Chemical Traffic Light

Polish version is here

In this article, I introduce another simple yet visually striking experiment: chemical traffic lights!

This experiment is named after its ability to mimic the colors of traffic lights: green, red, and yellow.

One key advantage of this experiment is its engaging introduction to chemical thermodynamics.

Required Substances

To conduct this experiment, we will need:

Indigo carmine C₁₆H₈N₂Na₂O₈S₂,
Sodium hydroxide (NaOH),
Glucose C₆H₁₂O₆.

Warning: Sodium hydroxide is a strong base and highly caustic! It can cause severe burns and serious eye damage. Indigo carmine is harmful if inhaled and can irritate the skin and eyes. The author assumes no responsibility for any potential damages. Proceed at your own risk!

Indigo carmine is a synthetic blue dye that is a sulfonated derivative of indigo. It also serves as a pH indicator: in strongly alkaline conditions, it turns yellow, while in other environments, it remains blue. This dye is also used in the food industry.

To prepare the experiment, we need to create two solutions:

Solution A - 1.5 g (0.053 oz) of glucose C₆H₁₂O₆ dissolved in 70 cm³ (2.4 fl oz) of water,
Solution B - 0.67 g (0.024 oz) of sodium hydroxide NaOH dissolved in 30 cm³ (1 fl oz) of water.

These solutions are stable and can be stored for an extended period.

Experiment

Setting up the experiment is simple. Solution A should be heated to approximately 35°C (95°F), after which a small amount — at most a few milligrams (less than 0.001 oz) — of indigo carmine should be added. The solution should take on an intense blue color.

Blue solution

Next, solution B is added, causing the liquid to turn green.

Green solution

However, after a short time, the solution suddenly turns red.

Red solution

The red color does not last long—within a few seconds, it changes to yellow.

Yellow solution

The resulting colors and their transitions are truly mesmerizing, especially under the right lighting conditions. But this isn’t the end—shaking the container vigorously causes the yellow solution to turn green again! This allows us to witness the striking color transformation once more.

The video below, which I recorded, demonstrates the entire experiment in action:

This experiment can be repeated several times, though the colors will gradually fade with each cycle.

Explanation

This experiment is fundamentally similar to the one presented here, but it produces a more visually engaging effect. The methylene blue used in that experiment, however, is easier to obtain and more affordable than indigo carmine.

The initial blue color occurs naturally in a neutral pH environment. When sodium hydroxide NaOH is added, the solution becomes highly alkaline, leading to a color change. At the same time, glucose exhibits its reducing properties under these conditions: the dye is first partially reduced, resulting in the red coloration. The process continues until the indigo carmine is fully reduced, which turns the solution yellow. Meanwhile, glucose oxidizes into gluconic acid, which, in the presence of sodium hydroxide, converts into sodium gluconate.

Why does shaking the solution cause a color change? The answer lies in the introduction of oxygen from the air into the solution. This oxygen reoxidizes the reduced dye, restoring the green color and allowing the cycle to repeat.

Have fun experimenting!

Weird Science