Chemical traffic light

Solution change colors as a traffic light

Scientific name: Indigo carmine in alkaline glucose solution undergoes a reversible redox reaction with oxygen from air


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Wear eye protection goggles. Prevent skin contact with base solutions.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

C16H8N2Na2O8S2 + HOCH2(CHOH)4COH + H2O =[OH-] → C16H10N2Na2O8S2 + HOCH2(CHOH)4COOH

2C16H10N2Na2O8S2 + O2 → C16H8N2Na2O8S2 + H2O

Step-by-step instruction

  1. Pour some distilled water (about 300 ml) into the flask and dissolve 8 grams of sodium hydroxide (or potassium hydroxide).
  2. Dissolve 10 grams of glucose in this solution.
  3. Prepare indigo carmine water solution (0.25 g in 25 mL distilled water) in another flask.
  4. Pour the indigo carmine solution into the flask with glucose and base solution.
  5. The solution changes color from green to yellow, passing through red.
  6. Shake the flask moderately – the solution becomes red and then returns to yellow.
  7. Shake the flask intensively – the solution becomes green, then red, then yellow. You can repeat shaking several times.

Scientific background

The oxidized form of indigo carmine is green in an alkaline solution. The indicator reduces in a glucose alkaline solution. The intermediate form of indigo carmine is red. The fully reduced form of the indicator is yellow.

When the flask is shaken, oxygen from air dissolves in the solution and oxidizes indigo carmine. Solution becomes red if a small amount of oxygen is dissolved (moderate shaking), and green if all of indigo carmine was oxidized (intensive shaking).


Published on 22 February 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst