Fantastic Four-Color Oscillator

Introduction

You won’t believe your eyes as you watch this amazing oscillating reaction! This four-color oscillator will get your students’ undivided attention as they observe a solution flash from green to blue to purple to red. And that’s not all—this four-color cycle will repeat itself for well over an hour!

Concepts

• Oscillating reactions
• Oxidation–reduction reactions
• Kinetics and catalysis
• Reaction mechanisms

Materials

Safety Precautions

A small amount of elemental bromine gas is released from the reactions in this demonstration; adequate ventilation is necessary. Potassium bromate is a strong oxidizing agent and poses a fire risk in contact with organic material; it is a strong irritant and moderately toxic. Malonic acid is a strong irritant, slightly toxic and corrosive to eyes, skin and respiratory tract. Potassium bromide is slightly toxic by ingestion and a severe body tissue irritant. Cerium(IV) ammonium nitrate is a strong oxidizer and a skin irritant. Ferrous sulfate is slightly toxic by ingestion and 1,10-phenanthroline is highly toxic by ingestion. Sulfuric acid solution is corrosive to eyes, skin, mucous membrane and other body tissue. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The reaction mixture should be neutralized with sodium carbonate and flushed down the drain with excess water according to Flinn Suggested Disposal Method #24a.

Procedure

1. Place a 1-L beaker on the magnetic stirring plate and place the magnetic stirring bar in the beaker.
2. Pour 250 mL of Solution A and 250 mL of Solution B into the 1-L beaker.
3. Adjust the stirrer speed to produce a vortex in the solution. The solution may become amber, and will turn colorless after about one minute.
4. Once the solution is colorless, add 250 mL of Solution C and 15 mL of Solution D. (Note: The solution composition is now 0.077 M BrO₃ ^–, 0.10 M malonic acid, 0.020 M Br^–, 0.0063 M Ce⁴⁺, 0.90 M H₂SO₄ and 0.17 mM ferroin.)
5. Keep stirring the green cloudy mixture and it will become a green solution. Over a period of about a minute, the color of the solution will change from green to blue, then to violet and finally to red-brown.
6. The color of the solution will suddenly return to green, and the cycle will repeat itself more than 20 times, lasting over an hour.

Student Worksheet PDF

12938_Student1.pdf

Teacher Tips

• There is enough of each solution to perform the demonstration at least seven times. The demonstration can easily be scaled up or down to suit your needs. Solution shelf life is good.
• The discussion and explanation of this oscillating reaction can be quite complicated for beginning chemistry or general science students. However, it is not necessary to fully understand the reaction mechanism in order to appreciate the spectacular chemistry that occurs in this demonstration. Thus, this demonstration can be performed at any level of science with the explanation suited to the level of the class. A complete discussion is included in this handout; however, further information can be found by reviewing the original reference (see References section).
• This reaction is very sensitive to chloride ion. Do not use tap water or rinse glassware with tap water.

Answers to Questions

1. Write the chemical equation for the overall reaction and describe what is happening.

   3CH₂(CO₂H)₂ + 4BrO₃^– → 4Br^– + 9CO₂ + 6H₂O
   The overall reaction in this demonstration is the cerium-catalyzed oxidation of malonic acid by bromated ions in dilute sulfuric acid. The bromated ions are reduced to bromide ions, while the malonic acid is oxidized to carbon dioxide and water.

2. This oscillating reaction mechanism involves two competing processes. Process A involves ions and two-electron transfers and process B involves radicals and one-electron transfer.
   1. What determines the dominant process at any given time?

      The dominant process is dependent upon the bromide ion concentration.

   2. Why do oscillations occur?

      The dominant process is dependent upon the bromide ion concentration.

   3. What is the name of the indicator used in this experiment?

      Ferroin is the indicator in this experiment.

3. The reaction oscillates between Processes A and B, triggered by changes in bromide ion concentration. During these processes cerium and ferroin also oscillate causing several color changes. Explain chemically what was happening between the cerium and iron as the following colored solutions were observed?
   1. Green solution: The yellow Ce(IV) is oxidizing Fe(II) to Fe(III) ; a small amount of Fe(II) has been oxidized to the blue Fe(III) complex, thus yellow and blue form a green solution.
   2. Blue solution: All Ce(IV) is reduced to colorless Ce (III); all Fe(II) is oxidized to the blue Fe(III)complex. Thus the solution is blue.
   3. Violet solution: The colorless Ce(III) is reducing the blue Fe(III) complex to the red Fe(II); the mixture of red and blue appears violet. d. Red solution: All of the blue Fe(III) is reduced to the red Fe(II) complex; colorless Ce(III) is present therefore the solution appears red.

