Sciencemadness Discussion Board

Vacuum Distillation & other vacuum uses

RogueRose - 11-1-2016 at 16:33

I was always confused as to how vac distill worked as I thought the dist pot was put under vacuum (the pot with the liquid to be distilled) and always wondered how the constant creation of vapor didn't nullify the vacuum.

Now I see that the vacuum is placed on the receiving flask which means that no vapor reaches it (theoretically) as it condenses before reaching flask.

Are there any alternatives to this standard setup or is that the only workable method.

I have also been looking at various reactions where heating a compound to X degrees will work best under vacuum (because the vac temp lowers BP to compound isn't damaged). Can the same type of vacuum unit be used as the distillation setup?

Are there any other applications where a vacuum is used to help the reaction?

BromicAcid - 11-1-2016 at 17:46

If the distillation flask was not under vacuum then it would not boil. The boiling point occurs when the vapor pressure of the liquid is the same as the pressure over the liquid. So although you are pulling vacuum from your receiver your distillation flask is under vacuum. Of course the hope is to condense all the vapors before they make it to your pump but they may not all be stopped along the way to your receiver and for fairly volatile things your receiver may be hot enough to allow them to continue to evaporate.

With regards to alternative setups it depends on what you are trying to accomplish.

For your last point are you talking about a thermally sensitive compound and continually stripping it off under vacuum as it is formed?

Zephyr - 11-1-2016 at 18:00

I have a similar question I am hoping someone can answer; could the vacuum be applied and then turned off and sealed before the is applied? I am hoping this could work because it would greatly reduce the fumes that reach the pump, but I suspect its not that easy?
Thanks.

Detonationology - 11-1-2016 at 19:23

Quote: Originally posted by Pinkhippo11  
I have a similar question I am hoping someone can answer; could the vacuum be applied and then turned off and sealed before the is applied? I am hoping this could work because it would greatly reduce the fumes that reach the pump, but I suspect its not that easy?
Thanks.

This only works for reactions that do not produce much off-gassing, and only for a short time before the pressure equalizes.

RogueRose - 12-1-2016 at 08:33

As an example, I need to evap the water from an ammonium sulfate solution without boiling as the sulfate decomposes at 212F to bisulfate. So if I could reduce the pressure and lower BP, that would allow me to do this without decomp to bisulfate. Reducing pressure to lowest BP would be great!

To do this, what kind of equipment is needed? Is the only way to do it a standard distillation setup?

chemrox - 12-1-2016 at 16:06

For water evaporation use an aspirator or an aspirator pump. The use of a trap is recommended although in practice these are usually absent. These questions are pretty basic and the topic is covered in all or almost all lab manuals. Vogel has a decent treatment of the subject. So does Organic Chemistry Lab Survival Manual. Read and learn.. read an learn. I don't know why this would be a new topic.

chemrox - 12-1-2016 at 16:08

Hey Bromic! Keep writing! Creative endeavors go beyond organic synthesis. Congratulations on your continuing efforts.