Classic Iodine Clock Reaction

A beaker of water magically turns into ink... or so you think.

Two clear liquids are mixed together and the audience is told to watch closely… “Don’t take your eyes off the liquid or you’ll miss it!” Then, in a flash, the water changes to ink (or so they think!). Clock reactions like this never fail to capture the student’s attention, and the Iodine Clock Reaction is one of the most startling chemical demonstrations you’ll ever see. “How did that happen?” is almost always the reaction, and when students ask “How?”, they’re ready to learn.

Experiment Materials

  • 1 250 mL beaker
  • 2 100 mL beakers
  • 2 1000 mL beakers or flasks
  • 1.2 g sodium metabisulfite
  • 10 mL ethanol
  • 4 g concentrated sulfuric acid
  • 4.3 g potassium iodate
  • 2 g starch soluble
  • Distilled water
  • Stopwatch


Solution A
In 1-Liter of distilled water add 1.2 grams of sodium metabisulfite. Carefully add 4 grams of concentrated sulfuric acid and 10 mL of ethanol.

Solution B
Add 4.3 grams of potassium iodate to 1000 mL of distilled water.

Solution C
Boil water and measure out 50 mL. Add 2 grams of soluble starch and allow to cool.

Presentation Time
In a 250 mL beaker, combine 50 mL of Solution A with 50 mL of distilled water. Now, add 10 mL of Solution C to the mixture.

In a 100 mL beaker, combine 50 mL of Solution B and 50 mL of distilled water.

Get ready to start your stopwatches! Pour the solution in the second beaker into the first beaker. Keep pouring the solutions back and forth into the beakers for about 8-10 seconds.

Watch in amazement as the combined solutions turn an inky black color instantly! Wait for the oohs, ahhs, and applause from your captivated audience.

How Does It Work?

This reaction is referred to as the Landolt Clock Reaction. There are three steps in the process that cause this amazing reaction. When you prepare the Solutions A, B, and C, the chemicals begin to mix and form new chemical compounds. This is a very slow reaction, so you don’t see any outward changes. When you begin to pour the solutions together a much faster reaction occurs, which leads to the third reaction which is instant. Suddenly, and immeasurably quickly, the clear liquids turn into a jet black iodine-starch complex. These reactions happen at different intervals because different chemicals react at different speeds.

Additional Info

Discussion of the Chemistry…
The sudden change from a colorless solution to the blue-black solution is the result of four sequential reactions. First, the bisulfite ions (HSO3-) reduce some of the iodate ions (IO3-) to form iodide ions (I-). Next, the iodide ions (I-) are oxidized by the remaining iodate ions (IO3-) to form triiodide ions (I3-). The solution now consists of triiodide ions (I3-) and soluble starch. In the third reaction, the triiodide ions (I3-) get reduced by the bisulfite ions (HSO3-) to become iodide ions (I-). That continues until all of the bisulfite has been consumed. Finally, the triiodide ions and starch combine to form the dark blue-black starch complex that looks like ink.