The Base Dissociation Constant, Kb, is the equilibrium constant for the reaction in which a weak base is in equilibrium with its conjugate acid in aqueous solution. For example,
NH3(aq) + H2O(l) = NH4+(aq) + OH-(aq)
Kb = [NH4+(aq)][OH-(aq)] / [NH3(aq)]
Therefore, the larger the value of Kb, the stronger is the base. The value is sometimes expressed as the logarithm of its reciprocal, called pKb. Therefore
pKb = -log Kb
The smaller the value of pKb the stronger the base. Kb is a better measure of the strength of a base than pH because adding more water to the base solution will not change the value of the equilibrium constant Kb, but it will change the H+ ion concentration.
In the above reaction, ammonium ions and ammonia form a conjugate acid-base pair.
See also Base | Equilbrium Constant.