The Pink Catalyst

Introduction

Add a pink cobalt chloride solution to a colorless solution containing potassium sodium tartrate and hydrogen peroxide and watch as a very obvious green-colored complex forms. As the reaction ends, the solution will return to its original pink color—mindicating that the cobalt chloride catalyst is not used up in the reaction.

Concepts

• Catalysts
• Kinetics

Materials

Safety Precautions

Cobalt(ous) chloride is toxic by ingestion (LD₅₀ 766 mg/kg) and causes blood damage. Hydrogen peroxide is an oxidizer and a skin and eye irritant. Although potassium sodium tartrate solution is considered non-hazardous, do not ingest the material. Avoid body tissue contact with all chemicals. Wear chemical-resistant goggles, chemical-resistant gloves and a chemical resistant apron.

Disposal

Dispose of the final solution according to Flinn Suggested Disposal Method #27d. Consult your current Flinn Scientific Catalog/Reference Manual for proper disposal procedures.

Procedure

1. Using a graduated cylinder, measure out 100 mL of 0.21 M potassium sodium tartrate solution. Pour it into a 600-mL beaker.
2. Warm the solution slowly to 70 °C on a hot plate.
3. While waiting for the temperature of the solution to increase, measure out 12–14 mL of 0.1 M cobalt chloride solution in a 25-mL graduated cylinder. Show this solution to the class so that the students can note the pink color of the catalyst.
4. When the temperature of the potassium sodium tartrate solution reaches 70 °C, add 40 mL of 6% hydrogen peroxide and the cobalt chloride catalyst to the 600-mL beaker. Stir continuously.
5. The solution will go through a series of color changes as the cobalt chloride begins to catalyze the reaction. The solution will start out pink (the color of cobalt chloride) and then darken to a brown before lightening up to a yellow-orange and finally becoming an olive green color. At this point, the reaction mixture is bubbling vigorously.
6. Once the bubbling subsides, the solution will progress back through the series of colors and return to the original pink color of the cobalt chloride solution.

Teacher Tips

• If you are going to time the reaction, the use of a hot plate-magnetic stirrer greatly aids the demonstration of this reaction. Otherwise, continuous stirring is necessary.
• Begin timing the reaction upon addition of the hydrogen peroxide and cobalt chloride solution. Complete the timing after the vigorous reaction subsides and the original pink color of the cobalt chloride solution has returned.
• The use of a large beaker is important so that the reaction does not froth over.

Discussion

The solution starts out pink due to the pink color of the cobalt chloride catalyst. The solution turns green forming an intermediate between the catalyst and potassium sodium tartrate. The solution returns to the original pink color of the cobalt chloride solution demonstrating and confirming the fact that a catalyst does not get used up in a chemical reaction.

Based on experimental findings, the following reactions can be suggested as taking place in the Co(II)–H₂O₂–tartaric acid system:

On the action of hydrogen peroxide, the cobalt(II)–tartrate complex becomes oxidized to a green, probably binuclear, Co(III)–tartrate compound. This cobalt(III)–tartrate is reduced both by tartaric acid and hydrogen peroxide to Co(II)–tartrate with a concomitant evolution of CO₂ and O₂, respectively. Since the color of the solution is green throughout the reaction, and most of the cobalt is present as Co(III), then the first step (oxidation) is most likely faster than the reduction of Co(III)–complex. (Toth, 1980).

This demonstration also allows you to demonstrate kinetics—the effect of temperature on the rate of a chemical reaction. For each 10 °C increase in temperature, the reaction rate will approximately double. The reaction may be timed at various temperatures. Suggested temperatures and their corresponding reaction times are:

50 °C—200 seconds; 60 °C—90 seconds; 70 °C—40 seconds

References

Deroo, Julius, Sci Teach., 1974, 41, 44.

Ruda, Paul T., J. Chem. Educ., 1978, 55, 652.

Toth, Zoltan, J. Chem. Educ., 1980, 57, 464.

Special thanks to Jim and Julie Ealy, The Peddie School, Hightstown, NJ, for bringing this demonstration to our attention.