In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction
known as dissociation in the context of acid–base reactions. The chemical species HA is an acid that dissociates into , the conjugate base of the acid and a hydrogen ion, . The system is said to be in equilibrium when the concentrations of its components will not change over time, because both forward and backward reactions are occurring at the same rate.
The dissociation constant is defined by
: or
where quantities in square brackets represent the concentrations of the species at equilibrium. As a simple example for a weak acid with K = 10, log K is the exponent which is -5, so that pK = 5. And for acetic acid with K = 1.8 x 10, pK is close to 5. A higher K corresponds to a stronger acid which is more dissociated at equilibrium. For the more convenient logarithmic scale, a lower pK means a stronger acid.