Ionic Product of Water

The Ionic Product of Water, K_w, is the equilibrium constant for the reaction in which water undergoes an acid-base reaction with itself. That is, water is behaving simultaneously as both an acid and a base.

H₂O_(l) + H₂O_(l) = H₃O⁺_(aq) + OH^-_(aq)

K_w = [H₃O⁺_(aq)][OH^-_(aq)]

At 298 K, the value of K_w is 1 x 10^-14 mol² dm^-6. This makes the concentration of H⁺ ions equal to 1 x 10^-7 mol dm^-3, and therefore the pH is 7. This is defined as 'neutral'.

From the above equilibrium expression, taking -log₁₀ throughout

pK_w = pH + pOH = 14

Ionic Product does not apply only to water. It applies, for example, to the equilibrium in liquid ammonia:

NH₃ + NH₃ = NH₂^- + NH₄⁺

See also pH.