Problems with extractions

There are several common sources of problems with extractions, which are outlined in this section.

1. Which layer is which? A convenient way to tell is to examine the separatory funnel to see the relative sizes of the layers, and then add a small volume of water – the aqueous layer will increase in size.

2. Only one phase is formed. Adding two immiscible solvents produces only one phase. This may be because the reaction mixture contains a water-miscible solvent such as ethanol, which partially dissolves the water and the organic solvent. Addition of more solvent, or brine, may help the layers form; this problem can likely be prevented by pre-concentrating the reaction mixture (by removing the solvent), before the work-up is attempted.

3. The interface is invisible. This may be because the phases are very darkly coloured, or they may be clear but with very similar refractive indices. It may be possible to observe the change in flow properties, from one layer to the next, as the lower layer is drained.

4. Insoluble material forms in the separatory funnel. It may be difficult to separate the layers cleanly, without taking some of the insoluble material. However, this material can be filtered out at a later stage, if necessary.

5. An emulsion forms. Instead of forming two clearly separate phases, the interface contains a mixture of droplets of one solvent dissolved in the other. This can be very difficult to deal with. Emulsions are more likely to form with chlorinated organic solvents, and strongly basic aqueous solutions, so their formation can sometimes be prevented by the choice of different reagents. If an emulsion forms, it must be broken (i.e. the phases properly separated) before the extraction can be finished. Some suggestions are given:

• allow the separatory funnel to stand for some time

• add some saturated brine solution

• stir the emulsion slowly with a glass rod

• filter the contents of the funnel by suction, then re-attempt the extraction

• add a few drops of ethanol

If the emulsion cannot be broken, the lower layer can be drained off, including or excluding the emulsion as required, with consequences for the recovery and purity of the material.