Self-Ionization of Water

Water can act as a very weak acid and also as a very weak base. In a sample of water, a small number of water molecules undergo auto ionization. Half of them acting as an acid while the other half acting as a base. As a consequence small concentration of H3O⁺ and OH^- are formed in water.

H₂O + H₂O ⇌ H₃O⁺ + OH^-

The constant, K_w, is called the dissociation constant or ionic product constant of water. The value of K_w at 298 K has been determined from the measurement of electrical conductivity of carefully purified water and has been found to be 1.0 × 10^–14 mol² dm^-6.

Since the concentrations of H₃O⁺ and OH^- ions is equal

[H₃O⁺] = [OH^-] = 1.0 × 10^-7 mol dm^-3 at 298 K