Traffic Light Reaction

Multi-color redox indicator changes

Difficulty: Easy | Time: 15 minutes | Visual Impact: Very High

Historical Context

The traffic light reaction is an extension of the classic blue bottle experiment, developed as a chemistry demonstration in the late 20th century. By using an indicator with multiple oxidation states, the reaction produces a sequence of colors reminiscent of traffic lights.

The reaction became popular in chemistry education because it demonstrates that redox reactions can involve multiple electron-transfer steps, not just simple two-state changes. Each color represents a different oxidation state of the indicator molecule.

Materials

Procedure

  1. Dissolve 8g dextrose in 300mL water
  2. Add 10g sodium hydroxide and stir until dissolved
  3. Add indigo carmine indicator solution
  4. Shake vigorously - solution turns green
  5. Let stand - changes to yellow/amber, then to red
  6. Shake again - cycles back through green → yellow → red
  7. Colors represent different oxidation states of the indicator

The Science

Indigo carmine has three oxidation states with different colors:

State Color Condition
Oxidized Green Just after shaking (oxygen present)
Intermediate Yellow Partially reduced
Fully reduced Red Fully reduced by glucose

The cycle: 1. Shake → Oxygen dissolves, oxidizes indicator to green 2. Stand → Glucose reduces indicator through yellow to red 3. Repeat → Shaking reintroduces oxygen

Variation: Using methyl red and methylene blue together produces similar color changes as each indicator responds to different redox potentials.

Reaction

\[\ce{Indicator_{ox} (green) + glucose ->[\text{slow}] Indicator_{red} (red)}\] \[\ce{Indicator_{red} (red) + O2 ->[\text{fast}] Indicator_{ox} (green)}\]

The intermediate yellow state is a partially reduced form.

Resources