It is very common for a desired compound to be dissolved in a solvent during regular manipulations in the laboratory. Solvents are used in separatory funnel extractions and column chromatography, and the solvent must be removed in order to isolate te desired compound. Solvents are regularly chosen that have lower boiling points than the compound of interest, so that there is some mechanism for their removal. In theory, a solution could simply be placed on a heat source to boil away the lower-boiling solvent, but this approach is not often used.
The principle of rotary evaporator remove solvent
The preferred method for solvent removal in the laboratory is by use of a rotary evaporator, also known as a “rotovap”. A rotary evaporator is essentially a reduced pressure distillation: a solution in a round bottomed flask is placed in the water bath of the apparatus , and rotated while the system is partially evacuated (by a water aspirator or vacuum pump). The reduced pressure in the apparatus causes the solvent to boil at a lower temperature than normal (see vacuum distillation), and rotating the flask increases the liquid’s surface area and thus the rate of evaporation.
The solvent vapor condenses when it comes into contact with a water condenser and drips into a receiving flask . When the solvent is removed, the concentrated compound is left in the flask. One difference between distillation and rotary evaporation is that the distillate is most often retained in distillation while the residue is retained in rotary evaporation.
Even though the rotary evaporator looks pretty complicated, it’s really pretty simple. The purpose of the rotovap is to remove low boiling organic chemicals, usually solvents, from a mixture of compounds. The rotary evaporator is the method of choice for solvent removal in the modern organic laboratory. The solvents or low boiling compounds are removed by a simple distillation. The rotovap is designed to be operated under a vacuum (to lower a compound’s boiling point) while heating the sample at the same time. A cold finger is used to condense the vapors to a liquid, which are trapped in a separate flask.
Important Features of the Rotary evaporator (Rotovap)
1) The vacuum pump is the source of reduced pressure in the system.
2) Spin speed and bath temperature control knobs.
3) Water bath.
4) Bump guard.
5)Cold finger. (Please do not use the rotovap if the finger is not cold. There should be excess solid dry ice in the finger.)
6)Vacuum release valve.
To use the rotovap, follow these simple steps:
The rotovap should be turned on and the trap cooled. If this is not the case, ask your TA for assistance.
1. Attach your flask to the bump guard of the rotovap using a Keck clip. The flask should not be more than half full of liquid.
Notice that the Keck clip has larger and smaller sides to accommodate the larger neck of the round bottom flask and the smaller stem of the bump guard.
2. Turn on the spin. (See left photo marker #2.) This will help prevent bumping during the distillation. Ask your TA if it is appropriate to turn on (or up) the heat to the bath.
3. Push down on the knob (marker #1, left photo) and slide the knob to the left to lower the flask into the water bath. The liquid in your flask should be below the level of the water in the bath, but the neck of the flask should be above the water line and the bump guard should not be rubbing the water bath..
4. Turn on the vacuum pump by flipping the switch on the pump to the ON position.
5. Close the stopcock (marker #1, right photo) to create a vacuum inside the rotovap. Stopcock #2 (right photo) should not need adjustment. Stopcock #2 should be open to the vacuum pump.
6. Within a minute or so you should see vapor condensing on the cold finger and collecting in the collection flask. If you do not, please ask for assistance sooner rather than later, especially if a line is developping for the rotovaps.
7. To remove your sample, (a) release the vacuum by turning the correct stopcock (marker #1, right photo.) (b) Turn off the vacuum pump. (c) Stop the spinning and raise your flask. (d) Once the pressure has equilibrated, THEN remove the Keck clip and take your flask.