Iodine Clock Reaction

Clock I

Hazards

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 (Na₂S₂O₅), 5mL 1M sulfuric acid in 1L solution

Materials

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

Procedure

  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.

Discussion

A simplified explanation of the reaction is as follows:

I- reacts with IO₃- to form I₂.

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

I₂ is immediately consumed by reaction with HSO₃-.

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

When HSO₃- has been consumed, I₂ accumulates. I₂ + starch forms a blue colored starch-I₂ complex.

When you dilute solution A (0.02 M KIO₃) in half, it takes twice as long to form the blue starch - I₂ complex. When solution A is diluted to 1/4 the concentration, it takes four times as long to form the blue starch-I₂ complex. The reaction is much slower at colder temperatures.

Disposal

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.

References

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

 

Clock II (Oscillating Clock)

Hazards

30% hydrogen peroxide is very reactive.

Chemicals and Solutions

Solution #1

36 mL of 30% H₂O₂ 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 MnSO₄ in 970 mL of water.
  2. Stir in 30 mL of a 1% starch solution.

Materials

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

Procedure

  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.

Discussion

The oscillations are due to the shifting concentrations of I₂ and I-. The amber color is due to the presence of I₂. When I- is present, it reacts with I₂ and starch to produce a dark blue complex. This color fades as iodine is consumed.

A very simplified explanation of this reaction is:

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

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

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

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

Disposal

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.