lordmagnus
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low pressure chemistry/ vacuum question
I have a question,
If I setup a soxhlet extraction, with 24/40 joints, high vacuum grease on those joints, and a vacuum adaptor on the top of the allihn condenser,
then pull a vacuum to the desired Torr, when I add heat to the setup to say boil water at 70 Deg. C. I hsouldn't need to be continuously be pulling a
vacuum with my pump right? or do I need to be constantly pulling a vacuum at the desired ammount throughout the whole process?
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Magpie
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Yes - you have to have the vacuum continuous.
I have never heard of an Allihn condenser being used under vacuum. I not sure it is designed for it - may not be strong enough. I would question the
use of the Soxhlet extractor even more. I'd hate to see you bust all that new glassware & get hurt in the process.
The single most important condition for a successful synthesis is good mixing - Nicodem
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lordmagnus
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Hummm, ok.. Thanks for the info.... I have the following condensors; Allihn, Graham, Liebig... As for distialltion collumns; Hemple, Vigereaux, Snyder
I\'ll kill a man in a fair fight, or if I think he is gonna start a fair fight, or if he\'s bothrn me allot, or if I am getin payed good, or probably
over a good woman.
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unionised
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I wouldn't want to try running a soxhlet extraction under vacuum- those fiddly shaped bits of glass are not built for it.
Anyway, the question of whether or not to leave the pump running depends on a number of things.
If you have a good pump and not a very cold condenser then you will suck all the solvent out through the pump. Not good for it and not any use for
extracting. If the pump cannot deliver a good enough vacuum to do this then you should leave it running to make up for any small leaks in the system.
If you are sure that the system doesn't leak then, once the system is under vacuum (and the liquid is boiling), you can close the tap between the pump
and the still and switch off the pump. If there isn't a tap there then leave the pump running - otherwise you risk sucking the pump oil (or tap water
if it's a water type pump) into your system.
Some questions just don't have a simple answer.
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Magpie
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Normally a soxhlet extraction is carried out with a low boiling solvent such as ether or dichloromethane. So in these cases temperature is low anyway
so there would be no need for a vacuum condition to keep the temperature down.
There are cases where you have a solute or solvent that's vulnerable to temperature breakdown and say you need to do a distillation. In this case you
need to do a vacuum distillation.
I believe all of the equipment in my organic lab kit is vacuum capable. This includes a range of round botom flasks, a distillation adapter, a
Claisen adapter, a vacuum adapter, a thermometer adapter, a Hempel column, and a West (or Liebig) type condenser.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Mr. Wizard
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If you are doing a soxhlet extraction with good sealed joints, you should be able to maintain the lower pressure without a constant running of the
vacuum pump. The pressure inside the extractor will be the vapor pressure of the liquid you are using at the temperature of the condenser. If you
don't keep the condenser cold enough to absorb all the heat used to boil the solvent, the pressure will rise, and your 'vacuum' will be lost. If you
run the pump constantly you risk pumping all the solvent, in the vapor phase, through the pump and it won't re-condense and extract what you want. In
short, if you don't generate any gaseous products, or have any leaky seals, your system, once pumped down to the vapor pressure of the condensate,
will not need any more assistance. The pressure inside will be dependent on the heat input and the temperature of the coolant. In theory, with high
vapor pressure solvents, you could just boil the solvent to fill the system with vapor at atmospheric pressure, turn on the coolant, and carefully
plug the system. This might be dangerous if the pressure got high before the cooling took effect ;-) With some other solvents that you wouldn't want
to boil at atmospheric, you could use the pump to get just below the desired pressure and then seal it up.
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