The Copper(l1 Iodide Law of Definite Proportions Revisited Several years ago, t h e following experiment was described in THIS JOURNAL as was shown to have many advantages over other approaches used in introductory chemistry.' Briefly, copper metal is reacted with iodine vapor to form CuI and its composition subsequently determined. We have been using this system in our freshman laboratories for the past eleven years, performing approximately 400-500 experiments per calendar year. (Adequately vented hoods appear to he satisisdary fur the elimination of any hazard posed by the iodine vapors.) We have, however, refined the original procedure by utilizing a one-meter length of copper wire (No. 24 gauge) coiled around a pencil instead of using a wire screen. T h e use of coiled wire, though less expensive than wire screen, presented problems with cryslallization of solid iodine on the wire. Failure to remove all of i t resulted in a higher mass ratio of combining elements, whereas removal by heating above a flame or with a heat gun gave lower values. Vibration of the coil and loss of CuI occurred with the heat gun. Both techniques led t a lower results of the relative deviation of the mean from t h e true value. To alleviate this problem, the reaction vessel (a test tube) containing solid iodine, was immersed in a mineral oil bath situated on a hotplate. T h e oil temperature was approximately 120'C. Exposure of t h e wire to the iodine vapor far 2 min was sufficient. Uniform heating prevented excessive crystallization of iodine and minimum post~heating.T h e absence of iodine crystals was our criteria for a sufficiently clean coil. During the last year, results have been obtained with only 1-2 percent relative deviation from the theoretical value as compared to 4-5 percent observed earlier. Furthermore, a n absolute standard deviation o f f 0.1 was determined from extensive student data.
' Wilhelm, D. L., J. CHEM.EDUC.,50,436 (1973)
6. D. Catsikis J. W. Goerner J. D. Goodrich Louisiana State University in Shreveport Shrevepon. LA 71115
148
Journal of Chemical Education
