Self-ionization of Water - Equilibrium Constant

Equilibrium Constant

Chemically pure water has an electrical conductivity of 0.055 µS∙cm−1. According to the theories of Svante Arrhenius, this must be due to the presence of ions. The ions are produced by the self-ionization reaction

H₂O + H₂O H₃O+ + OH−

This equilibrium applies to pure water and any aqueous solution.

Neglecting activities, the chemical equilibrium constant, K_eq, for this reaction is given by:

If the concentration of dissolved solutes is low, the concentration can be taken as being constant at ca. 55.5M.

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

which is numerically equal to the more traditional thermodynamic equilibrium constant written as:

under the assumption that the sum of the chemical potentials of H+ and H₃O+ is formally equal to twice the chemical potential of H₂O at the same temperature and pressure.

In infinitely dilute aqueous solution, the activity of water solvent is unity.

The ionization constant, dissociation constant, self-ionization constant, or ionic product of water, symbolized by K_w may be given by:

where is the concentration of hydrogen or hydronium ion, and is the concentration of hydroxide ion.

The ionic product of water can be also expressed on activity basis as:

At 25 °C K_w is equal to 1.0 × 10−14.