Photochemical decomposition of thionin. In this timelapse demonstration, the intense light causes the thionin, in an acidic solution, to be photochemically reduced by the ferrous or iron (II) ion. Thionin can exist in two forms, the oxidized form (purple), and the reduced form (colourless). When the reducing agent Fe2+ is added to the acidic thionin solution, the thionin molecule, in the presence of intense light, accepts two hydrogen atoms and is thereby reduced to its colourless form. When the light is dimmed, the thionin solution returns to its oxidised purple colour.