Base Dissociation Constant, K_b

The Base Dissociation Constant, K_b, is the equilibrium constant for the reaction in which a weak base is in equilibrium with its conjugate acid in aqueous solution. For example,

NH_3(aq) + H₂O_(l) = NH₄⁺_(aq) + OH^-_(aq)

K_b = [NH₄⁺_(aq)][OH^-_(aq)] / [NH_3(aq)]

Therefore, the larger the value of K_b, the stronger is the base. The value is sometimes expressed as the logarithm of its reciprocal, called pK_b. Therefore

pK_b = -log K_b

The smaller the value of pK_b the stronger the base. K_b 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 K_b, 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.