Introduction

Three colorless solutions are combined into a large beaker and the mixture becomes amber, then blue-black, and then colorless again. This sequence of color changes will repeat with a period of approximately 15 seconds at 25 oC. The reaction last about 5 min.

The Briggs-Rauscher Reaction

IO3- + 2 H2O2 + CH2(CO2H)2 + H+ --> ICH(CO2H)2 + 2 O2 + 3 H2O                            (1)

This reaction can be broken into two component reactions:

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

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

This reaction can occur by a radical process which is turned on when I- concentration is low, or by a non-radical process when the I- concentration is high. Both processes reduce iodate to hypoiodous acid. The radical process forms hypoiodous acid at a much faster rate than the non-radical process.

The second reaction (3) also consists of two component reactions:

I- + HOI + H+ --> I2 + H2O                                                                                           (4)

I2CH2(CO2H)2 --> ICH2(CO2H)2 + H+ + I-

The amber color results from the production of the I2. The I2 forms because of the rapid production of HOI during the radical process. When the radical process is occurring, HOI is created faster than it can be consumed. Some of the HOI is used while excess is reduced by hydrogen peroxide to I-. The increasing I- concentration reaches a point at which the non-radical process takes over. However, the non-radical process does not produce HOI nearly as fast as the radical process, so the amber color begins to clear as I2 is consumed more quickly than it can be created. Eventually the I- concentration drops low enough for the radical process to restart so the cycle can repeat itself.

The deep blue color is the result of the I- and I2 binding to the starch present in the solution.

Supplies

Short List

  •  4 x 2L beaker
  • 100 mL beaker
  • 50 mL beaker
  • 4 x Large magnetic stir bar
  • Stir plate
  • Stirring rod
  • White poster board
  • Ring stand
  • DI Water
  • Goggles, gloves, lab coat

Chemicals

  • 30% hydrogen peroxide (H2O2), 410 mL
  •  Conc. Sulfuric acid ( H2SO4) , 4.3 mL – or 8.6 mL 6.0M
  • Soluble starch, 0.3g
  • Potassium iodate (KIO3), 29g
  • Malonic acid (CH2(CO2H)2), 10.4 g
  • Manganese (II) sulfate monohydrate (MnSO4*H2O), 2.2g
  • Sodium thiosulfate (Na2S2O3), 20 g

Prior to Lecture  (prep time ~ ? min.)

Assemble the following:

  1. Prop the white poster board up with the ring stand$1§  Prop the white poster board up with the ring stand
  2. Place the stir plate on the base of the ring stand
  3. Place a 2L beaker with a large stir bar on the stir plate.

      Prepare the following chemicals:

  1.  Solution A: Pour 400 mL distilled water in a 2L  beaker. Wearing gloves, pour 410mL of 30% hydrogen peroxide into the beaker of water. Dilute the solution to 1L with distilled water. This solution is 4.0M H2O2.
  2. Solution B: Place 43 g of potassium iodate and approximately 800 mL distilled water in the second 2L beaker. Add 4.3 mL concentrated H2SO4 to this mixture. Warm and stir the mixture until the potassium iodate dissolves. Dilute the solution to 1L with distilled water. This solution is 0.2M KIO3 and 0.077M  H2SO4.
  3. Solution C: Dissolve 16 g of malonic acid and 3.4 g Manganese (II) sulfate monohydrate in approximately 500 mL distilled water in the third 2 L beaker. In the 100 mL  beaker heat 50 mL distilled water to a boil. In the 50 mL beaker mix 0.3 g soluble starch with about 5 mL distilled water and stir the mixture to form a slurry. Pour the slurry into the boiling water and continue heating and stirring the mixture until the starch has dissolved (1-2 min). The solution may be slightly turbid. Pour the starch solution into the solution of malonic acid and manganese (II) sulfate. Dilute the mixture to 1 L with distilled water. This solution is 0.15M malonic acid and 0.02M MnSO4.

To Conduct Demonstration

Pour 500 mL of solution A and 500 mL of solution B into the beaker on the magnetic stirrer. Adjust the stirring rate to produce a large vortex. Pour 500 mL of solution C into the mixture. The initially colorless mixture will become amber almost immediately, then it will suddenly turn blue-black, fade to colorless and the cycle will repeat with a period of about 15 seconds. After a few minutes the solution will remain blue-black.

Demo Time: ~5 minutes

Notes:

  1. Chloride ion concentrations in excess of 0.07M suppress the oscillations; therefore, the glassware used must be cleaned well.
  2. If contaminated (even from dust) 30% hydrogen peroxide will decompose and generate large quantities of oxygen gas.

Safety and Disposal

Safety:  This demonstration includes chemicals that are irritants and strong oxidizing agents. It also evolves large amounts of iodine in solution, in suspension and also as a vapor above the reaction mixture. The vapor and solid is irritating to eyes, skin, and mucous membrane. Avoid contact between 30% hydrogen peroxide and combustible materials..

Perform in a well ventilated  room, preferable in a hood. Wear goggles.

Disposal:  large amounts of elemental iodine (I2) are produced and need to be reduced to iodide ions before disposal. Carefully add 10g. sodium thiosulfate to each mixture and stir it until the mixture becomes colorless.Caution: the reaction between iodine and thiosulfate is exothermic.Once cool, the mixture may be washed down the drain with water.