Stoplight Reaction
Summary
Indigo carmine is a redox1 dye that cycles through yellow, red, green, and blue as glucose reduces it and shaking’s oxygen re-oxidizes it. You’ll be able to explain why the colours always appear in the same order, why the green is a mixture rather than its own compound, and why alkaline conditions are needed.
History
Indigo carmine is indigo’s water-soluble cousin — natural indigo treated with sulfuric acid, first made in the 18th century to give a dye that dissolves in water. It went on to colour textiles, then foods (it is still the food dye FD&C Blue No. 2), and even to trace kidney function in medicine, where the blue is injected and timed as it appears in the urine.
As a demonstration the “stoplight” (or “traffic light”) is a livelier descendant of the classic blue-bottle reaction — methylene blue, glucose, and alkali — popularised in early-20th-century classrooms. Swapping methylene blue for indigo carmine widens the show from two colours to four, and the red and green intermediates linger long enough to study. See the Blue Bottle for the two-colour original.
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.
Hazards & preparation
PPE: safety glasses and gloves.
- Sodium hydroxide is strongly caustic — add it to water, let it cool, and rinse any splash with lots of water.
- Keep the bottle sealed while shaking and vent carefully.
Disposal: neutralize the alkaline solution (dilute acid or plenty of vinegar) and flush down the drain with water. See the Safety page.
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 water in the bottle. It warms up — let it cool to room temperature.
- Add the dextrose and swirl until fully dissolved.
- Add the indigo carmine and seal. Swirl gently — the solution should turn yellow within a minute or two as glucose reduces the dye. If it stays blue, wait a few minutes.
- Once yellow, shake vigorously for a few seconds and watch the colour cycle up: yellow → red → green → blue.
- Set it down and watch it cycle back: blue → green → red → yellow.
- Repeat — the cycle works 10–15 times before the glucose is consumed and it stays blue.
Tip: if it goes blue and stays blue right after preparation, the glucose is exhausted or too dilute — start fresh with slightly more glucose.
What you should see
Left alone after mixing, the bottle settles to yellow. One good shake sends it racing yellow → red → green → blue in a second or two; set down, it unwinds back through green and red to yellow over a minute. Each cycle is a little slower until, after a dozen or so, it locks on blue.
| Symptom | Likely cause | Fix |
|---|---|---|
| Stays blue immediately | Glucose exhausted or too little | Start fresh with a bit more dextrose |
| Won’t turn yellow at rest | Too much oxygen, too little glucose | Add more glucose; leave it sealed and still |
| Cycles too fast to see colours | Warm / lots of glucose | Cool it, or use less glucose (4 g) for a slower cycle |
The reactions
Indigo carmine (blue, oxidized) accepts electrons from glucose in alkaline conditions, becoming leucoindigo carmine (yellow, reduced):
\[\text{Indigo carmine (blue)} + 2e^- + 2H^+ \rightleftharpoons \text{Leucoindigo carmine (yellow)}\]
blue dye + electrons (from glucose) ⇌ yellow reduced dye; oxygen from shaking drives it back to blue
The partially reduced semiquinone intermediate is red/orange, and the visual green arises from blue and yellow forms present simultaneously during the transition.
The Science
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.
Questions to Explore
Why do the colours appear in the same order every time? Always yellow → red → green → blue on shaking, reversing the same way. What does that say about the four forms, and why can’t blue jump straight to yellow?
Hint / answer
The dye moves through its oxidation states in sequence — fully reduced (yellow), part-oxidized (red), then oxidized (blue) — because electrons are added or removed step by step. You can’t skip a step, so the colours run in a fixed order in and out.
Why is the green a mixture rather than a pure compound? Green comes from blue and yellow coexisting, not a single green form. What does that imply about the transition?
Hint / answer
During the switch, some molecules are already blue while others are still yellow, and your eye blends them to green — so it’s a gradual equilibrium, not a sharp flip. A true single green intermediate would look green everywhere at once, not only mid-transition.
Why does alkalinity matter? At neutral pH the reduction is too slow. What does high OH⁻ do to glucose?
Hint / answer
Alkali opens glucose to its reactive open-chain form and boosts its electron-donating power, so it reduces the dye fast enough to watch. Without the base, glucose is a feeble reducer and nothing much happens.
How does this compare to the iodine clock? Both use glucose/alkaline redox and colour change, but the clock switches suddenly while this cycles. What differs?
Hint / answer
The stoplight is a reversible equilibrium you can push back and forth. The iodine clock is a one-way sequence of reactions timed so a colour appears abruptly when a reactant runs out — a clock, not a cycle. Different mechanism, different behaviour.
When will the cycle stop? It fails after 10–15 shakes. What’s consumed, and how would you test it?
Hint / answer
The glucose runs out (oxidized to gluconate); the dye and NaOH are essentially still there. Test by changing one ingredient at a time — more glucose gives more cycles, whereas more dye or more base does not.
Going further
- Slow it down. Use 4 g of glucose for a leisurely cycle that makes the red and green intermediates easy to study.
- Warm it up. Stand the bottle in warm water and watch the cycle speed up — a clear temperature-and-rate demonstration.
- Next in the Redox track: make and confirm elemental iodine in Iodine from Sodium Iodide.