JCE Classroom Activity: #81
Instructor Information
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pHantastic Fluorescence
Narra (species name: Pterocarpus indicus) is a tropical hardwood found in the Philippines, Burma, and nearby countries. It is one of two woods well known in the 16th and 17th centuries as Lignum nephriticum, whose water extract was thought to have medicinal value. It produces a blue tinge when the wood contacts water. Boyle noted the pH dependence of the blue emission. The fluorescent compound appears to be an Narra solution (with wood shaving) isoflavone of unknown structure. A companion article provides more information (1). illuminated by UV LED. Fluorescence results when a substance absorbs light of one wavelength and then emits light of a different, typically longer, wavelength. In incandescent room light, the wood solution appears yellow because it absorbs in the violet region of the visible spectrum. At pH < 6, the narra solution is nearly non-fluorescent; at pH ~ 7–10 it fluoresces blue. The pH effect is thought to be similar to the pH effect on the color of acid–base indicators: the structure of the molecule changes, altering its absorption of light and its ability to fluoresce.
Integrating the Activity into Your Curriculum This Activity gets students thinking about the interaction of light and molecules. The Activity demonstrates fluorescence of a natural material, the effects of pH, and the relationship between the color of light and color of solutions. Another JCE Classroom Activity and companion article discuss different types of luminescence (2).
About the Activity Narra is available from woodworking suppliers (3). For large groups, use a 4-ft. fluorescent lamp fitted with black light fluorescent bulbs (both sold at hardware stores).W Drape the fixture with black paper or cloth. Handheld black lights, including UV LED keychain flashlights (4), also work. If UV black lights are unavailable, fluorescent room lights or sunlight can be used: these require concentrated wood extract solution. Yellow filter sheets are available from various vendors (5). Any common acid and base may used, including HCl and NaOH (0.1 M). Vinegar and dilute clear ammonia are readily available, inexpensive, household products. Do not use sudsy ammonia; the soap interferes with the fluorescence. Use a balance to determine the number of shavings needed for ~ 0.5 g of wood. Give students guidelines for the number of shavings to use, or place shavings in plastic bags. A wood plane is useful for making shavings.
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More Things To Try 1. Adding wood shavings lowers the pH of distilled water below 6 where the extract is poorly fluorescent. In hard water from the tap, the natural buffers maintain a pH closer to 7 where the extract is fluorescent. 2. Taking time to make a more concentrated solution is worthwhile: the blue fluorescence is spectacular in sunlight. 3. A pH range of 4–10 with steps of 1 pH unit works well for the series. The fluorescence transition occurs at pH 6–8. Students will also notice a significant change in the yellow color of the solutions.
Answers to Questions 1. The fluorescence is blue. Base produces good fluorescence. 2. Answers may vary. Fluorescence with the UV black light can be spectacular, partly because the background is usually dark. Using room light is convenient. The fluorescence is a more striking blue in sunlight. Explore what the light sources have in common—they all produce some violet light that is absorbed, producing fluorescence. 3. The blue fluorescence is longer wavelength than the ultraviolet and visible violet light from the UV black light. 4. When the filter is between the UV black light and solution, the fluorescence is doused; the UV and violet light is blocked. When the filter is between the student’s eye and the solution, the filter passes emitted blue light; students see the fluorescence unaffected. This shows that the emitted light is a different wavelength from the absorbed light.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Background
photo by Mark Muyskens
Mark Muyskens Calvin College, Department of Chemistry and Biochemistry, Grand Rapids, MI;
[email protected] Students extract a fluorescent substance from shavings of narra wood. The pHdependent fluorescence can be turned on and off using household acid and base solutions. A yellow filter blocks the exciting light but not the fluorescent emission.
References and Additional Related Activities (sites accessed Mar 2006) 1. Muyskens, Mark A. The Fluorescence of Lignum nephriticum: A Flash Back to the Past and a Simple Demonstration of Natural Substance Fluorescence. J. Chem. Educ. 2006, 83, 765–768. 2. O’Hara, P. B.; Engelson, C.; St. Peter, W. Turning on the Light. J. Chem. Educ. 2005, 82, 48A–B; 2005, 82, 49–52. 3. One source is Curious Woods, 800/724–WOOD, http://www.curiouswoods.com/. See ref 1 for additional sources. 4. UV LED flashlight. Cost $15–20. http://www.teachersource.com/ (search “violet photon microlight”). 5. Yellow filter sheets (Roscolene Medium Lemon #806) are available in 20 in. ⫻ 24 in. sheets that can be cut into 10 in. ⫻ 12 in. pieces. See http://www.rosco.com/us/retail/index.asp for local dealers of Rosco products. JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 83 No. 5 May 2006 •
Journal of Chemical Education
768A
JCE Classroom Activity: #81
Student Activity
pHantastic Fluorescence What does a tropical hardwood have to do with fluorescence and pH? When placed in water, shavings of a wood called narra produce a remarkable solution. You will investigate how the appearance of this solution changes with light and pH. In general, a substance exhibits fluorescence if it absorbs light of one color, say violet, and at the same time glows with a completely different color. To observe fluorescence you will use a UV black light, which emits mostly invisible ultraviolet light. A yellow filter is a useful tool because it can block ultraviolet (and violet) light while allowing other colors of light to pass through. The properties you are about to observe have fascinated scientists for over 400 years.
