Chemical Education Today
Letters Sugar Dehydration without Sulfuric Acid
The author replies:
We find that the procedure for “Sugar Dehydration without Sulfuric Acid: No More Choking Fumes in the Classroom!” (1), which calls for igniting a reaction mixture of 25 g sugar, 12 mL ethanol, and 5 g KClO3 on a watch glass resting upon paper toweling, can lead to watch glass breakage and thereby a fire hazard. The watch glass breakage occurs through high temperature gradients generated by the exothermic reaction of KClO3 with sugar. Once the watch glass breaks, the reaction mixture can ignite the paper toweling. The demonstration, which combines an irreversible sugar dehydration reaction with an irreversible oxidation reaction, can be effectively and safely performed in a 3 in. clay flowerpot partially filled with sand and suspended from a ring-stand with a 3 in. iron ring. Filter paper placed over the drainage hole of the clay flowerpot prevents sand from leaking out. A large collection vessel, such as an iron pail or porcelain dish, set immediately below the flowerpot provides an extra measure of safety and collects any reaction products that leave the flowerpot. Further, we found that burning ethanol- or isopropanolsoaked sugar in the absence of KClO3 leads to a carbonaceous product similar to that reported in ref 1 as “small black upwellings of carbon” and “that smells for the most part like burnt sugar” and discussed in ref 2. This modification demonstrates sugar dehydration without the use of KClO3. Therefore, disposal of any unreacted and unstable sugar–KClO3 mixture (1) is no longer a problem. With our flowerpot demonstration setup, it was easy and safe to perform and contain the traditional, colorful, and highly exothermic sugar oxidation by KClO3 demonstration (3). Because of the large volume of hot gases released as well as the observation that KClO3 oxidation of sugar in the presence of sulfuric acid may yield toxic and reactive gases Cl2, ClO, and ClO2 (1, 3), we recommend performing this demonstration in a fume hood. To well mixed stoichiometric amounts of sugar (~1 g) and KClO3 (~3 g), we instantaneously (~1 s) initiated the reaction with a drop (~0.05 mL) of concentrated sulfuric acid. An alternative procedure, using a match to ignite an ethanol saturated reaction mixture, gives a one- to two-minute delay before initiating the highly exothermic sugar oxidation reaction.
I wish to thank Duhr et al. for raising two main issues with respect to our KClO3/EtOH dehydration of sugar demonstration (1): watch glass breakage leading to ignition of paper towels, and the option of omitting KClO3 from the mixture. Having performed this demonstration well over 50 times, I must report that I did once encounter a cracked watch glass, although that did not result in ignition of the paper towels underneath. Apparently, low quality glass and/or insufficient mixing of the solid KClO3/sucrose leading to hot spots at the bottom of the pile can on rare occasions crack the watch glass holding the mixture. I admire the ingenuity that Duhr et al. have shown in devising their 3 in. clay pot setup. Although clay lacks the transparency of glass, they overcome this problem by partially filling the pot with a bed of sand. This makes the KClO3/sucrose pile more visible to the class as it protrudes over the top of the pot, and it also serves to absorb heat from the combustion. For those who prefer to use equipment more commonly found in a chemistry stockroom when setting up their demonstrations, three other solutions to the cracked watch glass problem come to mind: 1. Use a Pyrex petri dish placed on top of a watch glass to hold the KClO3/sucrose pile. 2. Use two watch glasses: If the top one does crack, the bottom (cooler) one will still hold. 3. Use a nonflammable tabletop covering in place of paper towels, e.g., a metal tray or an asbestos pad.
Regarding Duhr et al.’s suggestion of performing the demonstration without KClO3, we have tested this many times and found it to be unsatisfactory. Because the combustion occurs at a substantially lower temperature in the absence of KClO3, sucrose dehydration is much less complete. The result, a rather unimpressive black mound approximately the same size and shape as the original solid mixture, does not grow into the dramatic black column afforded by the presence of KClO3. Literature Cited 1. Silverstein, T. P.; Zhang, Y. J. Chem. Educ. 1998, 75, 748.
Literature Cited 1. Silverstein, T. P.; Zhang, Y. J. Chem. Educ. 1998, 75, 748. 2. Dahn, J. R.; Zheng, T.; Liu, Y.; Xue, J. S. Science 1995, 270, 590–593. 3. Shakhashiri, B. Z. Chemical Demonstrations, A Handbook for Teachers of Chemistry; University of Wisconsin Press; Madison, WI, 1983; Vol. 1, pp 79–80.
Todd P. Silverstein Chemistry Department Willamette University Salem, OR 97301
[email protected] Edward F. Duhr, Allison S. Soult, John G. Maijub, Fitzgerald B. Bramwell* Department of Chemistry University of Kentucky Lexington, KY 40506-0055 *
[email protected] www.JCE.DivCHED.org
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Vol. 83 No. 5 May 2006
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Journal of Chemical Education
701
