Chemiluminescence (sometimes "chemoluminescence") is the emission of light (luminescence) with limited emission of heat as the result of a chemical reaction. Given reactants A and B, with an excited intermediate ◊,

[A] + [B] → [◊] → [Products] + light

For example, if [A] is luminol and [B] is hydrogen peroxide in the presence of a suitable catalyst we have:

luminol + H2O2 → 3-APA[◊] → 3-APA + light

where:

• where 3-APA is 3-aminophthalate
• 3-APA[◊] is the excited state fluorescing as it decays to a lower energy level.

The decay of the excited state[◊] to a lower energy level is responsible for the emission of light. In theory, one photon of light should be given off for each molecule of reactant, or Avogadro's number of photons per mole. In actual practice, non-enzymatic reactions seldom exceed 1% QC, quantum efficiency.

A standard example of chemiluminescence in the laboratory setting is found in the luminol test, where evidence of blood is taken when the sample glows upon contact with iron. When chemiluminescence takes place in living organisms, the phenomenon is called bioluminescence. A lightstick emits light by chemiluminescence.

Gas-phase reactions

A green and blue glowstick.

• One of the oldest known chemoluminescent reactions is that of elemental white phosphorus oxidizing in moist air, producing a green glow. This is actually a gas-phase reaction of phosphorus vapor, above the solid, with oxygen producing the excited states (PO)2 and HPO.
• Another gas phase reaction is the basis of nitric oxide detection in commercial analytic instruments applied to environmental air quality testing. Ozone is combined with nitric oxide to form nitrogen dioxide in an activated state.

NO+O3 → NO2[◊]+ O2

The activated NO2[◊] luminesces broadband visible to infrared light as it reverts to a lower energy state. A photomultiplier and associated electronics counts the photons which are proportional to the amount of NO present. To determine the amount of nitrogen dioxide, NO2, in a sample (containing no NO) it must first be converted to nitric oxide, NO, by passing the sample through a converter before the above ozone activation reaction is applied. The ozone reaction produces a photon count proportional to NO which is proportional to NO2 before it was converted to NO. In the case of a mixed sample containing both NO and NO2, the above reaction yields the amount of NO and NO2 combined in the air sample, assuming that the sample is passed through the converter. If the mixed sample is not passed through the converter, the ozone reaction produces activated NO2[◊] only in proportion to the NO in the sample. The NO2 in the sample is not activated by the ozone reaction. Though unactivated NO2 is present with the activated NO2[◊], photons are only emitted by the activated species which is proportional to original NO. Final step, subtract NO from (NO + NO2) to yield NO2  

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