The water molecule may either gain or lose a proton. It is said to be amphiprotic. The ionization equilibrium can be written
where in aqueous solution  denotes a solvated proton. Often this is written as the hydronium ion , but this formula is not exact because in fact there is solvation by more than one water molecule and species such as , , and  are also present.
The equilibrium constant is given by
With solutions in which the solute concentrations are not very high, the concentration  can be assumed to be constant, regardless of solute(s); this expression may then be replaced by
The self-ionization constant of water, K, is thus just a special case of an acid dissociation constant. A logarithmic form analogous to pK may also be defined
These data can be modelled by a parabola with
From this equation, pK = 14 at 24.87 °C. At that temperature both hydrogen and hydroxide ions have a concentration of 10 M.