Discussion

This oscillating reaction demonstrates the classic Belousov-Zhabotinsky (BZ) reaction which is a cerium-catalyzed bromate–malonic acid reaction.

The overall reaction occurring in this demonstration is the cerium-catalyzed oxidation of malonic acid by bromate ions in dilute sulfuric acid. The bromate ions are reduced to bromide ions, while the malonic acid is oxidized to carbon dioxide and water. The overall reaction can be represented by Equation 1:

In order to gain some understanding and appreciation for how this overall reaction can produce the amazing, repetitive color changes observed in the demonstration, it is necessary to look at the reaction mechanism or, in other words, how the reactants are transformed into products.

The mechanism involves two different competing processes—Process A involves ions and two-electron transfers; Process B involves radicals and one-electron transfers. The dominant process at any particular time is dependent on the bromide ion concentration. Process A (see Equation 2a) occurs when the bromide ion concentration rises above a certain critical level, while Process B (see Equation 3a) is dominant when the bromide ion concentration falls below a certain critical level. Oscillations occur because Process A consumes bromide ions, leading to conditions which favor Process B. Process B (indirectly) produces bromide ions, which leads to conditions which favor Process A.

Process A Bromate ions are reduced by bromide ions through a series of oxygen transfers (two-electron reductions) as shown in Equation 2a. This reaction occurs when Solutions A and B are mixed. The amber color which may develop is caused by the production of elemental bromine. This color soon disappears as the bromine reacts with malonic acid as shown in Equation 2b. Process A results in an overall decline in the bromide ion concentration and, once the necessary intermediates are generated and most of the bromide ions are consumed, the rate becomes negligible and Process B takes over.

Process B Bromate ions are reduced by cerium(III) ions to produce bromine through a simple redox reaction as shown in Equation 3a. Process B produces Ce(IV) ions and Br2. Both of these species react at least in part to oxidize the malonic acid (see Equation 2b) and the bromomalonic acid (see Equation 3b) to form additional bromide ions. As the concentration of bromide ions increases, the rate of Equation 2a increases until eventually Process A once again dominates. As the reaction oscillates between Process A and Process B, triggered by changes in the bromide ion concentration, concentrations of other species in solution oscillate as well—these concentration changes will explain the color changes observed. While Process A occurs, the cerium ions are in their reduced state, Ce(III). During Process B, some cerium ions are oxidized to Ce(IV) and thus the ratio of Ce(III) to Ce(IV) oscillates as well. 

The indicator used in this demonstration is ferroin, which is tris(1,10-phenanthroline) ferrous sulfate. As the concentration of Ce(IV) increases, the Ce(IV) oxidizes the iron in ferroin from Fe(II) to Fe(III). The Fe(II) complex is red while the Fe(III) complex is blue; thus the color of the solution changes as the iron is oxidized. As the concentration of Ce(III) increases, the Fe(III) is reduced back to Fe(II) and the color of the solution changes accordingly.

The color changes in this demonstration, however, are more complex than simple red-blue oscillations from the ferroin. There are also changes in color due to the cerium ions in solution—Ce(III) is colorless while Ce(IV) is yellow. A simplified equation to help explain the color changes is shown in Equation 4: A possible explanation for the appearance of the oscillating colors in solution is provided; however, a more complete understanding of the color changes might be gained by reviewing the original references.

Green = The yellow Ce(IV) is oxidizing Fe(II) to blue Fe(III); a small amount of Fe(II) has been oxidized to the blue Fe(III) complex; thus, the mixture of yellow and blue forms a green solution.
Blue = All Ce(IV) is reduced to colorless Ce(III); all Fe(II) is oxidized to the blue Fe(III) complex; thus, the solution is blue.
Violet = The colorless Ce(III) is reducing the blue Fe(III) complex to the red Fe(II); the mixture of blue and red appears violet.
Red = All of the blue Fe(III) is reduced to the red Fe(II) complex; colorless Ce(III) is present; the solution appears red.

References

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison;1985; Vol. 2, pp 257–261.