In the Laboratory edited by
Second-Year and AP Chemistry
John Fischer Ashwaubenon High School Green Bay, WI 54303-5093
Instrumental Analysis in the High School Classroom: UV–Vis Spectroscopy
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Walt Erhardt Battle Creek Area Mathematics and Science Center, 765 Upton Avenue, Battle Creek, MI 49015;
[email protected] How do we teach students to initiate their own learning? Require all students to initiate research they find personally interesting. The introduction of instrumentation can serve as a catalyst for personally interesting research in the high school classroom. A second-year high school chemistry course lends itself to the introduction of instrumentation and interesting research, so our analytical chemistry course includes both. A research project component in a course is not new, nor is it new that many research projects are prescriptive and do not reflect students’ own interests. Thus, it is difficult to get students to apply what they have learned in chemistry to their own lives. However, training our students to ask their own questions and teaching them to use modern chemical instrumentation to answer those questions has increased their interest in the way chemistry affects their own lives. This article presents a standard lab from our second-year chemistry course. The lab, “Determining Which of the Seven FD&C Food-Approved Dyes are Used in Making Green Skittles”, familiarizes students with the operation of the CHEM2000 UV–Vis spectrophotometer.1 Two subsequent examples describe students using this lab as a springboard for their own research. Green Skittles Lab
Experimental Procedure The Chem2000 UV–Vis spectrophotometer is capable of displaying the absorbance of wavelengths from 180 nm to
850 nm. This leads to a lab examination of all seven of the FD&C colors certified for use in foods (1).2 With directions for running the spectrophotometer in hand, students first look at the spectrum of the seven FD&C dyes permissible for use in food, drugs, and cosmetics. They then determine qualitatively whether green Skittles are made using FD&C Green #3 (the only green dye permissible in foods) or some combination of yellows and blues that makes green.
Determining Green Skittles Dye Makeup The absorbance spectrum of green Skittles is shown in Figure 1. The student, armed with the spectra of all seven FD&C food dyes, concludes that Mars, Inc., the maker of Skittles, uses Yellow #5 and Blue #1, rather than Green #3, to color green Skittles. A closer look at student reasoning leading to that conclusion is found in the Supplemental Material.W Before doing this experiment, students are unaware that Mars, Inc. does not list FD&C Green Dye #3 in either lake or dye form on the Skittles packaging. Finding A Researchable, Student-Initiated Question Each of the lab write-ups requires a lab extension entitled, “So What’s the Next Question?” This assignment has two purposes: 1. It forces students to take the initiative in using the lab to ask their own solvable question—solvable now because they have just had lab experience in answering a similar question. (For instance, in wet chemistry we did a gravimetric determination of the quantity of calcium in a popular mineral supplement. One student then wondered whether it would be possible to use a slightly modified procedure to determine the calcium content in calcium-fortified orange juice.) 2. The “Next Questions” found in a student’s own lab write-ups serve as a resource of possible research questions for that student to draw on when looking for a topic for a final project.
Figure 1. The absorption spectrum of green Skittles (λmax at 425 nm and 630 nm).
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The strategy in the course is first to learn to use one instrument with a prescribed procedure and write the lab report—including “So What’s the Next Question?” This first standard lab serves as a template for four subsequent labs using other instruments. Finally, the students each investigate their own student-initiated question using instrumentation skills learned during the five previous labs. This student-initiated lab happens during the last 10 of 45 class days.
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In the Laboratory
“So What’s the Next Question?” The green Skittles lab piqued the interest of two students. One student worked part-time in a pharmacy and wondered about dyes used to color capsules; the other student knew about the need to protect skin from the dangers of ultraviolet radiation in sunlight and asked about the effectiveness of sunscreen protection.
