Department of Chemistry

OSCILLATING CLOCK

Oscillating Clock pdf version

Through this reaction, students get to experience first hand a phenomenon that still puzzles scientists to this day. Combined in one beaker are: starch, hydrogen peroxide, and potassium iodate. In front of their eyes the solution changes color from a rich, deep blue, to colorless, then back to deep blue again (with a periodicity of about 1 minute).

Materials Preparation

  • 30% H2O2
  • KIO3
  • Concentrated H2SO4
  • CH2(CO2H)2, Malonic Acid
  • MnSO4∙H2O
  • Starch

Presentation

  • Solutions A, B, and C
  • 1 L Beaker
  • 250 mL Beakers (3)
  • Magnetic Stirrer with Stir Bar
  • Na2S2O3 (for quenching)

Preparation

Solution A: 3.6 M H2O2

  • Pour 400 mL of distilled water into a 2 L beaker.
  • Wearing gloves, pour 410 mL of 30% H2O2 into the beaker of water. Dilute the solution to 1 L with distilled water. (This solution should be prepared fresh for each day of demonstrations.)

Solution B: 0.2 M KIO3 and 0.08 M H2SO4

  • Place 43 g of KIO3 and ~800 mL of distilled water in a 2 L beaker.
  • Add 4.3 mL concentrated H2SO4 to this mixture.
  • Warm and stir the mixture until the KIO3 has dissolved.  Dilute to 1 L with distilled water.

Solution C: 0.15 M CH2(COOH)2 0.02 M MnSO4H2O, and Starch

  • Dissolve 16 g of malonic acid and 3.4 g of MnSO4 ∙ H2O in ~500 mL of distilled water in a 2 L beaker.
  • In a 100 mL beaker, heat 50 mL of distilled water to a boil.
  • In a 50 mL beaker, mix 3 g of soluble starch with ~5 mL of distilled water and stir to form a slurry. Pour the slurry into the boiling water and continue heating and stirring until the starch has dissolved.
  • Pour this starch solution into the malonic acid/MnSO4∙H2O mixture.
  • Dilute the mixture to1 L with distilled water.

Presentation

Put a stir bar in the 1000 mL beaker provided, and place it on the magnetic stirrer.

Add equal amounts (~200 mL) of solutions A and B to the beaker and turn on stirrer. Once a good vortex has been established, add an equivalent amount of solution C

At the end of the show, add 10 g of Na2S2O3 to the mixture and stir until the solution is colorless.  Flush down the drain with water.

*This procedure was taken from Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; The University of Wisconsin Press: Madison Wisconsin, 1985; Vol. 2, pp 248-256.

Background

The Oscillating Clock demonstration, based on the Briggs-Rauscher (BR) reaction, provides a visually impressive reaction in which a solution oscillates in color between amber-orange and blue-black. The solution will oscillate approximately 10-15 times, with the time between color changes increasing as the reaction proceeds.  After about 15 minutes, the oscillations will stop and the solution will remain blue-black in color.

The BR reaction was developed as a hybrid of two other oscillating reactions, the Bray-Liebhafsky (BL) reaction and the Belousov-Zhabotinsky (BZ) reaction. In the BL reaction, H2O2 was used as both an oxidizing and reducing agent to cause oscillations between I2 and IO3-.  In the BZ oscillating reaction, organic compounds such as malonic acid were used along with electron transfer agents such as Mn(II) ions. The BR reaction utilized the H2O2 and IO3- from the BL reaction and the malonic acid and Mn(II) ions from the BZ reaction to develop a reaction that caused the oscillation of the evolution of oxygen and carbon dioxide gases and the concentration of iodine and iodide ions, which accounts for the observed color changes.

The mechanism for the BR reaction has been studied extensively, and the origin of the [I2] and [I-] oscillations has been explained. However, the proposed mechanism is unable to account for the production of CO2 gas and does not identify the final organic products of the reaction.  This reaction, therefore, is a prime example of the ongoing exploration by chemists of phenomena that we can visibly observe, but not yet explain.

The color changes in the reaction are a direct consequence of the change in concentration of I- and I2. When the concentration of I- is low, I2 is produced rapidly by the mechanism, giving the solution an amber-orange color. With this increase in the concentration of I2, there is a subsequent increase in the production of I-.  The I- and I2 react with the starch in solution and together form a complex that is blue-black in color, thus causing the color of the solution to oscillate. As the reaction proceeds, the concentration of I2 decreases, while the amount of I- in solution remains fairly constant.  At this point, the I- concentration plummets causing the solution to return to amber-orange and the cycle to begins anew.  These oscillations continue until either the malonic acid or the IO3- is used up.

The overall reaction for the Oscillation Clock is shown below:

IO3- + 2H2O2 + CH2(CO2H)2 + H+ --> ICH(CO2H)2 + 2O2 + 3H2O

Reaction (1) is actually the sum of 2 component reactions, (2) and (3):

(2)IO3- + 2H2O2 + H+ --> HOI + 2O2 + 2H2O

(3)HOI + CH2(CO2H)2 --> ICH(CO2H)2 + H2O

Reaction (2) can occur by two different processes, a radical process and a nonradical process, and the occurrence of one rather than the other is determined by the I- concentration. Although not included in the above reactions, the Mn2+ ions in solution function as catalysts for the radical process. It is beyond the scope of this demonstration to explain the process in significant detail. However, if further information is needed, please see the reference in the Procedure section.

Safety Concerns

MSDS sheets for all involved chemicals can be found in the Appendix. 30% H2O2 is a strong oxidizing agent. In case of contact with skin or eyes, flush affected areas with water and seek medical attention (if eyes are affected).