Acid dissociation constant

An acid dissociation constant, K_a, (also known as acidity constant, or acid-ionization constant) 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. In aqueous solution, the equilibrium of acid dissociation can be written symbolically as:

where HA is a generic acid that dissociates into A⁻, known as the conjugate base of the acid and a hydrogen ion which combines with a water molecule to make an hydronium ion. In the example shown in the figure, HA represents acetic acid, and A⁻ represents the acetate ion, the conjugate base.

The chemical species HA, A⁻ and H₃O⁺ are said to be in equilibrium when their concentrations do not change with the passing of time. The dissociation constant is usually written as a quotient of the equilibrium concentrations (in mol/L), denoted by [HA], [A⁻] and [H₃O⁺]

In all but the most concentrated aqueous solutions of an acid the concentration of water can be taken as constant and can be ignored. The definition can then be written more simply

Dissociation constant

In chemistry, biochemistry, and pharmacology, a dissociation constant ( ) 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:

in which a complex breaks down into x A subunits and y B subunits, the dissociation constant is defined

where [A], [B], and [A_xB_y] are the concentrations of A, B, and the complex A_xB_y, 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_d has a simple physical interpretation: when , or equivalently . That is, K_d, 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.