The Acid Dissociation Constant, Ka, is the equilibrium constant for the reaction in which a weak acid is in equilibrium with its conjugate base in aqueous solution. Notice that in the equilibrium expression below the concentration of water is not included. This is because water is vastly in excess and the amount changes negligibly on equilibrium being established. Ka can be thought of as a modified equilibrium constant. For example,
CH3COOH(aq) + H2O(l) = CH3COO-(aq) + H3O+(aq)
Ka = [CH3COO-(aq)][H3O+(aq)] / [CH3COOH(aq)]
Therefore, the larger the value of Ka, the stronger is the acid. The value is sometimes expressed as the logarithm of its reciprocal, called pKa. Therefore,
pKa = -log Ka
The smaller the value of pKa the stronger the acid. Ka is a better measure of the strength of an acid than pH because adding more water to the acid solution will not change the value of the equilibrium constant Ka, but it will change the H+ ion concentration on which pH depends.
In the above reaction, ethanoic acid and ethanoate ions form a conjugate acid-base pair.
See also Acid | Equilbrium Constant | pH.