Orange Juice Clock Reaction
This clock reaction is initiated by adding household strength hydrogen peroxide to a mixture containing orange juice, tintcure of iodine, and starch. After about 30 seconds it suddenly changes from orange to black.
This demo is usually used in unit that deals with rates of reaction. Allow about 10 minutes for this demo.
The chemistry of clock reactions is rather complex, but this reaction is similar to the Landolt Iodine Clock Reaction with hydrogen peroxide replacing iodate ion as oxidizing agent and ascorbic acid taking the place of bisulfite ion as reducing agent. When iodine is added to the orange juice, the some of the ascorbic acid is used up reducing it to iodide ion. When the hydrogen peroxide is added to the solution a slow reaction occurs in which the hydrogen peroxide oxidizes the iodide ion to triiodide ion (I3-). Triiodide ion normally combines with starch to produce a deep purple color, but as long as any ascorbic acid is present, it will immediately reduce any triiodide ion back to iodide ion. Only when the ascorbic acid is all used up does the triiodide ion accumulate in sufficient quantity to produce a visible color change.1,2
1Shakashiri, Bassam Z. Chemical Demonstrations. vol.4. Madison, WI: The University of Wisconsin Press, 1992. p.22. 2Wright, Stephen W., "The Vitamin C Clock Reaction," Journal of Chemical Education. vol.70, no.1. Greeley, CO: Division of Chemical Education, Inc., of the American Chemical Society, January 2002. pp. 41-43
- a beaker containing a mixture of 280mL of orange juice, 5mL of tintcure of iodine, and 3mL of starch solution
- a small beaker containing 50mL of 3% hydrogen peroxide solution
- stir rod
- bottles of the household products used in this demo to show the students
Add the hydrogen peroxide to the orange juice solution. Stir. In about 30 seconds, the solution should turn dark.
Though these are all household products, remember that tincture of iodide is poisonous and flammable. Wear goggles.
Prepare solutions as per the JCE article cited above in "Discussion" section.