Hello,
how can I evaporate Et2O or pentane at low temperatures (room temperature or ice in the
rota vap) from volatile compounds the best way? And how long does it usually take to remove all the solvent at the evaporator? Anyone has good experience with this?
Thank you for any help!
By evaporating under vacuum then you are in fact, boiling a low temperature. The pressure in the system is directly proportional to the boiling point of the solvents. For example, at 100mbar pressure, the boiling point of pentane is ~ -18°C. AT 200mbar the boiling point is ~ -4°C.
The vacuum level only creates the conditions where solvents can boil. The level of heat in the water bath provides energy for boiling, the more heat, the faster it can dry .... PROVIDED THAT ... you can remove all that heat at the same rate in the solvent condenser, they must be in balance.
Please note, the temperture of the water bath does not control the temperture of the solvent under vacuum, only the drying time. Sure, the sample will get warm to the temperature of the bath when it is dry, but when it is boiling, pressure controls the temperature of the boiling system.
Diethyl ether and pentane both is very easy to evaporate even at cold temperatures cause the boiling point at atm pressure is 35/36°C.
There are a lot of tables where you can look up the temperature/pressure relationship for evaporation. From an educated guess I would start at ~650 mbar when using a water bath with rt. If you want to be sure you can start even higher and go down by 50 mbar steps. Good luck.
Thank you very much! Do you mean the temperature-pressure-nomograph from Sigma-aldrich?
Yes, you can use this one. I had a simple table in my "Organikum" back from the undergrad days. But this one from Sigma-Aldrich seams even more convenient!
All right. Thank you ver much!
Btw I tried to evaporate all Et2O from my sample at rt and it took me more than 2h at pump-down at the rota vap (1.0 mbar) to remove all the Et2O.
It was only a sample of around 60 mg and I evaporated it to 49 mL to have it solvent-free.
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