As i said it may take some mucking around getting it running but you never have to sweat that halfway through your anti-foam will stop being as
effective. The teflon tape against the ground glass joint provides a very effective seal and if your stirrer is good can be used under quite a decent
vacuum.
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