What is "dissociation constant"

n. the equilibrium constant for a reversible dissociation

In chemistry, biochemistry, and pharmacology, a dissociation constant (K ) is a specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex falls apart into its component molecules, or when a salt splits up into its component ions. The dissociation constant is the inverse of the association constant. In the special case of salts, the dissociation constant can also be called an ionization constant.

For a general reaction:

{A}_\mathit{x}{B}_\mathit{y} <=> \mathit{x}{A} + \mathit{y}{B}

in which a complex $\ce{A}_x \ce{B}_y$ breaks down into x A subunits and y B subunits, the dissociation constant is defined

K_d = \frac{[\ce A]^x [\ce B]^y}{[\ce A_x \ce B_y]}

where [A], [B], and [AB] are the concentrations of A, B, and the complex AB, respectively.

One reason for the popularity of the dissociation constant in biochemistry and pharmacology is that in the frequently encountered case where x=y=1, K has a simple physical interpretation: when $[\ce A] = K_d$, $[\ce B] = [\ce {AB}]$ or equivalently $\tfrac {[\ce{AB}]}{{[\ce B]} + [\ce{AB}]} = \tfrac{1}{2}$. That is, K, which has the dimensions of concentration, equals the concentration of free A at which half of the total molecules of B are associated with A. This simple interpretation does not apply for higher values of x or y. It also presumes the absence of competing reactions, though the derivation can be extended to explicitly allow for and describe competitive binding. It is useful as a quick description of the binding of a substance, in the same way that EC50 and IC50 describe the biological activities of substances.