Stoplight Reaction
Difficulty: Easy | Time: 15–20 min | Visual Impact: Very High
Background
Indigo carmine is a redox indicator: it exists in distinct colored forms depending on its oxidation state. In the presence of glucose and sodium hydroxide, the dye is slowly reduced to a pale yellow form. Shaking the bottle dissolves oxygen from the air, which reoxidizes the dye — and on the way back to fully oxidized blue, it passes through red and green intermediates. The sequence plays out vividly and reverses completely: a single bottle can be cycled ten or more times before the glucose is exhausted.
The reaction is closely related to the methylene blue Blue Bottle and Traffic Light experiments, but the color range is wider — four distinct colors rather than two — and the intermediate states are stable enough to observe clearly.
Colour Sequence
| State | Colour | What it represents |
|---|---|---|
| Resting (reduced) | Yellow | Fully reduced leucoindigo carmine |
| Intermediate | Red / orange | Partial reoxidation |
| Intermediate | Green | Blue + yellow mixed, near the transition |
| Shaken (oxidized) | Blue | Fully oxidized indigo carmine |
Shaking drives yellow → red → green → blue. Setting the bottle down drives blue → green → red → yellow. The speed depends on temperature and glucose concentration.
Materials
| What | How much |
|---|---|
| Indigo carmine | 0.1 g |
| Sodium hydroxide | 8 g |
| Dextrose | 8 g |
| Water | 500 mL |
| Sealable 500 mL bottle or flask | |
| Safety glasses |
Procedure
- Dissolve the sodium hydroxide in 500 mL of water in the bottle. The solution will warm up — let it cool to room temperature.
- Add the dextrose and swirl until fully dissolved.
- Add the indigo carmine and seal the bottle. Swirl gently. The solution should turn yellow within a minute or two as glucose reduces the dye. If it stays blue, wait a few more minutes.
- Once yellow, shake vigorously for a few seconds. Watch the colour cycle upward: yellow → red → green → blue.
- Set the bottle down and watch it cycle back: blue → green → red → yellow.
- Repeat. The cycle works 10–15 times before the glucose is consumed and the solution stays permanently blue.
Tip: If the solution goes blue and stays blue immediately after preparation, the glucose has already been exhausted or the concentration is too low. Start fresh with slightly more glucose.
What Is Happening
Indigo carmine (blue, oxidized) accepts electrons from glucose in alkaline conditions, becoming leucoindigo carmine (yellow, reduced):
\[\text{Indigo carmine (blue)} + 2e^- + 2H^+ \rightleftharpoons \text{Leucoindigo carmine (yellow)}\]
The partially reduced semiquinone intermediate is red/orange, and the visual green arises from a mixture of blue and yellow forms present simultaneously during the transition.
When you shake the bottle, dissolved oxygen reacts with the leucoindigo carmine and pushes the equilibrium back toward the oxidized (blue) form. When you set it down, glucose slowly tips the balance the other way. The alkaline conditions (from NaOH) are necessary — the reaction is much slower or doesn’t occur at neutral pH.
Eventually the glucose is entirely oxidized (to gluconate), no more reducing power is available, and the solution stays blue permanently.
Variations
Side-by-side comparison: Run this alongside the Blue Bottle Reaction (methylene blue version). The indigo carmine version shows four colors vs. the methylene blue version’s two — a vivid illustration of how the specific dye molecule determines the observable intermediates.
Slower cycling: Use less glucose (4 g) for a slower, more leisurely transition that makes the intermediates easier to observe.
Temperature effect: Warm the bottle slightly (place in warm water). The reduction is faster, and the cycle speeds up noticeably — demonstrating reaction rate dependence on temperature.