Electrochem -> Redox


Luminol reaction with oxygen by bkrieg564 https://www.youtube.com/watch?v=xlO1awr3-9E&feature=youtu.be

the following is from What is chemiluminescence? By Emma Welsh, Science In School: The European Journal for School Teachers, Issue 19- 25/05/2011

The release of a photon of light from a molecule of luminol is a fairly complex, multi-stage process.

In a basic (alkaline) solution, luminol exists in equilibrium with its anion, which bears a charge of -2.

The anion can exist in two forms (or tautomers), with the two negative charges delocalised on either the oxygens (the enol-form) or on the nitrogens (the ketol-form; see Figure 3, below).

Molecular oxygen (O2) combines with the enol-form of the luminol anion, oxidising it to a cyclic peroxide.

The required oxygen is produced in a redox reaction (i.e. one in which both reduction and oxidation occur)

It involves hydrogen peroxide (H2O2), potassium hydroxide and (for example) potassium hexacyanoferrate(III) (K3[Fe(CN)6], also known as potassium ferricyanide).

The hexacyanoferrate(III) ion ([Fe(CN)6]3-) is reduced to the hexacyanoferrate(II) ion ([Fe(CN)6]4-,

giving potassium ferrocyanide, K4[Fe(CN)6])

While the two oxygen atoms from the hydrogen peroxide are oxidised from oxidation state -1 to 0:

Chemiluminescence Luminol redox

The cyclic peroxide then decomposes to give 3-aminophthalate (3-amino-1,2-benzenedicarboxylic acid) in an excited state, along with a molecule of nitrogen (N2) – see Figure 3, below.

This decomposition reaction is favoured because the cyclic peroxide molecule is highly unstable, and the reaction involves breaking some weak bonds.

It is also favoured because of the increase in entropy (disorder) due to the liberation of a gas molecule.

When the excited 3-aminophthalate drops down to the ground state, a photon of blue light is released.

Figure 3: Reactions leading to the emission of light from luminol
Tautomers are molecules with the same molecular formula, but different arrangements of atoms or bonds.

The two tautomers can be interconverted; the curly arrows show the movement of electrons that brings about the change between the two forms. Click to enlarge image. Image courtesy of Chemistry Review


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