Iodine Clock Reaction

In chemistry, kinetics is the term we use when measuring and studying the rates of reactions. Chemical (or reaction) kinetics involves investigations of how different experimental conditions impact the rate of a chemical reaction. We also use kinetics to make reaction equations that can describe the characteristics of a chemical reaction, such as the physical states of the product(s)!

One classic experiment that showcases two chemical reactions taking place is the iodine clock reaction, discovered by Hans Heinrich Landolt in 1886. He took two colorless solutions and mixed them together. After a little time had passed, the solution suddenly turned dark blue! When he repeated the experiment, he found that the solution turned blue after the same amount of time had passed. Clock reactions (or oscillating reactions) are labeled as such because there is a sudden property change after a predictable amount of time. 

In this case, the change in property happens to be a colorless solution abruptly becoming a much darker color.


​Recreate the Experiment at Home!

Before we get into the reaction equations, it might be easier to see what’s going on instead of trying to imagine it. Luckily, we can recreate this famous experiment with a few items you might have in your medicine cabinet! All of the chemicals you’ll be using are completely safe, but iodine is very good at staining your skin and clothes, so you may want to wear some latex gloves just to be safe. All of the materials can be found in a drugstore or supermarket!


What You’ll Need:

• Distilled (or tap) water
  
• A few disposable cups
  
• One 1000 mg vitamin C tablet
  
• Tincture of Iodine (2%)
  
• Hydrogen peroxide (3%)
  
• Liquid laundry starch
  

First, prepare three solutions: a vitamin C stock, an iodine solution, and a hydrogen peroxide solution. For the vitamin C stock, crush the tablet and dissolve it in 2 oz (59 mL) of water.

The next solution will be one of the reactants. Mix 1 tsp (5 mL) of the vitamin C stock with 1 tsp of iodine and 2 oz of water. Label this as “Solution A”.

To prepare the second reactant, add 2 oz of water to 1 tbsp (15 mL) of hydrogen peroxide and 1/2 tsp (2.5 mL) of liquid starch solution. This will be “Solution B”.

Now you’re ready to begin the reaction! Carefully pour Solution A into Solution B. Then pour the entire solution into the now empty cup. Continue transferring the solution back and forth until you see the color change!


What is Happening?

Because the laundry starch contains sulfuric acid, we created the first reacting solution: hydrogen peroxide with sulfuric acid. Then we added it to a solution containing potassium iodide, sodium thiosulfate, and starch. When these are combined, it creates both the elemental and ion forms of iodine.

First Reaction:

In this reaction, iodide ions react with hydrogen peroxide to produce elemental iodine, which is blue in the presence of starch. However, before that can happen, the vitamin C quickly reacts and consumes the elemental iodine (that's what is happening in the second reaction below). That’s why we don’t immediately see the change of color!


Second Reaction:

The second reaction is much quicker than the first, so the reactions can proceed for a short amount of time before the thiosulfate (S4O62-) ion is exhausted in the experiment and the iodine-starch complex’s blue color is visible.
You can repeat this experiment as many times as you’d like! You could even try timing the reaction to see if the blue color appears at the exact same time in each trial.



References:

“Iodine Clock Reaction”. Imagination Station, www.imaginationstationtoledo.org.



Image Credits:

“Iodine Clock”. Released into the public domain under the Creative Commons Attribution-Share Alike 3.0 Unported license. Uploaded on 2/12/18 from commons.wikimedia.org

Hodan, George. “Laboratory Glassware”. Released into the public domain. Uploaded on 2/13/18 from publicdomainpictures.net