Iodine clock reaction
How to turn out the “light” in a solution
Wear protective gloves, eyewear, and a mask. Perform this experiment in a well-ventilated area. Observe safety precautions when working with acids and hydrogen peroxide.
Reagents and equipment
- 1 g sodium thiosulfate;
- 5 g potassium iodide;
- 1 L distilled water;
- 100 mL 15% hydrogen peroxide solution;
- 5 mL concentrated sulfuric acid;
- 5 g starch;
- 400 mL freshly-boiled water;
- 3 beakers;
- a pitcher.
Preparing a starch solution (solution A): Combine 5 g starch and 100 mL cold distilled water in the 500 mL beaker. Stir thoroughly. Add 400 mL freshly-boiled water and stir. Preparing a solution of sodium thiosulfate and potassium iodide (solution B): combine 1 g sodium thiosulfate and 5 g potassium iodide in a 500 mL beaker. Add 500 mL distilled water and stir thoroughly. Preparing a hydrogen peroxide solution (solution C): combine 400 mL distilled water, 100 mL 15% hydrogen peroxide solution, and 5 mL concentrated sulfuric acid in a 500 mL beaker. Stir thoroughly. The experiment: combine 100 mL of solution A and 50 mL of solution B in a pitcher. Stir thoroughly. The volume of solution C is variable – adding more or less of solution C will affect the speed of the reaction.
The “Iodine Clock” reaction (sometimes called “Egyptian Night”) is an oxidation-reduction reaction. Colorless solutions are mixed and, following a short pause, the liquid rapidly turns dark blue, almost black. Several oxidation-reduction reactions take place in the solution, which can be described as follows
Hydrogen peroxide oxidizes iodide ions to molecular iodine:
H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O
The solution remains clear because the forming iodine reacts with sodium thiosulfate:
2S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2I⁻
When the forming iodine has already reacted with all of the available sodium thiosulfate, it begins to react with starch, forming a dark-blue starch-iodine complex. This instant darkening effect, reminiscent of the sudden onset of night in Egypt, gave this reaction its poetic name of “Egyptian Night.”