overhead projector Demonstrations with Red Cabbage Indicator John J. Fortrnan and Katherine M. Stubbs Wright State University Dayton, OH 45435
Anthocyanin plant pigments such a s those found in red cabbaee have been extracted and used a s a n acid-base indicator in many educationill and enrrrtaining drmonstraol1 rannrs have been listcd 12.3, tions ( 1 -5,.. Aouroximatr .. 5).This paper extends the use of the red cabbage extract into demonstrations that illustrate the acidic or basic nature of solutions of gases and gives suggestions for modifications to project the beautiful range of colors and shades for better viewing in large lecture halls.
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Preparation of Red Cabbage Juice lndicator Chop up several leaves of a head of red cabbage in a blender with water. Filter the liquid and transfer it into a plastic bottle. (If you use tap water this will appear blueviolet, but if you use distilled water it will be more reddish.) Dilute with water to the desired intensity of wlor. (If you wish to stabilize it for storage for a week or more, add about one tenth volume of rubbing alwhol.) AcidIBase Concentrations On an overhead projector line up eight 100- or 250-mL beakers filled with sufficient (10-50 mL) red cabbage indicator to give a dark, but clear, color when projected. Mark the fiRh beaker from one end as the standard. Using a medicine dropper add one drop of household ammonia to the fourth beaker, two drops to the third, three drops to the second, and squirt a dropperful into the first. To the sixth beaker add one drop of white vinegar, to the seventh two drops, and to the eighth a dropperful. The results will be a beautiful variation in shades ranging from green to aqua to blue to violet to red. All additions must be made quickly because the heat from the projector will uolatillze the ammonia and the acetic acid over time. If too small a volume of red cabbage indicator is used in the beakers, a single drop of reagent will cause too great a pH change and much of the beauty of the gradual change in shades will be lost. This problem can be corrected by diluting the household ammonia and vinegar before addition. Effects of Gaseous Carbon Dioxide and Ammonia Outfit two small (250-mL) side-arm flasks with solid stoooers and rubber tubine conneded to Pasteur caoillarv (Alternatively two r a g e test tubes may be o;tfitt& with one-hole stoooers and elass tubine wnnected to rubber tubing and c&hary One 211 be used to generate C02 gas from vinegar and baking soda or Alka-Seltzer and water. The second will be used to generate small amounts of ammonia gas by the heat produced when about five pellets of NaOH are added to about 25 mL of concentrated ammonium hydroxide (6). On the overhead projector place three 250- or 400-mL beakers containing 50-100 mL of red cabbage indicator
Presented at the 11th Biennial Conference on Chemical Education in Atlanta, GA, August 8, 1990. 66
Journal of Chemical Education
edited by DORISKOLB Bradley University Peoria, IL61625
solution. Leaving the middle solution a s a comparison, add just enough drops of clear household ammonia to one of the side beakers to make the solution barely green. To the beaker on the other side add enough white vinegar to make the solution red. Begin generating the C02 and NH3 gases and simultaneously bubble the C02 into the green basic solution and the NH3 into the red acidic solution. ARer they have changed colors, reverse the gases and continue to alternate as often a s you wish or until the solutions become sufficiently buffered a s to slow the color changes. Similarly one may blow his breath through red cabbage indicator solution that is just barely basic enough to he green and the carbon dioxide in the breath will change the solution to blue and after a much longer time pink (7).I have found this exercise useful in keeping a troublesome student occupied for 5-10 minutes. Literature Cited 1.Asimoa I. In TheNew Boakofffiolukdg.; Gmlier: Denbury, CT. 1980; Vol3, p 217. 2. Shakhashui. B. 2. Chemiml Demonsfmlims: Univonit~ of Wisconsin:Madison. WI.
1989;v01'3pp ; 50-61. 3.Forsier.M. J. Chem Educ 1918.55, 101-108. 4. Benbou, A , Ed. Wondmsclem 1988.2 (11.7. 5. ICE Staff Ernlori~AeidFondfiosps;Madison. WI.1981: 2 w
Addition of IBr to Fatty Acids on the Overhead Projector Sally ~olornon,'Annarnarla Fulep-Poszrnik, Gary Kulp, and Heung Yu Drexel University Philadelphia, PA 19104
The intcrhalo~encompound IBr dissolved in CCli is addcd to petr~~leum ether solutions of fatty acids to test for unsaturation. IBr, a relatively inexpensive purple-black crystalline substance, is much less corrosive than the bromine normally used for this tesk2 This demonstration can be used to compare the reactivities of alkanes, alkenes, and benzene on the overhead projector, or may be adapted for a laboratory experiment on fats. Students, whose sole encounter with interhalogens is likely to be in solving general chemistry bonding and naming exercises, find it interesting to see what a mixed halogen compound such a s iodine monobromide actually looks like. Procedure A solution of IBr in CCld is made by dissolving 1g of IBr in 20 mL of solvent. The fatty acid solutions are prepared by placing 2 3 mL of oleic acid (CH3(CH2)7CH=CH(CH2)7COOH) in a 50-mL beaker and 1-2 g of lauric acid (CH3(CH2)&OOH) in another beaker, then diluting to 20 mL with petroleum ether.3 A 20-mL volume of the petroleum ether solvent alone occupies a third beaker. To each sample
'Author to whom correspondence should be directed. 2Aldrich Chemical Cornoanv. Inc. 1001 West Saint Paul Avenue. M Iwa-kee. W 53233. lhe'catilog nLrnDer is 22.484-7 (50g for $20) of low oo~mg a fianes, mostly pen'Petro e m etner s a rn~it~re tanes and hexanes
is added 5-10 drops of the 5% IBr in CCL solution. The reference solvent a n d lauric acid solution retain the purplish-red color of the 1Br solution. The oleic acid solution turns color and grows gradually paler.
Principles Interhalogen compounds, like bromine, can add across the double bonds of unsaturated fatty acids such as oleic acid, b u t fail to react with saturated fatty acids. I t i s
tempting to write a reaction analogous to the one ifl which bromine adds across double bonds, i n which the product would be the iodobromo adduct. However, i n the reaction of IBr with both oleic acid and linoleic acid (CHdCHd4CH=CHCH2CH=CH(CH2)7COOH)only the bromine adducts have been o b s e ~ e d . ~
%usuki,
Y. and Koizumi, H. BunsekiKagaku 1989,38(12), 718.
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January 1992
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