In the Classroom
Tested Demonstrations
Improvement of Sugar–Chlorate Rocket Demonstration† submitted by:
Robert Eliason,* Eric J. Lee, Darren Wakefield, and Adam Bergren Chemistry Department, Southwest State University, Marshall, MN 56258; *
[email protected] checked by:
Calvin E. Uzelmeier III Abrams Planetarium, Michigan State University, East Lansing, MI 48824 Wayne Wolsey Department of Chemistry, Macalester College, St. Paul, MN 55105-1899 Paul E. Smith Department of Chemistry, Purdue University, West Lafayette, IN 47907-1393
Background The combustion of sucrose using potassium chlorate as the oxidizing agent forms the basis of a well-known demonstration that has appeared in the literature over the years (1–5). One variation of the demonstration that we came across is called “Rocket Fuel” (6 ). The procedure for this demonstration called for the reaction to be carried out in a Carius tube to produce a rocketlike flame shooting up from the tube. It was suggested that iron filings or strontium carbonate can be added to give sparkle or color to the flame. During trials to test the suitability of this demonstration for inclusion in our repertoire, we found several procedural features that we judged to be unsatisfactory and potentially dangerous. Producing flame colors by the vaporization of metal salts also raises the question of potential hazards to the audience. We have conducted experiments to evaluate this hazard. The “Rocket Fuel” demonstration uses the common sulfuric acid method (1–6 ) for igniting the reaction mixture. In this method a few drops of concentrated sulfuric acid are added to the mixture. We found that this resulted in an immediate, violent reaction, causing material (including sulfuric acid) to spatter out of the reaction vessel. Moreover, ignition of the combustion reaction often failed, requiring more sulfuric acid to be added and resulting in more splattering. If too much sulfuric acid was used, a hard cover or plug occasionally formed on the surface before the reaction really started. We judged that if this plug were to become firmly seated in the tube while reaction commenced underneath, this could lead to an explosive ejection of material from the tube. We solved both of these problems by using a safety match and a charcoal grill butane lighter to ignite the mixture. This method never resulted in an ignition failure or in plug formation. Since we were unable to find a commercial source for Carius tubes, we tested various Pyrex test tubes as reaction vessels. The reaction produced such intense heat that ordinary tubes either shattered or cracked each time. Even the thickestwalled Pyrex tubes we could find did not survive the reaction. †
Poster presentations at the 205th National Meeting of the American Chemical Society, Anaheim, CA, March 1995 (CHED 377) and at the 217th National Meeting of the American Chemical Society, Anaheim, CA, March 1999 (CHED 594).
We found that an ordinary piece of galvanized plumbing pipe with an end cap worked very well as a reaction vessel. This vessel was shatter-proof and could be easily cleaned. However, we were worried that such a vessel, if shown to a public audience, might invoke an association with pipe bombs. To avoid any such suggestion the vessel was buried in sand so that the pipe rim was barely visible. An added advantage was that the sand caught any ejected ash material. The ratio of sucrose to potassium chlorate to be used in the reaction mixture was vaguely specified in the Rocket Fuel demonstration as “1:1” (6 ). Most of the published procedures specify a ratio of 1:1 by volume (1, 2, 4–6 ). Gilbert et al. (3) suggest a ratio of 1:3 by weight, which corresponds roughly to the molar weight ratio (1:2.86) calculated by assuming a stoichiometric conversion of the reactants to the products carbon dioxide, water, and potassium chloride. We observed that using the molar weight ratio produced the least amount of ash and ejected material. The use of metal salts to produce colored flames has been widely reported (7–15). Only one paper (12) reported the use of the sugar–potassium chlorate reaction for colored flames, and these authors used fireworks recipes for various colors. Since these recipes were somewhat complex and didn’t include one for green, we evaluated various salt mixtures. Intense red flames were easily produced from strontium nitrates or chlorides and intense yellow flames from sodium nitrate or chloride. Green, blue, and purple flames were more difficult to produce. Pyrotechnical theory (16 ) indicates that the best color intensity comes from the vaporized form of the metal monochloride. Therefore pyrotechnic mixtures often contained organochloro compounds as a source of chlorine atoms to form metal monochlorides. Polyvinyl chloride (PVC) was one of the compounds typically used, and we found that, indeed, the best results were obtained by adding PVC powder. A green flame was produced from a barium chloride–boric acid– PVC mixture, a blue flame from a copper(II) chloride–barium nitrate–PVC mixture, and a purple flame from a copper(II) chloride–strontium nitrate–PVC mixture. We investigated how much of the colorizing salt was vaporized by using barium salts because quantitative analysis for barium ion is straightforward. Our analysis indicated that about 23% of the barium salt was vaporized. (Experimental details and results are presented in Appendix 1.) If we assume
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In the Classroom
