Molecular autoionization

Molecular autoionization is a reaction between molecules of the same substance to produce ions. The oxidation number on all atoms in such a reaction remains unchanged. Such autoionization can be protic (H⁺ transfer), or non-protic. Some examples of each are:

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Protic:

2 H₂O <----> H₃O⁺ + OH^-

2 NH₃ <----> NH₄⁺ + NH₂^-

2 H₂SO₄ <----> H₃SO₄⁺ + HSO₄^-

3 HF <----> H₂F⁺ + HF₂^-

Non-protic:

2 PF₅ <----> PF₆^- + PF₄⁺

2 SO₂ <----> SO₃^2- + SO²⁺

4 AlCl₃ <----> 3 AlCl₄^- + Al³⁺

This type of autoionization can occur in gases or solids, but not as readily as in liquids. In gases, the molecules are so far apart that ion formation is very unfavorable, and in solids, there is not enough molecular movement. As long as the phase remains constant, autoionization normally increases substantially with increasing pressure.