Application
pH Acid & Bases

Theory
A colorless solution is added to each of six beakers. A second colorless solution is added and each of the six resulting solutions turns a different color of the rainbow. Add more of the first solution and the rainbow of colors disappears.

Materials
6 Beakers, 250-mL Sodium hydroxide solution, 0.01 M, 800 mL
1 Beaker, 1000-mL Hydrochloric acid solution, 0.01 M, 1 L
1 Beaker, 2000-mL 6 Dropping bottles

Safety
Hydrochloric acid solution, although dilute, is still severely corrosive to eyes, skin and other tissue. Treat with the same caution with which you would treat concentrated hydrochloric acid. Sodium hydroxide solution, although dilute, is corrosive; skin burns are possible; very dangerous to eyes; wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Indicator solutions contain ethyl alcohol which is a flammable liquid and a fire risk. Keep away from heat and open flame.

p-Nitrophenol is toxic by ingestion.

Preparation
Prepare 6 indicator solutions, 30 mL each:
Violet–0.45 g of phenolphthalein and 0.2 g thymolphthalein in 30 mL of 95% ethyl alcohol.
Blue–0.2 g of thymolphthalein in 30 mL of 95% ethyl alcohol.
Green–0.2 g of thymolphthalein and 2 g of p-nitrophenol in 30 mL of 95% ethyl alcohol. Add 5 drops of 1 M HCl to acidify.
Yellow–1 g of p-nitrophenol in 30 mL of 95% ethyl alcohol. Add 5 drops of 1 M HCl to acidify.
Orange–2 g of p-nitrophenol and 0.15 g phenolphthalein in 30 mL of 95% ethyl alcohol. Add 5 drops of 1 M HCl to acidify.
Red–1.5 g of p-nitrophenol and 0.75 g phenolphthalein in 30 mL of 95% ethyl alcohol. Add 5 drops of 1 M HCl to acidify.

Demonstration
1. Set up 6 beakers (250-mL) on an overhead projector or in front of the class.

2. Add 3 drops of "violet" indicator solution to the first beaker. Add 3 drops of "blue" indicator solution to the second beaker. Continue adding three drops of each of the other indicator solutions to the appropriate beakers.

1. Add approximately 75 mL of 0.01 M sodium hydroxide solution to each 250-mL beaker. Each of the six solutions should change from clear to a color of the rainbow! Pause for a few moments to allow your students to express their amazement, then continue.
2. Add approximately 100 mL of the 0.01 M hydrochloric acid solution to each beaker. The solutions will once again be clear. Further amazement will follow.

The specific amounts of acid and base solutions are not important. Each addition of acid or base solution must neutralize the solution in the beaker and drive the pH in the opposite direction. All solutions can be poured into the large two-liter beaker. The resulting solution will be of an acidic pH and clear.

Disposal
Solution may be poured down the drain and flushed with water according to Flinn Suggested Disposal Method #26b; consult your current Flinn Chemical Catalog/Reference Manual..

Reference
Shakhashiri, B. Z. Chemical Demonstrations; University of Wisconsin: Madison, 1989, Vol. III.