Luminol lighthouse

I was looking for details on oscillating reactions and discovered that not only do changes in colour, sound and heat oscillate, but flashes of light from the luminol reaction can also oscillate, creating an eerie lighthouse effect. Like all chemiluminescent reactions, this reaction works best in a darkened room. 

Preparation

These instructions make a 250 cm³ reaction, but it can be scaled up for a larger audience. Make up the following:

• Solution A: dilute 6 cm³ of 30% (100 vol) hydrogen peroxide to 50 cm³ water to make a 1 M hydrogen peroxide solution. 
• Solution B: dissolve 0.73 g of potassium thiocyanate in 50 cm³ of water to make a 0.15 M potassium thiocyanate solution. 
• Solution C: dissolve 0.015 g of copper sulfate pentahydrate salt in 100 cm³ of water to make a 6 x 10^–4 M copper(II) sulfate solution.
• Solution D: dissolve 0.06 g of luminol in 50 cm³ of 0.1 M sodium hydroxide to make a  7 x 10^–3 M alkaline luminol solution.

Before the demonstration, boil a kettle of water. Transfer solutions B, C and D into 100 cm³ beakers. Add hot water from the kettle to a larger beaker (I used a 2 L beaker, see Figure 1). Place the beakers of B, C and D into the water bath to heat to 50-60°C, taking care not to float them.

Place solution A into the demonstration flask with the stirrer bar. Darken the room.

In front of the class

When solutions B–D are between 50–60°C, remove them and adjust the temperature of the water bath to 40–50°C.

Ensure the water level is low enough not to float the demonstration flask.

Add solutions B and C to the demonstration flask, then place it into the water bath and stir (see Figure 2). To initiate the reaction, add solution D.

There will be an initial flash of blue light, then a faint afterglow associated with a separate reaction of luminol with superoxide ions in solution. Flashes will repeat. At ~40°C, the reaction lasts 10–15 minutes with steadily increasing periods ranging from approx 30 seconds initially to 70 seconds after 12 minutes (see Figure 3).

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Figure 2: reaction set-up for the demonstration

Source: © Declan Fleming

Figure 3: RGB plot at 40°C showing the increasing interval between flashes of light over time

Source: © Declan Fleming

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Teaching goal

This reaction is a great discussion starter, where students will find the predictable intervals of light captivating, like a slow-motion firefly. All oscillating reactions rely on competing autocatalytic steps broadly fitting the following scheme.

The overall reaction cycles between two central competing reactions that give rise to the oscillations. Over time, the concentration of A falls, so step (b) begins to fail, or the final product D inhibits the reaction, which slows and eventually stops the oscillations.

Briggs–Rauscher and Belousov–Zhabotinskii oscillating systems are more well known, but this reaction follows the Orban oscillating system. The mechanism with luminol has up to 30 different reactions and 26 different variables so we need to simplify it.

Although a dark room is best, typically the audience will see the initial burst of light in a classroom setting. Thereafter the solution turns yellow, which fades moments before the next flash of light. The yellow is attributed to copper(I) complexes and suggests that the copper sulfate has catalysed the decomposition of hydrogen peroxide, but before the copper(I) can be re-oxidised by dissolved oxygen or hydrogen peroxide, it is trapped by strong complexation from thiocyanate ions in the mixture. Copper(II) is needed to catalyse the separate luminol reaction (see Lighting up copper) in its own oxidation by hydrogen peroxide, so as long as the yellow colour is visible, the blue glow will not be.

Meanwhile, hydrogen peroxide and thiocyanate ions undergo a series of reactions to generate OS(O)CN^•, which re-oxidises the Cu(I)[SCN]_n complexes briefly before the copper(II) is reduced and trapped again.

A range of products are produced including oxygen from the decomposition of the peroxide, sulfate(VI) ions, hydrogen carbonate ions and, crucially, ammonium ions. Therefore, potassium thiocyanate and sodium hydroxide should not be substituted for ammonium compounds that can inhibit the oscillator.