Enhancement ol Visual Titration Endpoints The ordinary white expanded polystyrene foam coffee cup has proven to he an excellent titration vessel for a great variety of reactions and offers many advantages. The cups are very inexpensive and may be rinsed and re-used for several titrations prior to being discarded. The endpoint is visualized far better against the surrounding white background of the polystyrene cup than can be seen in ordinary glassware, such as Erlenmeyer flasks or beakers, with a white background, This is easily demonstrated by titrating to an endpoint in a beaker and then pouring the resulting Liquid into the white polystyrene cup and noting the difference in the perceived eolor intensity. A slight amount of red dye or ink (from a felt-tip pen) will illustrate the same color perception differential between the glassware endpoint and the polystyrene cup endpoint intensity. An easily perceived light color in the polystyrene container appears to he colorless in a glass vessel against a white background. Conversely a faint color solution in a glass looks much deeper in color when poured into the polystyrene cup. Even color photographic slides have shown the phenomenon to he real. In an attempt to ascertain factors that might explain this apparent difference, the polystyrene cups were examined under ultraviolet light, and optical brighteners were not observed to be present. The precision ahtained by a skilled analyst doing titrations in both glassware and polystyrene cups is comparable, hut the titrant volume delivered a t the endpoint can differ by as much as two drops between glass and styrofoam vessels using 0.10 M reagents and a 50.00-mL class"A" buret. Of course, one should not standardize using glassware and switch to the white ~olvstvrenecups, or about 1-2 parts per thousand error will be introduced in the analytical ;es;lG. These cups have been used for the past several years with success in both the teaching laboratory and routine titration work for industry. We used polystyrene cups extensively in a teacher enhancement project aver a three-year period. The teachers showed a preference for using the cups in performing titrations, and many have adopted use of the cups in their laboratories for enhanced endpoint detection, low cost, and safety advantage. Several different types of titrations have been checked and are well suited for the titration of dilute aqueous acid and base determinations, silver chloride precipitation titrations (Fajan's, Mohr, and Volhard), and potassium permanganate titration of sodium or calcium oxalate in acid solution. The teacher enhancement project referred to above was supported in part by the National Science Foundation (Grant No. TEI-8470135). The authors are indehted to the m a w teachers from Alabama, Arkansas, Louisiana, and Mississippi who participatedin the program. Howard P. Wllllams and J. Emory Howell University of Southern Mississippi Haniesburg. MS 39401
Joseph L. Russell Alcorn State University Lorman, MS 39976
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Journal of Chemical Education
