At standard temperature and pressure, has a value of . In pure water, the concentrations of the hydronium and hydroxide ions should be equal (every ionized water molecule creates one hydronium ion and one hydroxide ion according to equation (1)). These concentrations can be directly calculated:
Since the pH value is the negative log of the concentration of the hydronium ions, equation (6) implies that the pH value for pure water is 7.
Equilibrium constants for acids and bases can also be defined based on their reactions.
As with the autoionization of water, the concentration of water can be combined with the equilibrium constant to yield the acid ( ) or base ( ) ionization constants. For example, the acid ionization constant of equation (7) is calculated by
