Ionic Product of Water
The Ionic Product of Water, Kw, 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)
Kw = [H3O+(aq)][OH-(aq)]
At 298 K, the value of Kw is 1 x 10-14 mol^2 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
pKw = 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+
pH is related to hydrogen ion concentration. Since the H+ ion concentration in solution is often small, the concentration is generally expressed as the logarithm of its reciprocal, which is called a pH value. Therefore, pH is defined as
pH = -log[H+]
For a ten times increase in H+ ion concentration there is a decrease in the pH value of one unit.
Given the pH of a solution, its H+ concentration can be found:
[H+(aq)] = antilog -pH
[H+(aq)] = 10^-pH