Sciencemadness Discussion Board

HELP! Vacuum distillation!

DrNewbee - 4-8-2015 at 09:44

Hi guys!

Having problem with my vacuum distillation.

According to my vacuum pump i can pull 0.07 torr...or as it says on the vacuum 10 pascals.
I set up for vacuum distillation and i have calculated that my oil should come over at approximately 50c - 53c...
so i sat the RBF in a kettle and filled the kettle up with water...turned on the vac pump and i can see the effects of the pump without even turning on the heat...but when i come to the temp i should see distilate the oil is still not boiling...?i checked my seals and changed from silicone (anti key freeze) to vaseline and still nothing..? The thermometer shows no change during the entire process.. what am i doing wrong?

By the way im usig porcelain pieces as boiling stones and not a magnetic stirrer. .
I know this is kind of a newbee question but hey...look at my user name..

Any help is much appreciated. ..

[Edited on 4-8-2015 by DrNewbee]

Magpie - 4-8-2015 at 11:45

What is the thermometer measuring, ie, where is the thermometer bulb located?

[Edited on 4-8-2015 by Magpie]

DrNewbee - 4-8-2015 at 12:26

Quote: Originally posted by Magpie  
What is the thermometer measuring, ie, where is the thermometer bulb located?

[Edited on 4-8-2015 by Magpie]


The thermometer was positioned so that the top part of the mercury is "horizontal" to the outlet going to the condenser. ...but i did get a distilate at 75-80c wich confirmed that i had a leak...and the distilate had the right look and smell...so it worked in the end! :)

But for the future.. how would you guys seal the joint at 0.07 torr..?

Thx again for any answer. .

aga - 4-8-2015 at 13:35

Ground glass joints ?

Depends on what you're distilling.

If you were distilling Vaseline, then i have no idea what you'd use (lie).

Vaseline is the go-to when in doubt.

Mentioning what you wish to distill may generate other responses.

Magpie - 4-8-2015 at 13:41

With normal chemical glassware you can't expect to not have some leakage. For most vacuum distillations you don't need 0.07 torr. Most of my distillations are around 1-2 mmHg or up to around 20 mmHg. Standard vacuum pumps have sufficient capacity to hold this level of vacuum with normal leakage.

Why do you feel you have to hold 0.07 torr? That sounds like a condition some physics experiments might require - certainly not run-of-the mill vacuum distillations in chemistry.

I wouldn't even attempt to seal to 0.07 torr even if my pump nameplate claimed this as a specification.

The goal of vacuum distillations is to reduce the boiling point to the level that the product is not damaged by heat. It is not to reduce the bp to an absolute minimum.

[Edited on 4-8-2015 by Magpie]

[Edited on 4-8-2015 by Magpie]

BromicAcid - 4-8-2015 at 14:27

Few thoughts:

You're pulling 0.07 torr where? Before the traps, after the traps, no traps, on the pump itself sealed to the system... That's going to make a huge difference.

Boiling chips are not the best but they work.

Although I hardly ever disagree with Magpie, I would insist that with standard ground glass you can reach 0.07 torr in your setup <b>if</b> you know what you are doing (and of course if your pump is physically capable of going that low).

DrNewbee - 5-8-2015 at 07:21

Quote: Originally posted by Magpie  
With normal chemical glassware you can't expect to not have some leakage. For most vacuum distillations you don't need 0.07 torr. Most of my distillations are around 1-2 mmHg or up to around 20 mmHg. Standard vacuum pumps have sufficient capacity to hold this level of vacuum with normal leakage.

Why do you feel you have to hold 0.07 torr? That sounds like a condition some physics experiments might require - certainly not run-of-the mill vacuum distillations in chemistry.

I wouldn't even attempt to seal to 0.07 torr even if my pump nameplate claimed this as a specification.

The goal of vacuum distillations is to reduce the boiling point to the level that the product is not damaged by heat. It is not to reduce the bp to an absolute minimum.

[Edited on 4-8-2015 by Magpie]

[Edited on 4-8-2015 by Magpie]



Thx man...a lot of the points you give have allready run through my head but my insecurity with chemistry raises doubts about my logic..but yeah.it worked after a few more temperatures, raising the expected boiling point...it not that i wanted to pull in 007 torr but thats just what the pump should boil and i was addement on reaching that point...anyway..thanks for all your response...

DrNewbee - 5-8-2015 at 07:29

Quote: Originally posted by BromicAcid  
Few thoughts:

You're pulling 0.07 torr where? Before the traps, after the traps, no traps, on the pump itself sealed to the system... That's going to make a huge difference.

Boiling chips are not the best but they work.

Although I hardly ever disagree with Magpie, I would insist that with standard ground glass you can reach 0.07 torr in your setup <b>if</b> you know what you are doing (and of course if your pump is physically capable of going that low).


No traps...havent learned the technique yet..
24/40 ground glass joints...laboy glassware.
Vacuumpump is a rotary vane pump.

If i ever would need/want to reduce the presure pulling 007, what grease would you recomentd...i have some silicone and it seems it seals things a little better than the vaseline..? Any tip is much appreciated. .thx again

Bot0nist - 5-8-2015 at 08:51

High vacuum grease.
It's silicone based, if I recall correctly. Just search "vacuum grease" and "vacuum distillation seal" both on the sciencemadness search engine, and Google or the like. You'll find what you seek.

DrNewbee - 5-8-2015 at 11:21

Quote: Originally posted by Bot0nist  
High vacuum grease.
It's silicone based, if I recall correctly. Just search "vacuum grease" and "vacuum distillation seal" both on the sciencemadness search engine, and Google or the like. You'll find what you seek.


Yeah iv done some searching...i just thought that maybe a brand stood out...thanx for your help man....by the way...love the "joy of autodidactation" very true heheh