To better understand how a stopcock works, click here.

This photo shows an example of good separation between the two layers - the typical case.

•The ground glass joint of the separatory funnel is one of the few cases where grease is not used (it contaminates the organic layer).

•The student has removed the stopper prior to draining. Not removing the stopper is the most common mistake (not a serious one since nothing happens). [The second most frequent mistake is pouring the sample into the funnel with the stopcock open (then something surely happens).]

•If ever in doubt about which layer is which, do a simple test: use a pipet to add a few drops of one layer to 1 or 2 mL of water in a test tube. If it doesn't mix, that is the organic layer.

•Label the flasks into which the layers are drained to avoid confusion.



©2001,2002 Daniel A. Straus

  This photo shows a case in which an emulsion has formed, making it difficult to separate the layers. [The droplets of the two phases are slow to coalesce; it can take a long time for separation to occur.]

One method that is often used to break up an emulsion is to add some salt (use NaCl) to increase the ionic strength of the aqueous layer. Add some* solid NaCl, shake to dissolve, and let stand to separate.

*Avoid using too much - look up the solubility of sodium chloride in water, estimate your volume, and use well less than the maximum amount.

In the research laboratory, an emulsion can be one of the most annoying complications one encounters. Like a stripped bolt when doing an engine tune-up, a stubborn emulsion can turn a simple workup into a lengthy operation. No one technique works in all cases, but addition of a few milliliters of alcohol sometimes helps and gravity filtration through glass wool can be helpful.

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