Rainbow Connection
- Page ID
- 131421
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Required Training |
Required PPE |
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UC Lab Safety Fundamentals |
Lab coat, safety glasses/goggles, nitrile gloves |
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Equipment |
Chemicals |
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400 mL Beakers, 6 |
Violet (0.45 g of phenolphthalein and 0.2 g thymolphthalein in 30 mL of 95% ethanol) |
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2L Beaker |
Blue (0.2 g of thymolphtalein in 30 mL of 95% ethanol) |
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Glass Stir Rod |
Green (0.2 g of thymolphtalein and 2 g of p-nitrophenol in 30 mL of 95% ethanol. Add 5 drops of 1 M HCl to acidify) |
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6 Color Indicators in Dropper Bottles |
Yellow (1 g of m-nitrophenol in 30 mL of 95% ethanol. Add 5 drops of 1 M HCl to acidify) |
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Funnel |
Orange (2 g of p-nitrophenol and 0.15 g phenolphthalein in 30 mL of 95% ethanol) |
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Red (1.5 g of p-nitrophenol and 0.75 g phenolphthalein in 30 mL of 95% ethanol. Add 5 drop of 1 M HCl to acidify) |
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0.01M NaOH, 450 mL |
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0.01M HCl, 900 mL |
Procedure:
Steps 1-3 can be done before class
- Set up six 400 mL beakers on an overhead projector or in front of the class.
- Add 3 drops of “Violet” indicator into the first beaker. Add 3 drops “Blue” into the second beaker. 4 drops “Green” into the third. 4 drops “Yellow” into the fourth. 3 drops “Orange” into the fifth. 3 drops “Red” in the last.
- Add 10 drops of concentrated HCl into the 2L Beaker and set it aside until step 8.
- Fill each 400 mL beaker with approximately 50 mL of the 0.01 M Hydrochloric Acid solution. All 6 resulting solutions should be clear and colorless.
- Add approximately 75 mL of 0.01 M Sodium Hydroxide solution to each beaker. Each solution will change from colorless to a bright, vivid color of the rainbow!
- Add 0.01 M Hydrochloric Acid solution to each beaker until colorless (amounts will vary for each beaker).
- Add 3 M sodium hydroxide solution dropwise (4-5 drops) to each beaker until the color reappears.
- Empty the beakers into the 2L beaker containing 10 drops of concentrated HCl. Solutions will become colorless upon mixing with HCl.
Discussion:
The three indicators used in this demonstration—phenolphthalein, thymolphtalein and p-nitrophenol—are colorless in acidic solution. In basic solution phenolphthalein is red, thymolphtalein is blue, and p-nitrophenol is yellow. Any color in the spectrum may be prepared by combining these primary colors in different amounts. The indicator solutions added to each beaker readily evaporate, leaving the exact, minute proportions of the appropriate dry indicators on the bottom of each beaker. Students will not see this step of the procedure. They will only see the pouring of the acid and base solutions and the color changes
Hazards:
HCl and NaOH are both corrosive to eyes and skin. 95% Ethanol is flammable; keep away from flames and heat. p-Nitrophenol and m-nitrophenol are toxic by ingestion.
SOP: 
Carcinogen – Phenolphthalein
Corrosive – Hydrochloric Acid, Sodium Hydroxide
Potential Reproductive Hazard - Phenolphthalein
Disposal (by Storeroom)
Dispensary will submit waste solutions to EH&S.
Reference
Shakhashiri, B. Z. Chemical Demonstrations, Volume 3; University of Wisconsin: Madison