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Rotary Evaporator: Solvent Removal Cheat Sheet by

Rotary Evaporator Solvent Removal
cleaning     removal     solvant     rotary     evaporator

Introd­uction

A rotary evaporator is a specially designed instrument for the evapora-
tion of solvent (singl­e-stage or straight distil­lation) under vacuum. The
evaporator consists of a heating bath with a rotating flask, in which the
liquid is distri­buted as a thin film over the hot wall surfaces and can
evaporate easily. The evapor­ation rate is regulated by the heating bath
temper­ature, the size of flask, the pressure of distil­lation and the speed of rotation

Solvent removal

Solvent removal remains an unavoi­dable process that scientists and
engineers must perform on scales ranging from a few millil­iters to
thousands of liters. The use of a rotary evaporator in this process has become ubiquitous for single batch and continuous processes. Most scientists become familiar with the technique in a university experi­mental laboratory course. The technique can become trivial quickly – much like other daily tasks. Unfort­unately many rotary evaporator users forget the scientific principles of solvent evapor­ation. This is evidenced by pulling ultimate vacuums without control or knowledge of the pressure, the use of secondary conden­sers, proble­matic bumping and the use of bump traps. To achieve optimal distil­lation condit­ions, the distil­lation energy supplied by the heating bath must be removed by the condenser. An easy concept to remember for solvent removal via rotary evapor­ation is the 20/40/60 Rule.
These numbers refer to the D20ºC principle.

20/40/60 Rule

Use the 20/40/60 rule when utilizing your Rotary Evaporator for Solvent Removal: A Bath Temp of 60°C yields a 40°C Vapor Temp which will Condense at 20°C

Principles

Roto-e­vap­oration 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.
 

Rotary Evaporator

Procedure

1. Setup
Pour the mixture of solvent and desired compound in a round bottom flask. Best results are achieved when the flask is filled less than half full of the solution.
Fill the rotovap cold traps with dry ice.
Attach a glass "bump trap" which prevents any solution from entering the main part of the rotovap. Secure with a Keck clip.
With a Keck clip attach the flask and bump trap to the adapter portion of the roto-e­vap­orator.
Lower the flask into the water bath. This helps to prevent the flask from discon­nec­tion.

2. Rotary Evaporator Operat­ion
Start the rotation. Different speeds are preferable for different volumes.
Slowly start increasing the vacuum. The vacuum is at the proper strength when: 1) conden­sation of the solvent can be seen on the cold finger or in the receiving flask, or 2) the solvent begins to bubble.
Turn on the heat for the water bath. Recall from general chemistry that vacuum reduces the boiling point of the solvent, so signif­icantly lower temper­ature is needed to evaporate the solvent using a rotovap than at STP.
Adjust the vacuum setting as needed.
When all solvent has been removed turn off the vacuum and return the flask to atmosp­heric pressure.
Stop the rotation.
Raise the flask from the bath.
Remove the flask from the adapter.
If there is more solvent to remove it can be added it to the same flask and the procedure is repeated. Remember to empty the receiving flask when the evapor­ation is complete.

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