Iodine Clock

Two clear solutions are mixed, and after a short delay the mixture suddenly turns deep blue. The timing of the color change depends on concentration and temperature, making this a classic demonstration of chemical kinetics.

1. Prepare solution A: 0.02 M potassium iodate (KIO₃).
2. Prepare solution B: dissolve 4 g starch and 0.2 g sodium metabisulfite (Na₂S₂O₅) in water, then add 5 mL of 1 M sulfuric acid and dilute to 1 L.
3. Place 100 mL of solution A in a cylinder and 100 mL of solution B in a beaker.
4. (Optional) Chill one set in an ice bath for at least 15 minutes to observe temperature effects.
5. Mix equal volumes of A and B, stir, and watch for the sudden appearance of the blue color.
6. For concentration effects: dilute solution A to ½ or ¼ its concentration before mixing with solution B and compare reaction times.

Make the Iodine Clock Reaction (Chemistry) - BeardedScienceGuy:

IODINE CLOCK REACTION - University of Glasgow:

📄 Iodine Clock Reaction - University of Washington: https://chem.washington.edu/lecture-demos/iodine-clock-reaction

📄 https://www.flinnsci.com/api/library/Download/2eead53c42ed472ba2073c7141a53b52?srsltid=AfmBOooqLvLyRZPl-COd8cNznr040xwE1MI3o9tcDoL1CQb8We9awIkb - Flinn Scientific: https://www.flinnsci.com/api/library/Download/2eead53c42ed472ba2073c7141a53b52?srsltid=AfmBOooqLvLyRZPl-COd8cNznr040xwE1MI3o9tcDoL1CQb8We9awIkb

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• Show how dilution slows the reaction (½ concentration doubles the time; ¼ concentration quadruples it).
• Use an ice bath to slow the reaction noticeably.
• Compare results of warm vs. chilled solutions.

• Wear safety goggles and gloves.
• Do not leave solution B in an open beaker for more than an hour; discard if the reaction no longer works.
• Handle sulfuric acid carefully; it is corrosive.
• Dispose of solutions down the drain with water, as the reactive chemicals are consumed and iodine is complexed.

• Why does the mixture stay colorless at first, then suddenly turn blue? (Because iodine is initially consumed by bisulfite ions; once bisulfite is gone, iodine accumulates and forms a blue complex with starch.)
• How does concentration affect the rate of reaction? (Lower concentration slows the reaction; reaction time is proportional to dilution.)
• How does temperature affect the reaction rate? (Cooler temperatures slow the reaction; warmer temperatures speed it up.)
• Why is starch used in this reaction? (It forms a deep blue complex with iodine, making the endpoint very visible.)