Iodine clock reaction

The iodide clock reaction is a chemistry experiment designed to illustrate the different rates at which chemical reactions occur. The experiment can be performed in several different ways; each variation involves an iodine solution that is mixed with a reactive solution. When the solutions are combined, the liquid is initially colorless. After a few seconds, the solution spontaneously turns dark blue. The experiment is called a “clock” reaction because if the proper amounts of chemicals are used, the reaction will always occur after a predictable period of time. The reaction was first observed in 1886 by Swiss chemist Hans Heinrich Landolt. For that reason, it is sometimes referred to as the Landolt reaction.

Background

Chemical bonds are created through the interaction between electrons in the outer shell of an atom. Electrons from one atom can be shared with electrons in another atom, bonding those atoms together to form a molecule. A chemical reaction occurs when the chemical bonds between atoms in two or more substances are broken down and rearranged, resulting in the formation of a new chemical molecule or molecules. If an atom gains an electron during the process, that atom becomes negatively charged. Such atoms are referred to as anions and are renamed using the suffix -ide.

Iodine is a naturally occurring chemical element that plays an important role in regulating human metabolism. Iodine is necessary for maintaining a healthy thyroid gland, but in its elemental, or pure form, iodine is corrosive. In large doses, it can be toxic. Elemental iodine becomes safe for humans when it is in its ionic form, known as iodide (e.g., potassium iodide).  For example, iodized salt is a physical mixture of sodium chloride (table salt) and an iodide compound, such as potassium iodide.

Overview

Chemical reactions occur at different speeds depending on the properties of the molecules involved and the conditions under which the reactions take place. Scientists and chemistry teachers often use the iodine clock reaction to demonstrate how different reactions occur at different rates. The experiment uses an iodine solution and a reactive agent that contains starch, a complex carbohydrate that can be found in foods such as potatoes, wheat, and corn. The reaction can be accomplished using several different substances, as long as those substances contain the proper base chemicals.

A common method of demonstrating the iodine clock reaction uses products that can be found in the home or products that are easily purchased. The experiment requires distilled water, a 1000-milligram vitamin C tablet, hydrogen peroxide, liquid laundry starch, and an iodine solution. Iodine is typically found in most drug stores as an antiseptic. It is not pure iodine but is dissolved in a solution of water and alcohol. Laundry starch is made from grain products such as corn, rice, or wheat. Although the elements needed for the experiment are relatively safe, iodine and hydrogen peroxide can cause stains that are difficult to remove, so they should be used with care.

A crushed vitamin C tablet should be dissolved in 2 ounces of water. To make the first solution, 1 teaspoon of this mixture is added to 1 teaspoon of iodine and 2 ounces of water. The second solution is made by mixing 2 ounces of water, 3 teaspoons of hydrogen peroxide, and 1/2 teaspoon of starch. The laundry starch acts as an indicator, while the acidic environment helps the hydrogen peroxide react with iodide to produce iodine.

The first solution is poured into the second solution and then transferred to an empty cup. The combined solutions should be poured back and forth several times between cups to mix them thoroughly. When the solutions are mixed, iodine is produced. The vitamin C initially converts the iodine into iodide ions, keeping the solution clear. Once the vitamin C is used up, free iodine accumulates and reacts with the starch, causing the solution to suddenly turn dark blue..

This effect is caused by two separate chemical reactions occurring in the mixed solutions at the same time. In the first reaction, the hydrogen peroxide reacts with the iodide ions to produce elemental iodine. This form of iodine turns dark blue when exposed to starch. However, the solution initially remains clear because vitamin C rapidly converts the iodine back into iodide ions as quickly as it is produced. Once the vitamin C is consumed, iodine accumulates and the solution suddenly turns dark blue.

This second reaction occurs much faster than the first reaction. As soon as elemental iodine is produced by the first reaction, vitamin C rapidly converts it back into iodide ions. Because the amount of vitamin C in the solution is limited, it eventually becomes exhausted. When this happens, iodine is no longer removed as it is produced, allowing it to accumulate in the solution. The iodine then reacts with the starch, turning the solution dark blue.

If the proper amounts of chemical ingredients are used in the experiment, the color change will occur in a predictable amount of time. If the amount of hydrogen peroxide is increased, the first reaction will be sped up, and the time between color changes will be shorter. If more vitamin C is added, the second reaction will be extended and the color change will take longer.

Variations on the experiment can be done with other chemical ingredients; however, many of these ingredients are more dangerous to use or found primarily in professional environments such as chemistry labs. Chemists often use pure forms of the chemical ingredients to perform the experiment. For example, the experiment can be done directly with sulfuric acid and sodium thiosulfate, rather than getting those substances from starch and vitamin C tablets.

Bibliography

Group, Edward. “Understanding the Difference: How Are Iodide and Iodine Different?” Global Healing Center, 25 Feb. 2015, www.globalhealingcenter.com/natural-health/how-are-iodide-and-iodine-different/. Accessed 25 May 2026.

Harrington, Rebecca, and Dave Mosher. “Make a Clear Liquid Turn Black in Midair with This Bizarre Science Experiment.” Business Insider, 5 Dec. 2015, www.businessinsider.com/how-iodine-clock-experiment-works-ingredients-2015-12. Accessed 25 May 2026.

“Iodine.” Office of Dietary Supplements, 17 Feb. 2016, ods.od.nih.gov/factsheets/Iodine-Consumer/. Accessed 25 May 2026.

“The Iodine Clock Reaction.” LibreTexts, 31 Oct. 2024, chem.libretexts.org/Ancillary_Materials/Laboratory_Experiments/Wet_Lab_Experiments/MIT_Labs/Lab_3%3A_Chemical_Kinetics/1_-_The_Iodine_Clock_Reaction. Accessed 20 Nov. 2024.

Panzarasa, G. “Iodine Clocks: Applications and Untapped Opportunities in Materials Science.” Reaction Kinetics, Mechanisms and Catalysis, vol. 135, 2022, pp. 1349–64, doi:10.1007/s11144-022-02202-0. Accessed 25 May 2026.

Patterson, James E., et al. “Modified Iodine Clock Reaction to Introduce the Concept of Activity.” Journal of Chemical Education, vol. 101, no. 9, 14 Aug. 2024, doi:10.1021/acs.jchemed.4c00354. Accessed 25 May 2026.