Chemical Education Today
Letters A Simple and Inexpensive Salt Bridge for Demonstrations Involving a Galvanic Cell One obstacle to demonstrating the principles of a galvanic cell in a classroom is the trouble of preparing and maintaining an agar U-tube salt bridge. Even if one goes through the hassle of preparing a proper salt bridge one year, it may be dehydrated or moldy the by the next year. I have found that a standard laboratory sponge (2 × 8 × 14 cm), cut into the shape of a U, and saturated with a solution containing an appropriate electrolyte is a satisfactory salt bridge for classroom demonstrations (Figure 1). Admittedly, the sponge seems a bit crude, but it works reliably and the use of everyday materials may take some of the mystery out a demonstration that students may find a bit difficult to grasp. While other novel salt bridges such as the “worlds largest human salt bridge” (1), dialysis tubing (2), or the wet leather used to separate the silver and zinc discs in Davy’s voltaic pile (3), have been used, many demonstrations and lab resources rely on agar U-tubes. The purpose of a salt bridge is to allow ions to flow and a saturated sponge is a quick, economical, and reliable way to allow ions to transfer between the two half cells in a galvanic cell. The sponge can be dried out and saved for the next year when it can simply be re-wet and used again.
Figure 1. Diagram of galvanic cell made using a standard laboratory sponge.
Literature Cited 1. Silverman, L.; Bunn, Barbara B. J. Chem. Educ. 1992, 69, 309–310. 2. Summerlin, L. R.; Ealy, J. L. Chemical Demonstrations, a Resource for Teachers; American Chemical Society: Washington, DC, 1985; pp 117–118. 3. Marshall, James, M. Discovery of the Elements; Simon and Schuster: Needham Heights, MA, 1998; pp 41–45.
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Charles A. Liberko Department of Chemistry Cornell College Mount Vernon, IA 52314
[email protected] Vol. 84 No. 4 April 2007
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Journal of Chemical Education
597
