Use of a Separatory Funnel

Use of a Separatory Funnel - Experimental Chem I

Ref: Mohrig, pp. 149-155.
Proper use of a separatory funnel requires timing and dexterity. First, you should recognize that the purpose is to create the maximum possible intermingling of the two layers, then to allow them to separate (thus the name). A complication is the creation of pressure buildup that can come from either the inherent vapor pressure of the organic solvent (ether and dichloromethane are particularly nasty in this regard), or from gas evolution in the case of sodium bicarbonate washes. You need to be able to identify which layer is which (so you know which to keep!). Finally, you want to avoid contaminating the desired layer with stopcock grease or with any of the other layer.

Parts of a separatory funnel:

Stopper

Body

Stopcock

Drain Tip

1. Add the two liquids

Normally, you will be starting with an organic reaction mixture in an organic solvent. Close the stopcock. Add the organic liquid through the Stopper opening at the top. Then add the aqueous solution; if you anticipate a chemical reaction (e.g., formation of CO₂ from neutralization of an acid with bicarbonate) be sure to add the aqueous solution slowly.

2. Initial mixing

If gas evolution occurs, slowly swirl the contents of the separatory funnel until most of the foaming ceases. At this point, you must perform the most difficult maneuver: Place the stopper in the opening at the top, and pick up the separatory funnel. Hold it with one hand around the bottom of the flask with your thumb and forefinger on the stopcock handle, and with the other hand grasping the pennyhead of the stopper (if it has one):

In one motion, invert the separatory funnel and open the stopcock as it rises above the surface of the liquid. You shouldhear a "whoosh" of gas pressure escaping.

There are several challenges in this process:

If the stopper is not on tightly enough, the weight of the liquid will cause it to leak out the stopper opening. However, if you force the stopper in too tightly, it will become frozen.
If you open the stopcock too early, the liquid will leak from that opening.
If the entire process is not done rapidly, pressure buildup in the flask will force the stopper out as you invert the funnel.

3. The Extraction Process

At this point, you have not mixed the two layers sufficiently to get a good extraction. It is necessary to close the stopcock and shake the mixture vigorously. This will create more pressure buildup, so stop and vent periodically. Normally, three shake-and-vent cycles are sufficient for each extraction step.

4. Separation of the layers

Now you are ready to separate the layers. Place the separatory funnel back in the ring stand. The two layers should separate on their own; if not, or if they only partially separate, see Emulsions below. Take the stopper off, and set it on a clean paper towel. Once the last of the emulsion clears, drain the lower layer into an appropriate container (Organic liquids are kept in Erlenmeyer flasks; aqueous liquids can be stored in beakers).

How do you know which layer is which?

You should have some clues recorded in your notebook:

What is the density of your organic solvent?
What is the density of water?
What did you observe when you mixed the two liquids?

If you still aren't sure, separate the layers, and add a few drops of water to each. Water will be miscible with the aqueous layer, and immiscible with the organic layer.

Be careful: chlorinated organic solvents are usually denser than water, and other organic solvents are lighter than water. However, the density of the water solution is also affected by the dissolution of salts or of organic compounds.

Drain the lower layer into an appropriate container until the meniscus just reaches the stopcock plug. Then, drain the top layer out through the stopper opening into an appropriate container. This will prevent contaminating it with excess amounts of the lower layer solution.

Emulsions

Sometimes, the two layers do not easily separate due to the formation of an emulsion. (You have seen these before; milk is a suspension of fats and proteins in water.) Often, simply leaving the mixture alone for 5-10 minutes will allow it to separate. If this fails, try the following:

Use a glass rod to agitate the side of the funnel. Often you just need to get a nucleus of one separate layer to start the process. Or, there may be a film of emulsion preventing larger globules of solution from coalescing.
Add small amounts of saturated sodium chloride. The change in ionic strength tends to draw water into the salt solution. (This is also the principle behind using saturated NaCl as a final wash--to draw as much water as possible out of the organic layer.)
Add small amounts of the organic solvent to dilute the organic product and decrease surface tension.

In difficult cases, a combination of techniques must be used.

by K. Gable