A Plastic Dam Several years ago undergraduate chemistry students were occasionally introduced to the rubber dam' in the organic chemistry laboratory. The rubber dam was shown to he useful for pressing the last traces of solvent from a solid residue in a Biichner funnel during vacuum filtration. However, sheet ruhher was, and continues to be, very expensive while the technique was trauhlesome since the rubber sheet placed over the Biichner funnel had to be secured with rubber bands around the outside rim of the funnel. Today inexpensive plastic food wrap is available and can he conveniently and routinely used by classes for pressing solvent out of the filter cake during vacuum filtration. A piece of plastic wrap is torn slightly larger than the diameter of the Biichner funnel. By placing the wrap over the top of the funnel a seal is quickly established since the film clings. The vacuum draws the wrap down over the filter cake and additional pressing with a cork or stopper squeezes it nearly dry. Provided the distance from the rim of the funnel to the filter plate is less than half of the diameter of the funnel, the plastic wrap will not burst as it is stretched downward by the vacuum. Other advantages of plastic wrap over the rubber dam is its transparency, relative inertness to most solvents, and, of course, its disposability.
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Cummins, A. B., "Technique of Organic Chemistry 111," (Editor: Weissherger, A,), Interscience Publishers, Inc., New York, 1950, p. 579. Washington University School of Medicine St. Louis, Missouri 63110
Frederick Sweet