Try This
Be Safe! Keep
You will need: marker; narra wood shavings; two clear, colorless, plastic cups or beakers (~100 mL); stirring rod; tap water; dropper bottle with base (i.e., dilute ammonia: 1 part clear household ammonia your direct viewing to 3 parts water); dropper bottle with acid (i.e., household vinegar); yellow filter; UV black light; room of the UV black light to a minimum. light; fluorescent light; darkened room; paper filter (optional). __1. Label two clear, colorless, plastic cups or beakers “control” and “change”. Fill one cup or beaker two-thirds full of tap water. Add two drops of base, such as dilute ammonia. Darken the room. Under a UV black light, examine the solution in the cup, as well as a dry narra wood shaving. Record your observations. __2. In room light, place several narra wood shavings into the solution from step 1. Use a stirring rod to push the shavings into the solution. Soak the wood for a few minutes. Record your observations of the solution. __3. Transfer solution to the second labeled cup or beaker from step 1. Use a stirring rod or filter paper to prevent shavings from being transferred—it’s OK if a few bits of wood get carried along. Discard the wood in the trash; rinse or wipe out the first cup. Pour half of the solution back into the empty cup so that the two cups have equal amounts. Make changes to the solution in the “change” cup; use the other cup as a control. __4. Darken the room. Under the UV black light, examine the two wood extract solutions. Record your observations. Count as you add a few drops of acid, such as household vinegar, to the “change” cup. Record your observations. __5. In room light, observe the two cups. How has the solution’s appearance changed? __6. In room light, add drops of base to the “change” cup until the solution returns to its original appearance. How many drops did it take? Darken the room and observe the solutions under the UV black light. __7. Observe the “change” cup in other types of light, such as regular fluorescent light and in sunlight. (Tip: Under regular fluorescent light, hold the cup at eye level and look through the liquid just below the top surface, with the fluorescent light directly above the cup and a dark object behind the cup.) Record your observations. __8. Under the UV black light, add acid or base to the “change” cup so that the solution remains fluorescent. Place a yellow filter between the UV black light and fluorescent solution. Record your obserfilter placement vations. Place the yellow filter between your eye and the fluorescent solution. Is the effect for step 8b the same? wood extract solution __9. Add one or more drops of acid or base to the “change” cup and observe under the UV black light without stirring. Describe how you are able to see the mixing process.
More Things To Try
filter placement
__1. Compare narra wood solutions made with distilled water and tap water. for step 8a Repeat the above procedure with distilled water in one cup and tap water in the other, without adding the base as was done in step 1. Now try to make the UV LED solutions highly fluorescent by using acid or base. __2. Prepare a more concentrated solution: soak 3 g of shavings in 200 mL of tap water for an hour or more. Add acid or base to make it glow under the UV black light. Observe the solution under UV black light, fluorescent light, and sunlight. How does fluorescence in a concentrated solution appear different from a more dilute solution? __3. Use a pH meter or pH paper to determine the range of pH values over which the solution changes from very dim to very bright fluorescence. Use several cups to create a series and examine it under the UV black light.
Questions 1. 2. 3. 4.
What color is the wood extract solution fluorescence? Which reagent tends to aid bright fluorescence, acid or base? Which light source is best for showing the fluorescence? Does the solution fluorescence have longer or shorter wavelengths than those from the UV black light? Explain. How do your observations with the yellow filter show that the fluorescence is a different wavelength from the light emitted by the UV black light?
Information from the World Wide Web (accessed Mar 2006) NightSea (Fluorescence of ocean life). http://www.nightsea.com/ Thomas S. Warren museum of fluorescence. http://www.sterlinghill.org/warren/ This Classroom Activity may be reproduced for use in the subscriber’s classroom.
768B
Journal of Chemical Education •
Vol. 83 No. 5 May 2006 •
www.JCE.DivCHED.org
