Luminol is an organic compound which, when oxidized, emits light — a phenomenon known as chemiluminescence. This is similar to the reactions that fireflies uses to emit light, and to those used in "glow-sticks" and some roadside emergency lights.
In this reaction, a small amount of luminol (3-aminophthalhydrazide or 5-amino-2,3-dihydro- 1,4-phthalazinedione) is dissolved in a basic aqueous solution, which also contains a small amount of copper(II) sulfate. To this solution is added a solution of a mild oxidizing agent, which is 0.3% hydrogen peroxide in the demonstration below. (Bleach is also used in some recipes as the oxidizing agent.) The reaction is believed to occur by the following mechanism:
The luminol is converted by the basic solution into the resonance-stabilized dianion 1, which is oxidized by the hydrogen peroxide into the dicarboxylate ion 2, accompanied by the loss of molecular nitrogen, N2. When the molecule 2 is formed, it is in an excited (higher energy) electronic state, and sheds its "extra" energy by emitting a photon of light (hn), allowing the molecule to go to its ground state form (3).
In aqueous solutions, the luminol oxidation is catalyzed by the presence of a metal ion, such as iron(II) or copper(II). [In the video below, copper(II) sulfate is used in the luminol solution, which is why one of the containers is light blue.] For this reason, luminol can be used in the detection of blood, since it can be activated by the iron in hemoglobin.
Depending on how well the solution is buffered, the blue glow produced by this reaction can persist for a couple of minutes.
In the demonstration below, the light blue luminol solution and the colorless hydrogen peroxide solution are poured together in a funnel, to which a coil of clear tubing has been attached.
Video Clip: REAL, 5.39 MB
!!! Hazards !!!
This is a very safe reaction, as described in the procedures below. Some procedures require the use of more concentrated base solutions; the usual precautions should be observed with those.
Oxidations of Luminol: Bassam Z. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 1. Madison: The University of Wisconsin Press, 1983, p. 156-167.
Chemiluminescence: The Firefly Reaction: Lee R. Summerlin and James L. Ealy, Chemical Demonstrations: A Sourcebook for Teachers, Volume 1, 2nd ed. Washington, D.C.: American Chemical Society, 1988, p. 69-70.
F. Albert Cotton and Geoffrey Wilkinson, Advanced Inorganic Chemistry, 5th ed. New York: John Wiley & Sons, 1988, p. 152-153.
John Emsley, The Elements, 3rd ed. Oxford, Clarendon Press, 1998, p. 120-121.
David L. Heiserman, Exploring Chemical Elements and their Compounds. New York: TAB Books, 1992, p. 49-53.
Martha Windholz (ed.), The Merck Index, 10th ed. Rahway: Merck & Co., Inc., 1983.