You are here

Photochemical Reaction with Thionin


Sulfuric acid is corrosive

Chemicals and Solutions

  • 20 mL 0.001 M thionin solution (dissolve 0.23 g in 1 L water, prepare fresh)
  • 20 mL of 3M sulfuric acid
  • 4 g ferrous sulfate

Prior to presenting the demonstration mix together the following chemicals in a 2 L beaker:

  • 20 mL of 0.001 M thionin solution
  • 20 mL 3 M sulfuric acid
  • sufficient water to bring the volume to 1 L
  • 4 g ferrous sulfate


  • 2L beaker
  • Aluminum foil


  1. Place a (thick) piece of aluminum foil on the overhead projector such that half the screen is covered.
  2. Place the 2 L beaker containing the purple thionin solution on the overhead projector such that half of the beaker is exposed to light and half of the beaker rests on the aluminum foil and is protected from the light.
  3. Turn on the overhead projector. Any solution exposed to the light will become colorless. The beaker will have a colorless side and a purple side.

Hint: The reaction can be reversed by turning off the light.


"An intense light causes Iron II to reduce thionin in an acidic solution: the thionin is photochemically reduced. This demonstration is a visible example of the conversion of light to chemical energy. The reaction can be reversed by placing the solution in the dark."

(from Flinn Scientific, Inc., Batavia, IL)