Introduction to using a rotary evaporator

The standard method of removing large volumes of volatile solvent is to use a rotary evaporator. This may be to concentrate a solution, or to evaporate all of the solvent to leave an involatile component. To make the process faster, the evaporation is conducted at reduced pressure, with the result that the solvent will boil at a lower temperature. Rotary evaporators are also designed to allow the solutions to be conveniently heated, by lowering them into water baths. After the evaporation, the collected solvent can be disposed of. Rotary evaporators also allow flasks to be rotated during the application of reduced pressure, which is useful for two reasons:

• rotation helps to spread the solution around the flask thinly, which increases its surface area and thus speeds the evaporation

• rotation gently agitates the solution, which reduces the risk of the solvent “bumping” (undergoing uncontrolled boiling)

General principles of operation

A schematic diagram of a typical rotary evaporator is shown below. Be aware that there are many different designs of rotary evaporator, but they share common features.

Knowing the nature of the solvent to be evaporated allows the temperature of the water bath to be set correctly. Typically, the water bath is best left at room temperature; many organic solvents (e.g. ether, petrol, dichloromethane) have relatively low boiling points at atmospheric pressure, and hence under reduced pressure, their boiling points are significantly lower than room temperature. For this reason, great care must be taken in heating solutions on the rotary evaporator, as volatile solvents are likely to “bump” under these conditions. Heating a solution unnecessarily also increases the likelihood of decomposition reactions – it is not uncommon for compounds to be temperature-sensitive. In general, and depending on the solvent, it is usually not necessary to heat solutions to get them to evaporate – if you think you need to heat a solution, consult a demonstrator first. Even if the solution does not require heating, immersing it in a room-temperature water bath can be useful, as this offsets the effect of evaporative cooling. Solutions which become very cold during the evaporation process are also prone to bumping.

Schematic representation of a typical rotary evaporator

The solution to be evaporated is placed in a round bottom flask, so that the flask is less than half-full. If the volume of solution is too large to fit, selection of a different flask, or evaporating the solution in portions, are the best ways to proceed. Using a flask that is greater than half-full increases the likelihood of “bumping” which can result in contamination of the solution and the rotary evaporator.

The evaporation flask is fitted to the vapour duct of the rotary evaporator, using the ground glass joint. The use of vacuum grease is to be avoided here, due to the likelihood of contamination – for this reason, the joints should be checked carefully to ensure they are clean.

During the rotary evaporation process, the reduced pressure in the rotary evaporator will hold the evaporation flask on securely. However, if the reduced pressure is not active (i.e. before or after the procedure). the flask is at risk of slipping off, and falling into the water bath. For this reason, the flask should be secured with an intact, appropriately-sized Keck clip; it is also prudent to support the fitted flask by holding it gently, whenever the system is not under active vacuum.

A note on controlling the strength of vacuum

The vacuum tap, and the air bleed tap, can both be used to control the vacuum in the system. For example, during the evaporation, the vacuum can be increased (i.e. the pressure in the system can be reduced) by opening the vacuum tap fully, and closing the air bleed tap fully. Conversely, the vacuum can be weakened (i.e. the pressure in the system increased) by partially closing the vacuum tap, and partially opening the air bleed tap.

However, in the teaching labs, rotary evaporators and other devices use a communal source of vacuum. This means that if you open the air bleed tap to reduce the rate of evaporation for your flask, the rate of evaporation is reduced for everyone in the laboratory. For this reason, you should not control the vacuum with the air bleed tap – you should use the vacuum tap instead, as explained in the steps below. Other laboratories may use different vacuum systems.