Thionin—The Two-Faced Solution:Conversion of Light Energy to Chemical Energy

Introduction

A large beaker contains a bright purple solution. The beaker is placed on an overhead projector that is half-covered with aluminum foil—half of the purple solution is sitting on the piece of aluminum foil, the other half is sitting directly on the overhead stage. Switch on the overhead lamp and, in seconds, the solution on the side of the beaker exposed to light turns colorless, while the unexposed side remains purple. The result is sharp and stunning—a two-faced solution! The solution appears to be divided by an invisible line running vertically through the solution. Amazing enough, but turn off the overhead light and the entire process can be repeated many times!

Concepts

• Photochemical reaction
• Oxidation/reduction
• Reversible chemical reaction

Materials

Safety Precautions

Sulfuric acid solution is severely corrosive to eyes and skin and is toxic. Use extreme caution when handling. Iron(II) sulfat is slightly toxic by ingestion. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information

Disposal

The two-faced solution may be rinsed down the drain with excess water.

Prelab Preparation

Prepare 0.001 M thionin solution fresh by adding 100 mL distilled water to the bottle which contains 0.023 g of thionin. Stir to dissolve. The thionin solution has a poor shelf life—use within one week. 

Procedure

1. Mix together the following chemicals in a 1-L beaker: 10 mL of freshly prepared 0.001 M thionin solution, 10 mL of 3 M sulfuric acid, and distilled water to bring the total volume of solution in the beaker to 500 mL. Mix thoroughly.
2. Add 2.0 grams of iron(II) sulfate to the beaker and stir to dissolve.
3. Turn off the room lights and place the beaker on the overhead projector stage. Turn on the projector lamp. Observe that the solution changes from purple to colorless in a matter of seconds.
4. Now turn off the projector lamp and allow the purple color to return.
5. On the overhead projector stage, place a piece of aluminum foil several layers thick. The foil should not cover the entire projector stage.
6. When the solution is purple, place the beaker on the projector stage so that half of the beaker is sitting on the piece of aluminum foil. (The students should be in direct line with the bisecting line so they can observe the vertical division.)
7. Turn on the projector lamp, and observe the solution. A distinct vertical division between the purple side and colorless side should be clearly visible. (The vertical division indicates that the reaction is initiated by light and not heat.)
8. The reaction can be reversed by turning off the light. The reaction is reversible for several days.

Student Worksheet PDF

12995_Student1.pdf

Teacher Tips

• This kit contains sufficient materials to perform the demonstration as written three times. Once the solution has been prepared, however, the demonstration can be shown over and over again.
• The color change will fade over time from bright purple to a paler shade of blue or purple. This color change may still be effective after one week.
• Direct, bright sunlight may also be used as the light source.

Answers to Questions

1. Describe what happened in this demonstration. Include the apparatus and chemicals used.
   A 1-L beaker containing thionin solution, sulfuric acid, distilled water, and 2 g of iron(II) sulfate was placed on an overhead projector. When the projector light was turned on, the solution changed colors from purple to clear. When aluminum foil was placed underneath half of the beaker, the projector light was turned on again. The half that was exposed to the light turned clear. The other half remained purple.
   
2. Thionin can exist in two forms, a purple oxidized form and a colorless reduced form. What chemical served as the reducing agent in this demonstration?
   In this demonstration, the iron(II) ion from the ferrous sulfate served as the reducing agent.
   
3. Based on what you observed, what else needs to be present for the thionin to be reduced?
   An intense light source, such as the overhead projector light, also needs to be present in order for the thionin to be reduced.
   
4. Is this reaction reversible? What evidence do you have of this?
   This reaction is reversible. The thionin solution returns to its colorless state if the light source is removed, and can become purple again if the light is turned on again. The reaction can go back and forth several times.

Discussion

Thionin is an organic compound that can exist in two forms, an oxidized form which is purple and a reduced form which is colorless. When a reducing agent such as iron(II) ion (Fe²⁺) is added to an acidic thionin solution, the thionin (thio⁺) molecule accepts two hydrogen atoms and is reduced to its colorless form—but only in the presence of an intense light source. The reduction is a photochemical reaction that is catalyzed by light. This demonstration is a dramatic example of the conversion of light energy to chemical energy. The reaction can also be reversed; when the light source is removed the purple color due to the oxidized form of thionin returns.

The demonstration also provides a vivid example of the definition of equilibrium, a chemical reaction that can take place in both a forward and reverse direction. The equilibrium reaction is represented by the following reaction equation:

Remember that if one reactant in a balanced chemical equation is oxidized, another reactant must be reduced. Fe²⁺ is oxidized to Fe³⁺ in the forward reaction, while in the reverse reaction Fe³⁺ is reduced to Fe²⁺. Thio⁺ represents the monoprotonated form of thionin in acidic solution.

References

Robert J. Cairo, Horace Mann School, Bronx, NY.