Efficient Microscale Filtration We would like t o share with the chemical community a very simple technique for the rapid filtration of small liquid samples (4.5-2 mL). We are unaware of its demiption elsewhere in the literature. Perusal of several contemporary hooks written for a variety of audiences (e.g., Williamson, K. L. Microscale Organic Experiments; Heath: Lexington, MA, 1987. Ma, T. S.; Horak, V. Microscale Manipulations in Chemistry; Wiley: New York, 1976) reveals considerable attention devoted to filtration methods. These methods often become rather elaborate and involve specialized apparatus. A much simpler procedure makes use of a Pasteur pipet, a rubber bulh for the pipet, a small square (-24 mm an a side) cut from a 25 X 80-mm Sorhlet extraction thimble (Schleicher & Schull; Reference Numher 350217), and a 2-dr glass vial. The solution to he filtered is introduced into the vial followed by the thimble filter paper. The pipet with its bulb attached is then inserted into the solution so that the cleanly squared-off end of the pipet rests lightly against the filter paper. The rubber bulh is now squeezed (the flow of air bubbles serves to remove any adhering particles from the surface of the filter paper). Now, the glass tip of the pipet is placed snugly against the fdter paper, the rubher bulh is released, and the Pasteur pipet is allowed to fill slowly with liquid that is heing filtered through the paper. When the pipet is completely full, it is carefully withdrawn from the vial and used to transfer the liquid as desired. The key to this technique is the use of the relatively thick paper from the extraction thimble rather than regular filter paper, which is too flimsy for effective filtration when used in this manner. The filtered liquid is completely free of any visible particulate matter. We have found that this procedure works very well for filtering NMR samples, for filtering off small amounts of catalyst or drying agent, and even for filtering off activated charcoal (without the necessity for using a filter aid). There is some difference in the behavior of the extraction thimhle filter paper when using solvents of varying density. Thus, with deuterochloroform the filter paper squares tend to float. This requires that one "spear" the paper with the pipet in order to hold it against the bottom of the vial. In practice, this actually presents no problem. John L. Belletlre and Nosrat 0. Mahrnoodl Universily of Cincinnati Cineinnatl. OH 45221
964
Journal of Chemical Education
