Shed new light on the two-faced reaction

The classic two-faced thionine experiment is a visual demonstration of a reaction that is initiated by the absorption of light energy. But when I first attempted this demonstration in 2008, using one of the last remaining overhead projectors in my school, I was underwhelmed. The dazzling overhead projector lamp, and the curvature of the flask, made the division between the purple and colourless sections of the solution difficult to see. Then, when I switched off the lamp, my eyes did not adjust quickly enough to catch the wall of purple advancing back across the flask.

Kit

• 0.4 g iron(II) sulfate heptahydrate (irritating to skin and eyes)
• 0.4 g citric acid
• 0.5 cm³ methylene blue solution, 0.1% w/v
• 50 cm³ cell culture flask with cap
• LED work light (CLEAPSS members see PS052)
• Opaque card or similar (credit card sized) for masking the light
• Coloured filters (optional)

Preparation

Dissolve 0.4 g of iron(II) sulfate heptahydrate and 0.4 g of citric acid in approximately 100 cm³ of deionised water. Add about 0.5 cm³ of 0.1% methylene blue solution. The solution is similar in hue and intensity to a 0.5 M copper sulfate solution. Fill the cell culture flask with approximately 50 cm³ of this solution, close the cap tightly and let it settle.

In front of the class

Position the LED work light on a bench shining towards the ceiling. Use the opaque card to mask part of the light. Lay the cell culture flask on its side, with half of it on the card and half directly in the light. Position a white wall or screen behind the flask to help visualise the colour. Within a few seconds, the illuminated part of the solution will decolourise while the shaded section will remain blue. Switch the light back off and watch the colour return to the shaded section.

Teaching goal

This demonstration supports lessons on electrode potentials and the thermodynamics of redox reactions (e.g. spontaneity). With a small extension you can also show how kinetics often overrides a prediction made using thermodynamics alone.

In this experiment, methylene blue has a coloured cationic form (MB⁺) and a colourless reduced form, leucomethylene blue (LMBH). In the low pH of the citric acid solution (approximately pH 2.5), LMBH is further protonated to LMBH₃²⁺.

The pH of the demonstration is around 2.5 rather than 0, meaning the MB⁺ reduction potential is even less favourable. The Nernst equation shows that the potential at this pH ought to be closer to +0.3 V than +0.53 V.

Either way, the ∆E is negative and the dye will not spontaneously decolourise without energy being supplied to the system. In this demonstration, the energy required to nudge the dye towards spontaneity comes from light.

Methylene blue absorbs efficiently in the red end of the spectrum thanks to the delocalised π system extending over all three rings with λ_max of 664 nm.

When MB⁺ absorbs a photon, a short-lived, highly energetic excited state forms and its full two electron reduction potential increases to +1.43 V. Now the ∆E is large and positive – the solution bleaches. You can demonstrate the importance of light at the red end of the spectrum with the use of coloured filters. Red or yellow filters allow the correct wavelengths through and the dye will bleach, while blue filters will prevent the reaction from occurring.

Once light is removed, the backward reaction wins out and the Fe³⁺ oxidises the leucomethylene blue back to its coloured form again. This happens slower than in the original demonstration that uses thionine, meaning it’s easier to observe the effects.