Linette Guare

Vitamin C Clock Reaction

Concept:

A delayed chemical reaction occurs with the mixture of Iodine and starch by adding Vitamin C.

Materials:

• 2 - 250 mL beakers Liquid laundry starch
• Distilled water 100 mL graduated cylinder
• 500 mg Vitamin C tablet (C6H8O6)
• 25 mL graduated cylinder
• Tincture of iodine - (2%) (I2)
• Hydrogen peroxide - (3%) (H2O2)
• Light Box

Pre-Demo Preparation:

Vitamin C stock: Crush Vitamin C tablet in 30 mL distilled water. Break up so no pieces of solid can be seen. (Solution will be slightly hazy.)

Solution A: 1. Place 60 mL distilled water in beaker A.

2. Add 3.5 mL Vitamin C stock.

Solution B: 1. Place 60 mL distilled water in beaker B.

2. Add 15 mL hydrogen peroxide.

3. Add 2 mL laundry starch.

Directions: Place beakers on light box.

1. Add 5 mL tincture of iodine to solution A and stir. This solution is now colorless

2. Add solution B to solution A.

3. Stir a few times and wait. It should take approximately 1 - 2 minutes for the solution to suddenly turn blue-black.

Introduction: Clocks have been around to measure time thousands of years. They have been made from different materials, like stone on a sundial, and liquid crystals in watches. A clock can be formed from molecules that react at a rate from the time the chemicals are mixed to the time the reaction occurs.

Explanation: There are two forms of iodine:

There are two reactions taking place simultaneously in the solution:

#1. 2I- + H2O2 ---(slow)-------> I2 + H2O

#2. I2 + Vitamin C ----(fast)----> I (colorless)

In Reaction # 1 iodide ions react with hydrogen peroxide to produce iodine element which is blue in the presence of starch. BUT, BUT......

In Reaction # 2 The Vitamin C is immediately reacting with any iodine formed in reaction # 1. The net result, at least for part of the time is that the solution remains colorless with excess of iodide ions being present.

Now after a short time as the reactions keep proceeding in this fashion, the Vitamin C gets gradually used up. The Vitamin C creates a clock reaction (1 - 2 minutes) and once it is used up, the solution turns blue, because now the iodine element and starch are present.

Safety Precautions: Be careful when working with the iodine - it stains.

Waste Disposal: Dispose all liquids down the drain with plenty of water.

Reference: Wright, Stephen W. Tick Tock, a Vitamin C Clock J. Chem. Educ. 2002 79 40A. (January 2002)

Wright, Stephen W. The Vitamin C Clock Reaction J. Chem. Educ. 2002 79 41. (January 2002)