Self-ionization of water

The self-ionization of water also autoionization of water, in addition to autodissociation of water is an ionization reaction in pure water or in an aqueous solution, in which the water molecule, H₂O, deprotonates loses a nucleus of one of its hydrogen atoms to become a hydroxide ion, OH^{−. The hydrogen nucleus, H+, immediately protonates another water molecule to form hydronium, H₃O+. this is the an example of autoprotolysis, & exemplifies the amphoteric quality of water.}

Isotope effects

Heavy water, D₂O, self-ionizes less than normal water, H₂O;

This is due to the equilibrium isotope effect, a quantum mechanical case attributed to oxygen forming a slightly stronger bond to deuterium because the larger mass of deuterium results in a lower zero-point energy.

Expressed with activities a, instead of concentrations, the thermodynamic equilibrium constant for the heavy water ionization reaction is:

Assuming the activity of the D₂O to be 1, and assuming that the activities of the D₃O^{+ and OD− are closely approximated by their concentrations}

The following table compares the values of pK_w for H₂O and D₂O.

In water–heavy water mixtures equilibria several style are involved: H₂O, HDO, D₂O, H₃O^{+, D₃O+, H₂DO+, HD₂O+, HO−, DO−.}