Landolt Reaction

Materials

purified water
two cups
two small cups
2.5g of potassium iodate
1.2g of sodium metabisulfite
0.5g of starch
stick

What is Landolt Reaction?

Landolt Reaction, also known as The iodine clock reaction, was discovered by Hans Heinrich Landolt in 1886. Clock reactions show a distinct observation after a certain time. Two colorless solutions are mixed and then after a certain period of time a sudden color change occurs. It's like magic that turns water into Coke. Follow me and try this magical science at home.

Procedures

Label the two cups, solution A and solution B respectively.
Repeat this for two small cups.
Now let’s make two different solutions. In cup A, pour 250ml of purified water and add 2.5g of potassium iodate. Mix well.
In cup B, pour 250ml of purified water. Add 1.2g of sodium metabisulfite and 0.5g of starch. Mix well.
In the small cup A, pour 50ml of solution A.
In the small cup B, pour 50ml of solution B.
Pour solution A into solution B. And watch what’s happening.

Science Behind It

Two colorless solutions are mixed and then after a certain period of time a sudden color change occurs. When solution A meets solution B, two reactions start occurring. The first reaction has no visible reaction. After a certain time, when the first reaction has all reacted, the second reaction starts. As a result of the second reaction, liquid suddenly turns to dark color.

The reaction rate varies with concentration. Concentration means how 'crowded' the solute particles are in a given volume. The higher the concentration, the faster the reaction rate.