that all salts are vaporized to the same extent as barium salts, an audience could be exposed to a significant amount of the metal salt. The threshold limit value (tlv) sets the maximum amount of chemical to which individuals can be exposed (17 ). Barium salts have the lowest tlv of the salts used in this demonstration. To keep the amount of metal ion vapor below the tlv using the amount of colorizing salt (0.8 g) given in the Experimental Procedures, the size of a room with a ceiling height of 9 ft would have to be at least 40 × 40 ft (or >1600 ft2) (see Appendix 2 for calculations.) A room this size is very large, and prudent safety practice would require an even larger room. Our recommendation is, therefore, that this demonstration always be done in a hood when using colorizing salts. An interesting variation may be created by adding layers of reaction mixture containing different colorizing salts. We found that the maximum number of layers that would produce distinct colors for our reaction vessel was three. In addition, because of the intensity of red and yellow, it works best if these colors are the last ones. A five-color demonstration ranging from purple to red can be constructed if the length of the reaction vessel is increased to 1.5 in. In this case it is imperative to ensure that the hood capacity is large enough to accommodate the volume of product gases. Experimental Procedures Materials Sucrose was purchased in a grocery store. Potassium chlorate and the colorizing salts were obtained from commercial chemical supply houses. Powdered polyvinyl chloride (PVC) was purchased from Aldrich Chemical Company (Cat. 18,958-8; powder; inherent viscosity 0.68; density 1.400 g/mL).
Reaction Vessel The reaction vessel was made from a threaded galvanized plumbing pipe 1 in. long and 0.5 in. in diameter and a threaded galvanized end cap with an inside diameter of 0.5 in. The pipe was screwed very tightly into the end cap. The pipe and end cap can be purchased at any hardware store selling plumbing supplies. The pipe assembly was filled with 5 g of standard Ottawa sand to give the final configuration of the reaction vessel. The sand was added to reduce the amount of reaction mixture needed to fill the vessel.
Reaction Mixture Reaction mixtures were prepared in a constant ratio of 2.86 g of KClO3 to 1 g of sucrose. The reaction vessel holds 5 g of the reaction mixture, and this amount was prepared by adding 3.70 g of KClO3 and 1.30 g of fine-grained sucrose (table sugar) to a 100-mL beaker. Never grind the sugar and potassium chlorate together! Never grind the potassium chlorate at all! The compounds were mixed well together using a plastic spoon. (Do not store the mixture, as KClO3 is a strong oxidizing agent and spontaneous ignition may occur.) Colorizing Salt Mixtures Red-producing colorizing salt consisted of only strontium nitrate and yellow-producing salt of only sodium nitrate. The appropriate mixtures for green-, blue-, and purple-producing salts are as follows:
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Green: 5 g barium chloride, 5 g boric acid, 1 g PVC Blue: 7 g copper(II) chloride, 7 g barium nitrate, 1.5 g PVC Purple: 4.5 g strontium nitrate, 7 g copper(II) chloride, 1 g PVC
Reaction Mixture with Colorizing Salts The amount of colorizing salts in the reaction mixture was kept at a constant ratio of 0.2 g salt to 1.0 g reaction mixture. The amounts needed to fill the reaction vessel were, therefore, 4 g reaction mixture (2.97 g KClO3 and 1.04 g sucrose) and 0.8 g colorizing salts. These were placed in a 100-mL beaker and thoroughly mixed together with a plastic spoon. Demonstration Procedure This demonstration must be done in a hood. The mixture was poured into the reaction vessel, which was gently tapped on the bench to bring the reaction mixture level with or below the top of the pipe. The reaction vessel was then buried in sand up to the rim of the pipe. (The sand was in a container 16 in. in diameter and 4 in. high.) The top fourth of a safety match was broken off and pushed into the reaction mixture until the match head just emerged above the surface. The reaction was started by lighting the safety match with a butane lighter of the type used for starting a charcoal or propane grill. Disposal For reactions not containing any metal salts the small amount of ash should be collected and flushed down the drain with copious amounts of water. For reactions containing colorizing salts the ash should be disposed of according to standard practices (19). Literature Cited 1. Tested Demonstrations in Chemistry, 6th ed.; Alyea, H. N.; Dutton, F. B., Eds.; Journal of Chemical Education: Easton, PA, 1965; p 20. 2. Ford, L. A. Chemical Magic; Fawcett World Library: New York, 1959; p 110. 3. Gilbert, G. L.; Williams, L. G.; Shakhashiri, B. Z.; Dirreen, G. E.; Juergens, F. H. In Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 1; Shakhashiri, B. Z., Ed.; University of Wisconsin Press: Madison, 1983; p 79. 4. Summerlin, L. R.; Ealy, J. L. Jr. Chemical Demonstrations: A Sourcebook for Teachers; American Chemical Society: Washington, DC, 1985; p 161. 5. Tested Demonstrations in Chemistry; Gilbert, G. L.; Alyea, H. N.; Dutton, F. B.; Dreisbach, D., Eds.; Journal of Chemical Education and Division of Chemical Education, Inc., American Chemical Society: Granville, OH, 1994; p G-1. 6. Sharpe, S. The Alchemist’s Cookbook. 80 Demonstrations; 2nd ed.; Instructional Development Centre, McMaster University: Hamilton, ON, 1977; p 23. 7. Summerlin, L. R.; Ealy, J. L. Jr. Chemical Demonstrations: A Sourcebook for Teachers; American Chemical Society: Washington, DC, 1985; p 149. 8. Solomon, S.; Hur, C.; Lee, A.; Smith, K. J. Chem. Educ. 1995, 72, 1133. 9. Thomas, N. C.; Brown, R. J. Chem. Educ. 1992, 69, 326.
