Separatory Funnel

Separatory Funnel

Preparing to use the separatory funnel

A separatory funnel (sep funnel) is used to separate immiscible liquids. When two immiscible liquids are placed in a separatory funnel, two layers are seen. The denser solvent will be the bottom layer. Most halogenated solvents are denser than water, most non-halogenated solvents are less dense than water. If you are not sure which layer is which, add a drop of water and see which layer it joins.

Before performing an extraction the sep funnel should be thoroughly checked. Ask yourself:
Does the stopper fit?
Is the stopcock assembled correctly? Does it leak?
Are there any cracks or sharp edges?
Is the funnel large enough? (it should be twice the total volume of the liquids used)

Performing an Extraction

The following is an example in which iodine (I2) in being extracted from an aqueous solution into dichloromethane (methylene chloride). Iodine, while slightly soluble in aqueous solutions, is more soluble in dichloromethane. Therefore, when aqueous solution containing iodine is placed in contact with dichloromethane, the iodine migrates into (is extracted by) the dichloromethane layer. Iodine in an aqueous solution appears brown. Dichloromethane is a colorless liquid. Since dichloromethane is a volatile solvent, this extraction should be done in a hood.

1. The aqueous iodine solution (the solution to be extracted) is placed in the sep funnel (held in ringstand) and a small amount of dichloromethane poured into a beaker.

2. A portion of the dichloromethane is added to the sep funnel and the funnel shaken gently. To shake the funnel, invert it while holding the stopcock in with your hand. When volatile solvents are being used the sep funnel needs to be vented occasionally. Do this by pointing the inverted funnel away from yourself and others and opening the stopcock.

3. Return the sep funnel to the ringstand and allow the layers to separate. Because most of the iodine has been extracted into the dichloromethane, the aqueous layer becomes lighter brown. The iodine in the dichloromethane layer makes it appear purplish.

4. Remove the stopper and drain the lower layer (the dichloromethane layer in this case) through the stopcock into a beaker or flask. The upper layer should remain in the sep funnel.

5. To extract additional iodine, steps 2 - 4 can be repeated by adding a second portion of dichloromethane to the sep funnel containing the aqueous layer. Additional extractions can be performed until most of the iodine is removed from the aqueous layer. For a majority of situations this can be accomplished if three extractions are performed.

6. Combine the dichloromethane portions. The dichloromethane now contains almost all the iodine, while only a small amount of I2 remains in the aqueous layer.

Notes and Hints

- In the above example the layer being extracted was the top layer. As such, it did not have to be removed from the sep funnel during each extraction. If it were the bottom layer a slightly different procedure would be used. The bottom layer is drained from the funnel into a clean beaker. The top is drained into a different beaker. The bottom layer is returned to funnel and the extraction process continued.

- NEVER throw away either layer until the extraction is complete and you are absolutely sure you have the correct layer.

- Extracting with several small portions is more efficient than with one large one.