A rotary evaporator is a specially designed instrument for evaporation of solvents (single-stage or straight distillation) under vacuum condition.

The evaporator consists of a heating bath with a rotating flask, in which the liquid is distributed as a thin film over the hot surface and the liquid can be evaporated easily.

The evaporation rate is regulated by factors like the heating bath temperature, the size of flask, the pressure of distillation and the speed of rotation.

Main parts of a rotary Evaporator and function of each part

  1. Evaporation Part

The Solvent is heated by the heating bath. A thin film of solvent forms on the inner surface of rotating evaporating flask, resulting in a higher rate of evaporation. Rotation ensures homogenous mixing of sample and prevents the solvents to be overheated inside the flask.

  1. Rotary Drive

The drive unit provides constant rotation of the evaporating flask, so that the solvents can be continuously evaporated.

  1. Condensation Part

The solvent vapor flows at high speed into the condenser of the rotary evaporator. At this point the temperature inside the solvent vapor is transferred to the cooling medium (usually water) and the solvent condenses. The condensed vapor then becomes liquid and flows into the receiving flask by the force of gravity.

  1. Receiving Flask

The receiving flask is used to collect the condensed solvent.

  1. Vacuum

Vacuum is used to lower the boiling point of the solvent and increase efficiency of distillation process.

General Rules for Using A Rotary Evaporator

1. The collecting flask should always be emptied before use to prevent accidentally mixing of incompatible chemicals. SAFETY FIRST! 

2. The flask should be lowered into the water bath (or the water bath should be 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.

3. 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.

4. 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 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 diaphragm pump is used as vacuum source.

5. Once all the solvent has been evaporated (or whatever is desired at this point), the vacuum should be released. The flask should be raised out of the water bath and all power supply should be disconnected.

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