Heavy water, DO, self-ionizes less than normal water, HO;
:DO + DO  DO + OD
This is due to the equilibrium isotope effect, a quantum mechanical effect 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 DO to be 1, and assuming that the activities of the DO and OD are closely approximated by their concentrations
The following table compares the values of pK for HO and DO.