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Iodine Clock Reaction

Clock I


Warning: never let solution B stand in open beaker for over an hour. Test existing stock solutions. If demonstration does not work, discard solution B.

Chemicals and Solutions

  • Iodine clock solution A
  • Iodine clock solution B

Solution Preparation

Solution A

0.02 M KIO3 

Solution B 

4g soluble starch, 0.2g sodium metabisulfite (NaSO₅), 5mL 1M sulfuric acid in 1L solution


  • Electronic stop clock
  • Ice bath in large refrigerator dish
  • 100 mL cylinders
  • 400 mL beakers
  • Magnetic stirrer and stirring bars for both beakers


  1. Into cylinders, put 100 mL of solution A.
  2. Into beakers, put 100 mL of solution B.
  3. Put one cylinder and beaker into ice bath at least 15 minutes before expected use.
  4. Mix together and observe the color change.
  5. ALTERNATIVE: to illustrate effect of concentration, put differing amounts of solution A in cylinders and top off with DI water, mixing each cylinder with an equal volume of solution B (e.g. 50 mL solution A with 50 mL DI water mixed with 100 mL solution B). Measure rate of reaction with a stop clock.


A simplified explanation of the reaction is as follows:

I- reacts with IO- to form I.

\( \ce{ 5HI_{(aq)} + HIO3_{(aq)} -> 3I2 + 3H2O } \)

I is immediately consumed by reaction with HSO-.

\( \ce{ I2 + HSO3- + H2O -> 2I- + SO42-_{(aq)} + 3H+_{(aq)} } \)

When HSO- has been consumed, I accumulates. I + starch forms a blue colored starch-I complex.

When you dilute solution A (0.02 M KIO) in half, it takes twice as long to form the blue starch - I complex. When solution A is diluted to 1/4 the concentration, it takes four times as long to form the blue starch-I complex. The reaction is much slower at colder temperatures.


The solutions can be poured down the drain. (The bisulfite and iodate are consumed and the iodine is complexed with starch so there is no oxidizer hazard). Demo generates 800 mL of 0.2% starch iodine complex aq.


Summerlin and Ealy, Chemical Demonstrations, pp. 75-76.


Clock II (Oscillating Clock)


30% hydrogen peroxide is very reactive.

Chemicals and Solutions

Solution #1

36 mL of 30% HO to 100 mL (make fresh)

Solution #2

43 g KIO₃, 4.3 mL conc. sulfuric acid, in 1L solution

Solution #3

  1. 15.6 g malonic acid, 3 g of MnSO in 970 mL of water.
  2. Stir in 30 mL of a 1% starch solution.


  • magnetic stirrer and bar
  • Three 100 mL graduated cylinders
  • 400 mL beaker


  1. Measure out 100 mL of each solution into graduated cylinders.
  2. With stirring, quickly add each solution to the 400 mL beaker. The solution will oscillate between colorless, amber and dark blue.
  3. Clock will oscillate for about 5 minutes typically.

Hint: When no stir bar is used, regions of the solution will change first.


The oscillations are due to the shifting concentrations of I and I-. The amber color is due to the presence of I. When I- is present, it reacts with I and starch to produce a dark blue complex. This color fades as iodine is consumed.

A very simplified explanation of this reaction is:

\( \ce{ $\underset{\text{gold}}{\ce{ 2HIO3 + 5H2O2 -> I2 + 5O2 + 6H2O }}$ } \)

\( \ce{ I2 + CH2(COOH)2 -> ICH2(COOH)2 + H+ + I- } \)

\( \ce{ $\underset{\text{dark blue}}{\ce{ I2 + I- + starch -> starch-iodine complex }}$ } \)

\( \ce{ $\underset{\text{colorless}}{\ce{ I2 + 5H2O2 -> 2HIO3 + 4H2O }}$ } \)


The hydrogen peroxide and iodate are consumed and the iodine is complexed with starch so there is no oxidizer hazard. Therefore the solution can be rinsed down the drain. Demo generates 300 mL of 2% starch iodine complex aq.