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FeSO4 is dissolved in H2SO4, and methylene blue is added.
The magnetic stirrer is turned on and a high-intensity lamp is made to shine on the solution.
When the lamp is turned on, the blue color of the oxidized form of methylene blue gradually disappears and the solution becomes colorless. When the light is shut off, the solution gradually becomes blue again.
Unlike the original blue bottle demonstration, the mechanism for this demonstration is not yet clear. It doesn't seem to involve O2, because the photochemical reduction occurs equally well in solutions that have been degassed via repeated freeze/pump/thaw cycles.
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