Thionin—The Two-Faced Solution:Conversion of Light Energy to Chemical Energy
Publication No. 12995
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!
(for each demonstration)
Iron(II) sulfate, FeSO4•7H2O, 2.0g
Sulfuric acid solution, H2SO4, 3 M, 10 mL*
Thionin solution, 0.023 g*
Water, distilled water, 500 mL
Beaker, glass, 1-L
Graduated cylinder, 10-mL
Stirring rod, glass
*Materials included in kit.
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
The two-faced solution may be rinsed down the drain with excess water.
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.
Student Worksheet PDF
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesAsking questions and defining problems
Developing and using models
Constructing explanations and designing solutions
Engaging in argument from evidence
Disciplinary Core IdeasMS-PS1.A: Structure and Properties of Matter
MS-PS1.B: Chemical Reactions
MS-PS4.B: Electromagnetic Radiation
HS-PS1.A: Structure and Properties of Matter
HS-PS1.B: Chemical Reactions
HS-PS3.B: Conservation of Energy and Energy Transfer
HS-PS4.B: Electromagnetic Radiation
Crosscutting ConceptsCause and effect
Systems and system models
Energy and matter
Stability and change
MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
Answers to Questions
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.
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.
Robert J. Cairo, Horace Mann School, Bronx, NY.