Water - Ionization Constant, pKw, of Normal and Heavy Water

Autoprotolysis describes the reaction of water with itself:

2 H₂O ↔ H₃ O⁺+ OH¯          (1)

and an equlibrium constant can be defined as

K_eq = a(H₃ O⁺) * a(OH^-) / a(H2O)²             (2)

where a is the chemical activity of the water ions.

Because most acid–base solutions are typically very dilute, the activity of water is generally approximated as being equal to unity. In dilute aqueous solutions, the activities of the solute particles are approximately equal to their concentrations.

Then, and an autoprotolysis constant or water ionization constant can be defined as

K_w = (H₃ O⁺) (OH¯)            (3)

where (H₃ O⁺) and (OH¯) represents the concentration of H₃ O⁺and OH^-ions in water.

Often, a general value of 1.00×10¯¹⁴ is used. However, pK_w varies with the temperature as shown in the figures and tables below.

From the chemical equation (1) above, it can be seen that H₃ O⁺and OH¯ concentrations are in the molar ratio of one-to-one. This means that (H₃ O⁺) = (OH¯). Therefore the values of (H₃ O⁺) and (OH¯) can be determined by taking the square root of K_w .

Heavy water, D₂O, self-ionizes less than normal water, which gives larger pK_w values for D₂O compared to H₂O.

See Water and Heavy Water for thermodynamic properties at standard condtions.
See also other properties of Water at varying temperature and pressure : Boiling points at high pressure, Boiling points at vacuum pressure, Density and specific weight, Dynamic and kinematic viscosity, Enthalpy and entropy, Heat of vaporization, Melting points at high pressure, Prandtl number, Properties at Gas-Liquid Equilibrium Conditions, Saturation pressure, Specific gravity, Specific heat (heat capacity), Specific volume, Thermal conductivity, Thermal diffusivity and Vapour pressure at gas-liquid equilibrium.