Chemical Traffic Light Experiment

 

The chemical traffic light experiment is a redox reaction that changes colors from amber to green to red.

The chemical traffic light experiment is a dramatic redox reaction that changes colors between yellow or amber, green, and red. Shaking the solution then reverses the reaction, so the color change goes from red to green to yellow (like a traffic light). Here is how you perform the chemical traffic light experiment and a look at its chemistry. Also, explore chemical substitutions.

Chemical Traffic Light Materials

The classic color change demonstration uses glucose, indigo carmine, sodium hydroxide, and distilled water:

• 6 grams glucose
• 0.01 grams indigo carmine
• 40 milliliters 1M sodium hydroxide (NaOH) solution
• distilled water

It’s fine using indigo carmine indicator solution. Preparing the solutions several days in advance works well and actually increases the color transition speed.

Perform the Chemical Traffic Light Experiment

1. Dissolve about 6 grams of glucose in 200 milliliters of water distilled water.
2. Add 40 milliliters of the sodium hydroxide solution (3.75 g NaOH in 125 ml water or 1M NaOH).
3. In a separate container, dissolve the indigo carmine in water. The amount you use depends on how deep you want the color. Indigo carmine in water is blue.
4. Mix the indigo carmine indicator solution and the glucose with sodium hydroxide solution. The alkalinity turns the blue solution green.

As the green solution stands, it changes color and becomes red and then yellow. Shaking the solution and mixing it with air and changes the color green. Upon resting, the color becomes yellow or red.

You can repeat the transitions several times before the colors fade. Adding a bit more indicator solution extends the display up to around 50 cycles.

How It Works

Basically, the chemical traffic light is a variation of the blue bottle chemistry demonstration, except using indigo carmine instead of methylene blue. Both demonstrations are examples of redox reactions and are useful for studies of chemical kinetics because temperature affects the color change rate. The chemical traffic light is an example of a clock reaction.

Indigo carmine is a redox indicator that changes from blue in water to green in the alkaline glucose solution. Shaking the solution dissolves oxygen from air into the liquid and oxidizes indigo carmine. Dissolving a small amount of oxygen by lightly swirling the flask turns the liquid red. Vigorous shaking dissolves a lot of oxygen into the solution, oxidizes all of the indigo carmine, and turns the liquid green. As the oxygen concentration drops, the color returns to yellow.

• Blue: pH < 11.4
• Green: pH between 11.4 and 13
• Yellow: pH > 13

The red color comes from the sugar, which is glucose or dextrose. The reducing sugar converts to an enolate. It first reduces indigo carmine into a red semiquinone intermediate and then into a yellow reduced form. Introducing oxygen into the solution by shaking the flask repeats the cycle until all of the sugar is gone.

Substitutes in the Chemical Traffic Light Experiment

There are multiple variations of this demonstration:

• The colors of the reaction depend on pH. Lowering the starting pH to 11.4 changes the color shift to blue, purple, orange, and yellow.
• Substitute potassium hydroxide (KOH) in place of the sodium hydroxide.
• Substitute dextrose instead of glucose.
• You can use other redox indicators in place of indigo carmine and get different color changes. For example, methylene blue changes between blue and colorless.
• A variation of the chemical traffic light uses indigo carmine, ascorbic acid or vitamin C, sodium bicarbonate, sodium chloride, copper(II) sulfate, sodium hydroxide, and water. The ascorbic acid replaces the glucose in the original project. The copper ions act as a catalyst.
• Another variation uses potassium sodium tartrate (Rochelle salt), hydrogen peroxide, and a cobalt(II) salt as a catalyst. This reaction changes colors between green and pink.

Safety

• Indigo carmine stains skin and clothing, so either wear gloves or else stopper the container to avoid splashes.
• Sodium hydroxide is a strong base, so wear goggles and gloves and avoid skin or eye contact.

References

• Engerer, Steven C.; Cook, A. Gilbert (1999). “The Blue Bottle Reaction as a General Chemistry Experiment on Reaction Mechanisms”. Journal of Chemical Education. 76 (11): 1519–1520. doi:10.1021/ed076p1519
• Rajchakit, Urawadee; Limpanuparb, Taweetham (2016). “Greening the Traffic Light: Air Oxidation of Vitamin C Catalyzed by Indicators”. Journal of Chemical Education. 93 (8): 1486–1489. doi:10.1021/acs.jchemed.5b00630
• Shakhashiri, Bassam Z. (1985). Chemical Demonstrations. Madison, Wis.: Univ. of Wisconsin Press. pp. 142–143. ISBN 978-0-299-10130-5.
• Wellman, Whitney E.; Noble, Mark E.; Healy, Tom (2003). “Greening the Blue Bottle”. Journal of Chemical Education. 80 (5): 537. doi:10.1021/ed080p537