In the Laboratory
Delivery of Uniform and Countable Drops David Koster Department of Chemistry and Biochemistry, Southern llinois University, Carbondale, IL 62901
Many experiments require the delivery of uniform and countable drops. This is particularly true in microscale experiments using Beral pipets. It is often difficult to keep from getting partial drops containing air bubbles or from getting more than one drop at a time. The difficulty is compounded when one is delivering liquids with low densities. An excellent article by Early and Pickering (1) has examined the reproducibility of counting drops and concluded that if uniform drops are needed, the best way is to use a buret. A simple method that overcomes many of the problems when using a Beral pipet is to place the bulb of the pipet in a clamp (a 3-finger microclamp works well, but most any clamp will do). By slowly tightening the screws of the clamp
one can deliver uniform and easily countable drops. The clamp maintains a constant pressure on the bulb so that air is not sucked into it. It also holds the pipet at a constant angle, ensuring drops of uniform size. If the volume per drop is needed, the usual procedure is to count the number of drops in a milliliter. Alternatively, one could weigh 10 drops or so on a milligram balance and from the density of the liquid, determine the drop volume. We have found that both of these procedures give very reproducible results. Literature Cited 1. Early, J.; Pickering, M. J. Chem. Educ. 1991, 68, A120.
JChemEd.chem.wisc.edu • Vol. 75 No. 5 May 1998 • Journal of Chemical Education
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