Rotary evaporation is a technique most commonly used in organic chemistry to remove a solvent from a higher-boiling point compound of interest. The rotary evaporator, or “rotovap“, was invented in 1950 by the chemist Lyman C. Craig. The primary use of a rotovap is to dry and purify samples for downstream applications. Its speed and ability to handle large volumes of solvent make rotary evaporation a preferred method of solvent removal in many laboratories, especially in instances involving low boiling point solvents.
Principles of rotary evaporator
Roto-evaporation (rotovap) requires mechanical rotation of a flask under vacuum. The rotation of the flask increases the surface area of the solvent to be removed, increasing the rate of evaporation, and reducing the risk of “bumping”: when a large pocket of solvent vapor forms rapidly and displaces the surrounding liquid. The vacuum reduces the boiling point of the solvent, as well as providing a means to separate the solvent from the compound of interest.
This video will explain the process of rotary evaporation, including the key components of a rotary evaporator, or “rotovap”. Advice for the most common organic solvents and crucial safety considerations will be presented.
General rules for usage of rotary evaporators
1. The solvent collection flask of the unit should always be emptied prior use to prevent accidentally mixing of incompatible chemicals. SAFETY FIRST!
2. The flask with the solution is placed on the rotary evaporator. The use of a bump trap prevents the solution from accidentally splashing into the condenser (and being contaminated). It is highly advisable to start with a clean bump bulb in case something bumps over after all! This would allow the experimenter to recover the solution or solid.
3. A metal or Keck clip is used to secure the flask and the bump trap. The green one shown below fits 24/40 ground glass joints. Similar blue clips fit 19/22 joints and the yellow ones fit 14/20 joints, which will most likely used in the lab. If you break the bump trap, you will have to pay for it!
4. The dial on the motor is used for speed control of the flask rotation. A typical rotavap uses a variable speed sparkless induction motor that spins at 0-220 rpm and provides high constant torque. A good setting here is 7-8.
5. The aspirator vacuum is turned on. On most models, the vacuum on/off control is managed by turning a stopcock at the top of the condenser (left side of the above diagram). This stopcock is later also used to vent the setup after the solvent is removed.
6. The flask is lowered into the water bath (or the water bath is raised to immerse the flask in the warm water. (On most models, a convenient handle (with height locking mechanism) moves the entire condenser/motor/flask assembly up and down. Often the tilt of the condenser assembly can also be adjusted. The water bath temperature should not exceed the boiling point of the solvent!! For small amounts of common solvents the bath heater is not needed.
7. The solvent should start collecting on the condenser and drip into the receiving flask. Some solvents (such as diethyl ether or dichloromethane) are so volatile that they will also evaporate from the receiving flask and be discharged down the drain. To prevent this, a cooling bath on the receiver or (on some models) use a dry-ice condenser can be used. In addition, an additional trap (with dry-ice or liquid nitrogen) can be placed between the vacuum source and the condenser unit. This is particularly important of a membrane pump is used as vacuum source.
8. Once all the solvent evaporated (or whatever is desired at this point), the vacuum is released,. The flask is raised out of the water bath and the spinning is discontinued.
9. The bump trap has to be cleaned and the receiving flask is emptied upon completion of the evaporation.
Applications and Summary
Rotary evaporation can be used to separate solvent from many organic, inorganic, and polymeric materials. It is crucial that the desired compound has a lower boiling point than the solvent and that the compound does not form an azeotrope with the solvent. If these conditions are true, rotary evaporation may be a very efficient technique to separate solvent from the compound of interest. Lower boiling solvents work best, however, rotary evaporation is commonly used to remove water. Higher boiling solvents such as DMF and DMSO are more easily removed using other techniques such as lyophilization, however, with a very good vacuum pump, they may be removed using rotary evaporation.