Determining Pharmaceutical Capsule Color Makeup The first student observed that many medicines come packaged in swallowable, colored capsules. She noticed the original capsule packaging did not list the dyes used. She decided to use the spectrophotometer to determine whether the capsule maker was using only the approved seven FD&C food colors in their manufacturing process. After dissolving several capsules of one color in 0.10 M HCl, to simulate stomach acid, she used the spectrophotometer to obtain the absorption spectrum. The spectrum she collected for the bluecolored capsule is shown in Figure 2. Upon comparing this spectrum with those of the seven food dyes, she concluded that FD&C Blue #1 is probably the main color ingredient in the capsule’s blue dye. Her conclusion for all capsules tested: All capsules used colors or some combination of colors permitted by the Food and Drug Administration for use in foods. The assumption underlying this comparison is that the seven FD&C food dyes will give the same spectra whether in water or in an acid solution. Determining UV-Light Protection in Sun-Protection Lotions The second student wondered what the absorbance pattern looked like for sun-protection lotions and in what areas of the UV spectrum the least protection was given. Using UV-transparent cuvettes, he looked at the spectrum of Banana Boat Quick-Dry Sport SPF 30.3 Trying to mimic the thickness of actual application on skin, he varied the lotion’s concentration by dissolving 1, 2, and 3 drops, respectively, of the SPF 30 lotion into 50 mL of isopropyl alcohol. The spectra are shown in Figure 3. Dilution was necessary to discern the features in the spectra. The relatively poor absorption, and therefore protection, from UV rays in the 200–290 nm range was clearly observed. Known as the UV-C range, this is the highest energy wavelength and is most damaging to living tissue (2). Fortunately, it is also the range that is absorbed by the atmospheric ozone and theoretically never reaches the earth’s surface (2). Dilution lowers the absorbance of all three areas of ultraviolet light: UV-B (290–320 nm), UVA (320–400 nm), and the UV-C range (2). The obvious “hole” in the UV-C range stands out. Perhaps this is the reason that Banana Boat Quick-Dry Sport SPF 30 advertises protection only against UV-A and UV-B ultraviolet radiation. Clearly, there was a difference in the degree of absorbance in all ranges as the concentration of the solution varied. The student’s conclusion: Wearing sunscreen makes a difference in the type of UV protection given, and wearing more sunscreen gives better protection in all three areas of the UV spectrum.
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Figure 2. Student’s spectrum for blue pharmaceutical capsules (λmax at 630 nm and 410 nm).
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Figure 3. Spectra of drop(s) of Banana Boat Quick Dry Sport SPF 30 in 50 mL of 100% isopropyl alcohol: (A) 1 drop, (B) 2 drops, and (C) 3 drops.
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In the Laboratory
Presentation After completing their own lab procedures, all students wrote papers following a prescribed rubric and made a PowerPoint presentation of their research to part of the student body and faculty at our annual research symposium. Analytical instrumentation in the classroom provided the means for students to ask and answer their own personally interesting, researchable questions. Hazards Safety concerns about the use of dilute solutions of the seven food dyes are minimal, except for the possibility of temporary staining of skin and clothing.
Frustration, a guaranteed component in most experimental processes, can be a great teacher. These descriptions of the work of the two students do not convey the frustrations they faced. The student exploring the colors used in capsules, especially the light and dark green, eventually used column-chromatography and varying concentrations of isopropyl alcohol in water to separate the dyes, before making a determination of color from the spectrum of the individual, separated dyes. The student exploring sunscreens had no idea which sunscreens to start working with and had false starts before finding a sunscreen that was both adaptable to dilution in isopropyl alcohol and free of colloidal suspensions that initially block the passage of all electromagnetic radiation. The instructor’s greatest frustration is in allowing students freedom to make choices of their own (inefficient as they may be) in initiating and executing personal research.
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Acknowledgments I thank Colleen Davis and David Squires for permission to use their lab data in the writing of this article and the late Marc Percovic of Western Michigan University for help in preparing the manuscript. W
Supplemental Material
A student handout, instructor notes, and the spectra of the seven FD&C food dyes are available in this issue of JCE Online.
Challenges
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Instructors may know that a more efficient technique or instrument is available; the instructor’s biggest challenge is treading the line between telling the students too much and telling them enough to help them meet their presentation deadline.
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Notes 1. The spectrophotometer was purchased from Ocean Optics, Inc., 380 Main Street, Dunedin, FL 34698; 727/733-2447; http://www.OceanOptics.com (accessed Feb 2007). 2. Two-gram samples of the seven FD&C food colors are available from Rainbow Colors, 1116 Hartford, Turnpike, Vernon, CT 06066. Phone: 860/871-2033. 3. Banana Boat Quick Dry Sport SPF 30 is an over the counter sunscreen available at many drug stores.
Literature Cited 1. VanBlaricum, Ann. Food Dyes. http://www.doggedresearch.com/ chromo/dyes.htm (accessed Feb 2007). 2. Robbins, David K. ChemMatters 1984, 2 (2), 4–7.
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