Hello everyone,
I'm trying to evaporate ethanol from a water/ethanol mixture in the rotary evaporator. The ethanol content is 13% (v/v) in the mixture. I tried to use a bath temperature of 40 ºC and vacuum pressure of 100 mbar, however the process is too slow. How can I determine the right vacuum pressure to eliminate ethanol more effectively? Keeping the temperature at 40 ºC.
Thank you!
Giovana i have found previously that increasing the surface area of the liquid is the best way to shorten evaporation time, maintaining the same temperature.
Dear Giovana Colucci , may be the best idea would be to apply the rule of thumb ∆ 20 °C rule, proposed by the rotavap manufacturer Buchi. It implies 20ºC differences between temperatures of bath, vapour or boiling solution and condenser.
(It is usually refered as 20-40-60 rule, and in your case due to you cannot surpass 40ºC for your bath, the rule would be: 0-20-40ºC (condenser- boiling solution-bath). Dealing with 0ºC water in a condenser is complicated and it would require a chiller connected to the input port of your condenser instead of tap water.
Adjust the needed vacuum for the solvent boiling point of 20ºC (in this case ethanol because, I understand you don´t want to dry up your sample completely, just eliminate the ethanol) 97mbar or 100mbar would work well.
However, consider to increase the bath temperature to 50 or 60ºC, if your chemical of interest doesn´t degrade at 30 or 40ºC (the temperature of the boiling solution when the bath is at 50-60ºC). If your compound is very labile, keep your conditions!
Here you can find a guide for properly use the rotovap and a table with solvents and pressures (vacuum).
https://static1.buchi.com/sites/default/files/downloads/List_of_solvents_en.pdf?e88f9352d46a9351cb506acdc781a20ec52eb322
As Robert Gal pointed, increasing the size of the boiling flask would increase the area of contact of your solution with the walls and hence with the bath, improving the evaporation rate.
Also it is a good idea to keep the level of liquid inside the boiling flask under the level of water in the bath, to improve the heath transmission.
The faster the rotation speed the better.
Applying more vacuum could cause splashes, specially if your compound is tensioactive (forms bubbles or foam).
Don´t worry about water:EtOH azeotropes, they will form on the receiving flask as pretty well explained Patrick Wilson at the end of this query on RG:
https://www.researchgate.net/post/How-do-I-evaporate-water-and-ethanol-using-a-rotary-evaporator
Hope it helps! Good luck!
Thank you Manuel for your very complete answer! It helped me to understand better the process. I think it is okay for me to increase the temperature of the bath at 60 ºC. I'll try this!
And thank you Robert for you idea, I'll try that too!
Hello Manuel and Robert,
I applied the recomendations you gave me and it work really well! In few minutes I could remove the ethanol from my sample. Thank you!
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