Condensation in TFlask/feeder flask in rotovap

my bath is 40C … pulling vac to 0.1 MPa then 100RPM… then down to 0.075. vac when starts pissing… I keep getting a buildup/condensation of ethanol in my feeder/Tflask so when i drop pressure at end of run it drips back down into my product then im having to go into vac oven… what am i doing wrong? ive dissasembled after finding leaks and fixed also.

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Can you post a better picture? maybe a little further back so we can see the whole setup? I assume your rotovap isnt assembled correctly if you are having solvent drop back into your flask. Also side note, are you sure its not water?

Also how are you feeding your solution into your flask with the feed tube not going into the neck of your flask? i assume you removing your flask to add in more solution?

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no… i dont know its NOT water… although when i introduce the oil to a bigger area on a pyrex and into vac oven @ 29.9Hg and 100F im getting alot of reaction bubbles…

Yes i normally vac down and remove flask and add solution. although im currently not doing big batches requiring this.

This was asked last week. Your evaped etoh is condensing on the cold sides and dripping back into the work flask.

Also, install a line from the glass feed tube and route 2’ past the work flask neck.

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I’m assuming this is you as well? Optimizing your RotoVap - #294 by seedof6ix

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Thanks guys. I appreciate the help even if u HAVE to be assholes doing it.

Let us know if you find a solution.

Try wrapping the glass in some form of insulator to carry the vapor more efficiently to the condensing coils.

ok thank you.

WE get called assholes after we help…with both threads asking the same question with the same issue, and given the same responses to fix it.


Maybe reddit has your answers next time.


Agree with @Demontrich, it’s a few posts short to get pissy.

For the sake of anyone else looking to solve the problem, the answer is this:

Your bump trap is cold enough that its condensing the vapor coming from the boiling flask. You can fix this by insulating as @WolfeXtracts suggested or simply heat gunning the bump trap before breaking vacuum. In all likelihood though if you have enough vapor being generated that it’s condensing noticeably in the bump trap, you’ll need to either keep running the roto longer or purge in an oven anyways. Pro tip: letting a tiny bit of gas in through the feed tube near the end will help push the last bit of vapor from the BF to your pump


To be honest your probably right i shouldnt be like that excuse me. I tried the heat gun… works fine… yes im still putting in the vac oven AFTER roto and getting substantial bubbling at 30 Hg 105F… im taking out few times a day while at work and stirring and this goes on for 3 days… im STILL seeing bubbling… could there still be ethanol left ?

Try using a food dehydrator for the last bit of residuals. The forced air will dry it out

For final purge

Turn heat on rotovap bath up

Turn rotational speed down

Gradually decrease vacuum level, and hold, then gradually increase vacuum level
Fluctuate vacuum depth, up and down


Small % of volatiles in a highly viscous solution does not have the ability to generate enough vapor pressure to release themselves from a thick solution

  • the surface tension of the oil + G-force from rotation of the flask is keeping your last bit of solvent from efficiently evaporating.

You need more heat, which serves dual purpose of lower viscosity and pushing solvent to exit the system at a higher rate, less G force means the bubbles don’t have to fight as hard to break surface tensions, and vacuum fluctuation ensures that you aren’t pulling so hard that the bubbles spend all their time moving towards one another instead of pushing out and into a vapor…

You are coaxing, think of the word coaxing.