Stop-’N-Go Light

Introduction

The color of this chemical “traffic light” reaction changes from yellow to red to green and then back to yellow again.

Concepts

• Oxidation–reduction
• Redox indicator

Materials

Safety Precautions

Indigo carmine is moderately toxic by ingestion and is a body tissue irritant. Sodium hydroxide solution is a corrosive liquid and skin burns are possible. It is very dangerous to eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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 Stop-’N-Go Light solution may be neutralized according to Flinn Suggested Disposal Method #10.

Procedure

1. For best results, prepare the indigo carmine indicator solution fresh within one week of use.
2. Place 100 mL of dextrose solution and 100 mL of sodium hydroxide solution into a 500-mL Erlenmeyer flask.
3. Add 10 mL of the indigo carmine solution to the flask. Firmly insert the stopper.
4. Allow the solution to sit undisturbed until it is fully reduced (yellow). It can then be shaken to obtain all of the colors (see Tips).

Student Worksheet PDF

12535_Student1.pdf

Teacher Tips

• The solution will be green initially. Allow the solution to sit undisturbed until it becomes a bright amber (yellow). This may take 20–30 minutes. To decrease this initial waiting period, gently warm the solution to about 35 °C.
• Show the amber solution to the class and then place the flask behind your back and shake it gently. Show the students that the solution is now red. When it changes back to amber, again place the flask behind your back, shake it vigorously, and show the students the green color. A little practice will quickly determine how many shakes are needed for the red color and how many additional shakes for the green color.
• The “traffic light” solution will repeat this yellow to red to green cycle for 20 minutes or so depending on how often it is shaken and how much oxygen is reintroduced by opening the bottle.

Answers to Questions

1. What is an oxidation–reduction reaction?

An oxidation/reduction (or “redox”) reaction occurs when one or more electrons are transferred between molecules. Oxidation refers to a loss of electrons (and rise in oxidation state), and reduction refers to a gain of electrons (and subsequent decrease in oxidation state).

2. Once the indigo carmine is added to the flask containing dextrose solution and sodium hydroxide solution, what physical change is observed to indicate the indigo carmine as been fully reduced?

The color is changed from green to bright amber (yellow).

3. Once the solution has been reduced what do you observe as

a. The solution is shaken gently?

Upon gentle shaking the solution turns red.

b. The solution is shaken more vigorously?

Upon vigorous shaking the solution turns green.

4. What compound reduces and what compound oxidizes the indigo carmine and what color will the solution turn in each scenario?

Indigo carmine is reduced by alkaline sugar and turns amber. Indigo carmine is oxidized by the oxygen in the bottle and turns green.

Discussion

Indigo carmine is an organic redox and acid–base indicator. It exists in different oxidation states having different colors. The reduced form of indigo carmine is yellow, while the fully oxidized form is blue (green in the pH range 11–13 in the “traffic light” solution). There is also an intermediate red form whose structure has not been fully determined. In this demonstration, the blue (oxidized) form of indigo carmine is first added to a solution of dextrose and sodium hydroxide. It immediately turns green due to the pH change. Dextrose is a reducing sugar—it reduces the indigo carmine indicator to the yellow (reduced) form in the presence of base. When the yellow solution containing the reduced form of indigo carmine is then shaken, oxygen from the air above the liquid dissolves in the solution and re-oxidizes the indigo carmine. The overall oxidation occurs in two steps, first to the intermediate red form, and then to the fully oxidized (green) form. This two-step oxidation sequence can be controlled by the rate of shaking, which influences the amount of oxygen that dissolves in the solution. When the solution is no longer being shaken, dextrose molecules again reduce the indigo carmine back to the yellow color. This sequence of oxidation–reduction color changes can be repeated many times. After about 10 or 15 color cycles, when all of the oxygen in the flask has been used up, the redox reaction will cease. (Removing the stopper will introduce more air so the process can be repeated.) See Figure 1 for the structures of the oxidized and reduced forms of indigo carmine, and the proposed structure of the red intermediate (Shakhashiri).

References

Special thanks to Jim and Julie Ealy (retired), The Peddie School, Hightstown, NJ, for providing the instructions for this activity.

Ferguson, H. W.; Schmuckler, J., et al. Laboratory Investigations in Chemistry, Silver Burdett: Parsippany, NJ, 1970.

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers in Chemistry; Univ. of WI, Madison; Vol. 2, pp 145–146.

Soifer, M. and Garber, M., (Students of Joseph Schmuckler, Haverford High School, Haverford, MA), 1969.