Luminol is a chemical that produces a beautiful blue fluorescence when oxidized by hydrogen peroxide. In addition to providing one of the best-known examples of chemiluminescence, it is also a valuable crime scene investigation tool whose blue glow reveals the presence of blood.
For teachers, demonstrating the luminol reaction can add to discussions of oxidation-reduction reactions, conservation of energy, and electron energy levels. The following demonstration is ideal for middle and high school students.
Materials
- 1 g Luminol
- 20 mL Sodium Hydroxide Solution (1 M)
- 10 mL Hydrogen Peroxide (3%)
- 0.2 g Potassium Ferricyanide
- 4-ft Piece of Rubber Tubing
- Funnel
- Ring Stand
- Support Ring
- Clamps
Preparation
You will need a separate beaker for each of the 2 stock solutions you’ll prepare. Prepare the solutions immediately before use. Don lab coat or apron, goggles, and gloves before preparing solutions. Caution: Hydrogen peroxide is a strong oxidizer. Avoid skin contact. Sodium hydroxide and its solutions are caustic and can irritate skin. Avoid skin contact.
- To prepare stock solution A, fill a beaker with 100 mL of water. Add 0.18 g of luminol and 3.0 mL of sodium hydroxide solution (1 M).
- To prepare stock solution B, fill another beaker with 100 mL of water. Add 1 mL of hydrogen peroxide (3%) and 0.03 g of potassium ferricyanide.
To set up the apparatus, follow the steps in the figures below.
Procedure
- Dim the lights.
- Simultaneously pour an equal amount of solution A and solution B into the funnel.
- As the 2 solutions mix, a blue light is emitted that is relatively bright and should last for several minutes.
Discussion
Reactions that produce light without heat are called chemiluminescent reactions. Perhaps the most familiar chemiluminescent reactions are those that occur in living organisms and are referred to as bioluminescence. A classic example of this is the light produced by fireflies.
The reaction in this demonstration is an oxidation-reduction reaction in which a photon of light is released from an excited molecule. In the reaction, luminol is oxidized and its electrons elevated to an excited state. When the electrons return to the ground state, visible light is emitted.
Light’s wavelength determines its color. Light at a wavelength of 680 nm is red; at 500 nm, green; and at 425 nm, blue. The energy of one quantum (one photon, one particle) of light is inversely proportional to its wavelength. Thus:E = hc/l
where E is the energy of one quantum of light of wavelength (l), h is Planck’s constant and c is the speed of light.
In the reaction, hydrogen peroxide oxidizes luminol to produce aminophthalic acid, nitrogen gas, water, and light.
Whether from fireflies or luminol, visible light is produced by the release of light energy from energized atoms. Our chemistry kits below include material along with complete instructions and background information for this interactive activity.