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In the Classroom 10. Ragsdale, R. O.; Driscoll, J. A. J. Chem. Educ. 1992, 69, 828. 11. Gouge, E. M. J. Chem. Educ. 1988, 65, 544. 12. Ager, D. J.; East, M. B.; Miller, R. A. J. Chem. Educ. 1988, 65, 545. 13. Peyser, J. R.; Luoma, J. R. J. Chem. Educ. 1988, 65, 452. 14. Pearson, R. S. J. Chem. Educ. 1985, 62, 622. 15. McKevly, G. M. J. Chem. Educ. 1998, 75, 55. 16. Conkling, J. A. Chemistry of Pyrotechnics; Dekker: New York, 1985. 17. The American Conference of Governmental Industrial Hygienists defines tlv as the concentration of a chemical in air to which nearly all individuals can be exposed without adverse effects. Prudent Practices in the Laboratory: Handling and Disposal of Chemicals; National Academy Press: Washington, DC, 1995; p 203. 18. Kolthoff, I. M.; Sandell, E. B. Textbook of Quantitative Inorganic Analysis; Macmillan: New York, 1952; p 322. 19. Prudent Practices in the Laboratory: Handling and Disposal of Chemicals; National Academy Press: Washington, DC, 1995; pp 166–170.
Appendix 1 Standard mixtures of sugar, potassium chlorate, and barium nitrate or chloride were prepared. We checked our analysis procedure by preparing reaction mixtures containing a known quantity of barium salt and conducting the analysis on the unreacted mixtures. This was done by dissolving the mixture in water and precipitating the barium ion as barium sulfate with sulfuric acid (18). The average
recovery of triplicate samples was 100.3 ± 2.2% barium ion. Next we conducted experiments to estimate the amount of barium vaporized during the combustion reaction. Reaction mixtures containing known amounts of barium chloride were prepared and ignited. All the ash and ejected material were collected and the unvaporized barium ion was extracted into aqueous solution. Again, the amount of barium in this aqueous solution was determined by precipitation as the sulfate. The average recovery from triplicate samples was 77.6 ± 0.1% barium ion.
Appendix 2 In the calculation of a minimum room size we assumed that all metal ions are vaporized to the same extent as we determined for barium chloride and that the smallest tlv (17 ) found for any of the salts must be used. Barium salts have the smallest tlv (0.5 mg/m3 for either barium chloride or barium nitrate from MSDS data) of any of the salts in our mixture. Therefore the total amount of vaporized metal ion in a room must be kept below 0.5 mg/m3 (1.4 × 10᎑3 mg/ft3). Since 23% of the salt in a sample is vaporized and since the amount of vaporized salt cannot exceed the tlv limit of 0.5 mg/m3, the total amount of salt that can be used in a sample is limited to the tlv divided by 0.23, which gives 2.2 mg of salt per m3 of room volume (or 6.1 × 10᎑2 mg/ft 3). If a sample contains 0.8 g of salt as described in the Experimental Procedures section, the minimum size of room for the demonstration is calculated by 0.8 g/6.1 × 10᎑2 g/ft3 = 1.31 × 104 ft3 (364 m3). If the room has a ceiling height of about 9 ft (3 m), the room must have an area of 1460 ft2 (121 m2). This corresponds roughly to a 40 × 40-ft room (11 × 11 m).
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