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Author Topic: Vacuum Distillation of High Boiling Point Solvent  (Read 15174 times)

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beheada

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Vacuum Distillation of High Boiling Point Solvent
« on: July 07, 2007, 05:53:57 AM »

I've been working with DMSO lately. When I attempt to vacuum distill it at roughly 1 torr my vacuum actually suctions the gas through the condenser and out its exhaust. Not good considering the horrible smell of DMSO. Does that mean that the water in the condenser isn't cold enough (perhaps try it with an ice pack wrapped around the column?) or does that mean I need to vary the vacuum pressure?
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billnotgatez

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Re: Vacuum Distillation of High Boiling Point Solvent
« Reply #1 on: July 14, 2007, 12:25:09 PM »

http://en.wikipedia.org/wiki/Dimethyl_sulfoxide

I am probably grasping at straws but the first trap would be very cold and the second trap would be a compound that would absorb it. That way you would get no fumes.

I was hoping for one of the organic people to comment but since they did not I decide to give it a guess.

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beheada

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Re: Vacuum Distillation of High Boiling Point Solvent
« Reply #2 on: July 17, 2007, 01:56:47 PM »

Should I repost in organic? I'm having difficulty finding a solution. I even wrapped the condenser column with an ice pack and it still doesn't condense before getting sucked through the vacuum adapter. Suggestions on compounds for absorption?
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billnotgatez

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Re: Vacuum Distillation of High Boiling Point Solvent
« Reply #3 on: July 19, 2007, 03:33:14 AM »

Quote
Should I repost in organic
?

That is cross posting and it is frowned upon.
i am seeking a way to lure someone here from organic
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AWK

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Re: Vacuum Distillation of High Boiling Point Solvent
« Reply #4 on: July 22, 2007, 08:15:24 PM »

I've been working with DMSO lately. When I attempt to vacuum distill it at roughly 1 torr my vacuum actually suctions the gas through the condenser and out its exhaust. Not good considering the horrible smell of DMSO. Does that mean that the water in the condenser isn't cold enough (perhaps try it with an ice pack wrapped around the column?) or does that mean I need to vary the vacuum pressure?
You should use liquid nitrogen trap or (dry ice + acetone or methanol). Otherwise DMSO goes to oil and you loss pressure in the next distillations
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AWK

bucknerwh

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Re: Vacuum Distillation of High Boiling Point Solvent
« Reply #5 on: July 30, 2007, 09:22:38 AM »

I'm not sure if you are still working on this... I am an organic chemist that just happened across this topic.

The problem is not that you are failing to trap DMSO, which is very high boiling and thus trapped very easily even with a regular (H2O) ice trap. In fact, in most cases a glass bottle of DMSO left outside on a cold day will literally freeze inside the container and must be melted in order to be used!

The compound responsible for the smell is DMS (dimethylsulfide), a gas that almost always accompanies DMSO in very minute quantities. DMSO itself is odorless, so any smell you detect is coming from DMS or chemically related sulfides. Generally, DMS is a by-product of the DMSO synthetic process and is so commonly associated with DMSO that people often attribute the bad smell to DMSO itself.

Since it is gaseous, DMS is very difficult to trap even with a liquid nitrogen trap, as suggested above. You are better off treating the exhaust of your vacuum pump so that any DMS gas that passes through is neutralized before it can offend your nostrils. I would recommend attaching one end of a plastic or rubber hose to the exhaust of your vacuum pump. Stick the other end of the hose into a solution of household bleach (i.e., Clorox) and water. The bleach will oxidize the DMS back into DMSO and get rid of the smell.  When working with sulfides in organic chemistry, it is usually necessary to wash all lab equipment with a bleach solution to get rid of the foul odor. Spoiled food products and human waste contain similar sulfides, so its no wonder the odor has been troubling you. I would hasten to add that the actual amount of DMS we are talking about here is very tiny, so there is virtually no risk of being harmed by it or the bleach solution once it has been used to neutralize the exhaust gas.

A suggestion for the bleach concentration is something like 9 parts water, 1 part bleach. It doesn't take very much to neutralize DMS chemically. Just make sure the exhaust tube is completely submerged in order to assure all of the gas passes through the bleach solution before escaping into ambient air.

Hope this is helpful. Good luck!

William Buckner
bucknerwh@hotmail
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bucknerwh

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Re: Vacuum Distillation of High Boiling Point Solvent
« Reply #6 on: July 30, 2007, 09:40:12 AM »

I forgot to mention...

The oil in your vacuum pump may already have DMS dissolved in it. If the smell persists even when you are not actively vacuum distilling DMSO, the vacuum pump could probably use an oil change. The bleach solution suggestion I made previously should cut down on the smell, and when you are finished distilling DMSO, I'd recommend an oil change. I don't think DMS (gas) dissolved in pump oil will significantly hurt your vacuum pump's effectiveness, but if you are actually getting DMSO (liquid, high boiling, unlikely) or other solvents (liquid, low boiling, likely) that are escaping the trap and ending up in your pump oil, your pump's performance will suffer. However, the same thing (high bp) that makes vacuum distillation of DMSO necessary make it unlikely that it would carry past your ice traps into the pump oil. Ethyl acetate, THF, dicloromethane, and other low-boiling organic solvents are much more likely to foul your pump oil than DMSO, as long as there is some sort of cold trap in use at all times. After completing the distillation, check the ice trap to see if there is solvent condensed in it. With a dry ice or liquid nitrogen trap, expect DMSO to remain in solid form until it warms up closer to room temperature. Be sure to pour off this trapped DMSO before turning the pump back on, or a small amount might be sucked into the pump oil if someone tries to use the pump without filling the ice trap (which happens ALL the time).

Good luck!

William Buckner
buckner@hotmail
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billnotgatez

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Re: Vacuum Distillation of High Boiling Point Solvent
« Reply #7 on: July 30, 2007, 12:16:36 PM »

Thank you AWK and bucknerwh
I hope the original poster will return and comment
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