autoprotolysis of water

Autoprotolysis of water, also known as self-ionization of the same, is the transfer of a proton (H⁺) between the two H₂0 molecules. One of the molecules which donates proton, then, behaves as Bronsted acid while the molecule which accepts the same behaves as Bronsted base.

H₂O + H₂O  OH^– + H₃O⁺