Chemistry for Everyone edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
Simple Flame Test Techniques Using Cotton Swabs submitted by:
Michael J. Sanger* and Amy J. Phelps Department of Chemistry, Middle Tennessee State University, Murfreesboro, TN 37132; *
[email protected] checked by:
Catherine Banks Department of Chemistry, Peace College, Raleigh, NC 27604
Flame test experiments and demonstrations are used by most high school and college general chemistry instructors, and have discussed recently on the ChemEd-L listserve (1). Most traditional flame test techniques involve the use of aqueous solutions and Nichrome wires, but there are several disadvantages of this technique including the fact that it delivers only a small quantity of chemical to the flame (2), the wire must be thoroughly cleaned with hydrochloric acid between subsequent tests (3), and the production of volatilized HCl in the cleaning process requires the use of ventilation hoods (4). Because of these disadvantages, several alternative flame test methods have been proposed (1–17). Many of these involve the use of alcohol burners or Sterno cans (5–6), spray bottles (7–10), or devices that must be specially constructed (5, 8, 10, 11). Some also involve the use of flammable solids and liquids such as nitrocellulose (12, 13) or methanol (5– 9, 14, 15). In this article, we describe three alternative methods for performing flame tests using cotton swabs. These methods have several advantages: cotton swabs can be cheaply purchased at any grocery or convenience store, the other equipment (Bunsen burners, candles, rubbing alcohol) can be easily obtained, these methods work well either as chemical demonstrations or in laboratory experiments, and the cleanup and disposal are simple. As with all flame test experiments, these experimental methods require adequate ventilation and should be performed in a hood. Method 1: Bunsen Burner and Solid Salts
Equipment
into the container. Instead, they should pour out a small sample of the solid onto a spot plate, watchglass, or weighing paper. Students should also be asked what part of the flame color is due to the metal salt and what part is due to the cotton swab. A control test (cotton swab dipped in distilled water only) imparts no color to the flame, so all of the flame color is due to the metal salts. Method 2: Bunsen Burner and Aqueous Solutions
Equipment Bunsen burner Cotton swabs 1 M Solutions of metal salts
Procedure In this flame test method, a cotton swab is dipped directly into the 1 M aqueous solution of the metal salt and is placed in the flame of the Bunsen burner for several seconds. Initially, there is no color in the flame, but after 5–10 seconds, the colors associated with the metal ions appear in the flame. The cotton swab does not ignite while in the flame for 20–30 seconds, but a faint orange color may appear in the flame from the cotton swab. In general, these colors take longer to appear and are less intense than the flame tests using the solid salts. However, this method eliminates the mess occurring in the first method when the solid sample occasionally falls off the swab and onto the benchtop or into the burner. The contamination and disposal methods are the same as in the first method.
Bunsen burner
Method 3: Candle, Rubbing Alcohol, and Solid Salts
Cotton swabs
Equipment
Distilled water
Candle
Solid metal salts
Cotton swabs
Procedure In this flame test method, a cotton swab is dipped into distilled water and then dipped into a sample of the solid metal salt. The cotton swab is then placed into the flame of the Bunsen burner. The colors are usually very bright and intense and are seen immediately. The used cotton swab may be discarded in the trash. If this method is used in a laboratory experiment, students should be cautioned against contaminating the solid sample by placing the wet swab directly www.JCE.DivCHED.org
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Rubbing alcohol Solid metal salts
Procedure Although the first two methods are preferred, some high schools do not have access to Bunsen burners. The third flame test method only requires rubbing alcohol (70% isopropyl alcohol by weight), solid metal salts, and a source of ignition (candle, match, butane lighter, etc.). In this method, the cot-
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A orangish-black flame
flame test color
bluish-purple flame
cotton swabs should not be placed in the flame since they are flammable, and the cotton swabs should be properly extinguished before being discarded in a trash receptacle. Users should not substitute more flammable liquids (such as methanol or ethanol) for rubbing alcohol in Method 3. Summary
cotton swab
Figure 1. Flame tests using rubbing alcohol, Method 3: (A) cotton swab alone and (B) cotton swab and solid metal salt.
The three methods for performing flame tests mentioned in this article are particularly useful for chemical demonstrations or laboratory experiments because they are quick and easy to perform, require inexpensive and easily obtained materials, and the cleanup and disposal of materials is easy. Literature Cited
ton swab is dipped in rubbing alcohol and then into a sample of the solid metal salt. The swab is placed in the candle flame, ignited, turned vertically, and allowed to burn until it extinguishes. It is possible that the heated metal salts may spatter off the swab, so one might want to advise students to hold the swab using metal tongs. We tried this method using ethanol, but the cotton swab ignited and quickly burned out of control until extinguished in the sink. In addition, we did not see any flame color indicative of the metal salt used. The other two methods are preferred because when the cotton swab is dipped in water (or an aqueous solution), it does not produce a color inside the flame. When the cotton swab is dipped in rubbing alcohol and ignited, the flame burns with a bluish-purple base and an orangish-black top (Figure 1A). When a metal salt is included, the flame shows an additional color associated with the metal ions on the outer part of the flame (Figure 1B). This method can be used to introduce students to the importance of performing background (or control) experiments. The contamination and disposal issues are the same as with the other two methods. Hazards Although this demonstration or laboratory experiment poses no significant safety hazards, users should be cautioned to be careful around ignited Bunsen burners or candles. Dry
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1. Archives of ChemEd-L Listserve. http://mailer.uwf.edu/archives/ chemed-l.html (accessed Apr 2004). 2. Barnes, Z. K. J. Chem. Educ. 1991, 68, 246. 3. Bare, W. D.; Bradley, T.; Pulliam, E. J. Chem. Educ. 1998, 75, 459. 4. Boueher, J. H. J. Chem. Educ. 1986, 63, 158. 5. Dragojlovic, V. J. Chem. Educ. 1999, 76, 929–930. 6. McKelvy, G. M. J. Chem. Educ. 1998, 75, 55–56. 7. Johnson, K. A.; Schreiner, R. J. Chem. Educ. 2001, 78, 640– 641. 8. Dalby, D. K. J. Chem. Educ. 1996, 73, 80–81. 9. Mattson, B. M.; Snipp, R. L.; Michels, G. D. J. Chem. Educ. 1990, 67, 791. 10. Gouge, E. M. J. Chem. Educ. 1998, 65, 544–545. 11. Marsden, S. R. ChemEd-L Listserve. http://mailer.uwf.edu/Lists/ wa.exe?A2=ind0304&L=chemed-l&D=1&F=&S=&P=5115 (accessed Apr 2004). 12. Thomas, N. C.; Brown, R. J. Chem. Educ. 1992, 69, 326– 327. 13. Peyser, J. R.; Luoma, J. R. J. Chem. Educ. 1988, 65, 452– 453. 14. Smith, E. T. J. Chem. Educ. 1995, 72, 828. 15. Ragsdale, R. O.; Driscoll, J. A. J. Chem. Educ. 1992, 69, 828– 829. 16. McRae, R. A.; Jones, R. F. J. Chem. Educ. 1994, 71, 68. 17. Pearson, R. S. J. Chem. Educ. 1985, 62, 622.
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