Luminol

Luminol (C₈H₇N₃O₂) is a chemical that exhibits chemiluminescence, with a blue glow, when mixed with an appropriate oxidizing agent. Luminol is a white-to-pale-yellow crystalline solid that is soluble in most polar organic solvents but insoluble in water.

Forensic investigators use luminol to detect trace amounts of blood at crime scenes, as it reacts with the iron in hemoglobin. Biologists use it in cellular assays to detect copper, iron, and cyanides as well as specific proteins via western blotting.

When luminol is sprayed evenly across an area, trace amounts of an activating oxidant make the luminol emit a blue glow that can be seen in a darkened room. The glow only lasts about 30 seconds but can be documented photographically. The glow is stronger in areas receiving more spray; the intensity of the glow does not indicate the amount of blood or other activator present.

Synthesis

Luminol is synthesized in a two-step process, beginning with 3-nitrophthalic acid. First, hydrazine (N₂H₄) is heated with the 3-nitrophthalic acid in a high-boiling solvent such as triethylene glycol and glycerol. A condensation reaction occurs, with loss of water, forming 3-nitrophthalhydrazide. Reduction of the nitro group to an amino group with sodium dithionite (Na₂S₂O₄), via a transient hydroxylamine intermediate, produces luminol.

The compound was first synthesized in Germany in 1902 but was not named luminol until 1934.