If hydrogen peroxide foam in presence of blood, luminol produces a glossy blue light, it's called luminol chemiluminescence. Chemiluminescence more generally refers to any chemical reaction that produces light. This reaction can be represented by:
A + B -> [I] * -> + LIGHT PRODUCT
With [I] * being a molecule in an excited state. This phenomenon allows different uses of the light, such as fluorescent bracelets or emergency light sources but of course its most common use is trought luminol, to detect blood. But how is this light produced?
Luminol chemiluminescence is a redox phenomenon that is producing light, and a redox above all.
The reduction-oxidation of luminol involves hydrogen peroxide and luminol. The equation can be representated by this:
H2O2 + luminol -> H2O + X*
Two couples can be differentiated: H2O2/H2O and luminol/X*
As explained before, the reaction between H2O2 and H2O is a reduction, with H2O2 being the oxidant. Thus, the other half-equation has to be the oxidation, with the luminol being the reductant.
The reaction beetween H2O2 and luminol produces water and a species we called X*. The equation and the balancing being to complicated, we will focus on the products of the reactions. H2O is water, but what is this X*?
X* is th oxidant of luminol and actually a complexe ion we won't study: C8H5NO42- *
Though, we notice the asterisk. This asterisk means the ion is in an excited state, a high energy level.
Energy level are a quantum mechanical system which claims each atoms possess different energy levels: a stable ground state, with the lowest energy level. If it is at a higher energy level, it is said to be excited. Those species can be excited to a higher level by absorbing photons. Conversely, a decrease in energy level will result in the emission of a photon.
In our case, the species X is in an excited state because of the redox reaction. But once the reaction is done, it will return to a lower energy level by itself.
Returning to the ground state, its energy level will drop and a packet of energy (a photon) will be release in the form of electromagnetic radiation. The amount of energy will make the wavelenght vary, and when it is within the range of visible light, electromagnetic radiations are percieved as light of a particular colour determinated by the electromagnetic spectrum.
With luminol the electromagnetic radiation of the excited species has a wavelength of about 380 nm, this, explaining the blue colored light.
