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Luminol is a versatile chemical that exhibits chemiluminescence, with a striking blue glow, when mixed with an appropriate oxidizing agent. It is a white to slightly yellow crystalline solid that is soluble in water and most polar organic solvents.
Additional recommended knowledge
Luminol may be synthesized beginning from 3-nitrophthalic acid. First, hydrazine (N2H4) is heated with the 3-nitrophthalic acid in a high-boiling solvent such as triethylene glycol. A condensation reaction occurs, with loss of water, forming 5-nitrophthalhydrazide. Reduction of the nitro group to an amino group with sodium dithionite (Na2S2O4) produces luminol.
To exhibit its luminescence, the luminol must first be activated with an oxidant. Usually, a solution of hydrogen peroxide (H2O2) and a hydroxide salt in water is used as the activator. In the presence of a catalyst such as an iron compound, the hydrogen peroxide is decomposed to form oxygen and water:
In a laboratory setting, the catalyst used is often potassium ferricyanide. In the forensic detection of blood, the catalyst is the iron present in hemoglobin. Enzymes in a variety of biological systems may also catalyze the decomposition of hydrogen peroxide.
When luminol reacts with the hydroxide salt, a dianion is formed. The oxygen produced from the hydrogen peroxide then reacts with the luminol dianion. The product of this reaction, an organic peroxide, is very unstable and immediately decomposes with the loss of nitrogen to produce 5-aminophthalic acid with electrons in an excited state. As the excited state relaxes to the ground state, the excess energy is liberated as a photon, visible as blue light.
Use by crime scene investigators
Luminol is used by crime scene investigators to locate traces of blood, even if it has been cleaned or removed. The investigator prepares a solution of luminol and the activator and sprays it throughout the area under investigation. The iron present in any blood in the area catalyzes the chemical reaction that leads to the luminesence revealing the location of the blood. The amount of catalyst necessary for the reaction to occur is very small relative to the amount of luminol, allowing the detection of even trace amounts of blood. The glow lasts for about 30 seconds and is blue. Detecting the glow requires a fairly dark room. Any glow detected may be documented by a long exposure photograph.
Luminol has some drawbacks that may limit its use in a crime scene investigation:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Luminol". A list of authors is available in Wikipedia.|