overhead projector Jemonstrations Teas as Natural Indicators Dianne N. Epp East rl gh Scnoo 1000 So~lh701n Lincoln, NE 68510 The introduction of chemical concepts to first-year students by use of familiar phenomena is a well-accepted pedagogical method. Generally students are aware that the addition of lemon to iced tea causes the tea to become significantly lighter in color, more so than would be explained by a simple dilution factor. The following demonstration uses that knowledge as a springboard to introduce the general concept of acidhase indicators. Tea leaves contain a tannin, tannic acid, a s the brown coloring material which, being a polyphenol, is pH sensitive. While the color change is not extreme, it is distinct enough to introduce the concept that some plant pigments Herbal Teas and Their Colors in Acid and Base Tea Black Green Hibiscus Flowers Jasmine Red Clover Rose Hips Pennyroyal Red Raspberry Spearmint Camomile Eucalyptus Licorice Lavender Red Zinger Yellow Zinger Mandarin-Orange Cinnamon-Apple Spice
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Acid Color light yellow yellow red yellow peach yellow colorless light yellow light yellow light yellow colorless milky light yellow pink red red red red-orange
Journal of Chemical Education
Base Color dark brown dark brown green dark brown bright yellow dark yellow yellow green dark tan dark yellow green brilliant yellow yellow orange yellow orange green green green green green
edited by DORISKOLB Bradley Universify Peoria, lL61625
are affected by acids and bases. I n many plants the colors (especially the reds, blues, and violets) are due to anthocyanins whose pH sensitivity has led them to be used a s acidmase indicators (1).Several herbal teas, notably Red Zinger, Yellow Zinger, Maudarin-Orange, and CinnamonApple Spice (registered trademarks of Celestial Seasonings Inc., Boulder, CO), produce deep red colored infusions. The red color is due to the presence of hibiscus flowers which contain a n anthocyanin that shows a remarkable redlgreen color change in acidhase solutions. In the tea section of some health food stores one may procure pure hibiscus flowers that will give an intense red solution, but all of the herbal tea that contained hibiscus have given similar results. Other pure herbal teas have been tested, many of which show some pH sensitivity. Procedure Prepare separate infusions of the teas to be tested (two tea bags or a spoonful of loose tea in 100 mL of boiling water, allowing them to steep until a n intense color is obtained). A24-well plastic tissue culture plate is a convenient container to use on the overhead for demonstration of a number oftea samples. Use a pair ofndjacent wells lillingeach half full of the tea sample to be tested. Add dropwise, 6 M HCI to one s n r n ~ l eand 6 .M NaOH to the other. An altemative method would be to use three wells and leave the center well neutral for comparison. Common teas, both black and green, that give a tan infusion will turn yellow in the presence of a n acid and very dark brown in the presence of a base. Herbal teas containing hibiscus flowers will be bright red in a n acid and green in a basic solution. Other herbal teas tested are listed in the table. An extension of this demonstration is the common student act~vitytesting the pH senhitivity of pigments from various flowers or plants. Most of the anthocvanins that are pH sensitive a i e water soluble and can de extracted with boiling water. Some will prove more soluble in a n ethanol or waterlethanol solution. In addition to many others cited in the literature, infusions of red geranium petals, purple petunias, and the leaves of purple basil, all ofwhich may be extracted with water, have shown vivid acidmase color changes. ~
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Literature Cited 1. Shakhsshui, B. 2. Chrmieol Demonsfmflona; University of W~~lsmnsin: Madison, 1989:Vol. 3; pp 5 M 7 .
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