 Dissociation constant

A dissociation constant is an equilibrium constant for a reaction in which a combined species separates into components. For $AB\rightleftharpoons A+B$, a concentration convention writesIt has units of concentration. The dimensionless thermodynamic form instead uses thermodynamic activities, $K_d^{\circ }=a_A{a}_B/a_{AB}$. For ideal concentration activities, $K_d^{\circ }=K_d^{(c)}/c^{\circ }$, with specified standard concentration $c^{\circ }$. The choice of quantity and reference is therefore part of quoting the constant.^[ref]

In the simple dilute $1:1$ model with free $c_A$ held fixed, the fraction of B present as AB is $c_A/(K_d^{(c)}+c_A)$. A smaller $K_d^{(c)}$ therefore means more association at the same free concentration. The inverse association constant describes the reversed reaction only when conventions, solvent conditions and temperature match. A dissociation rate is a kinetic quantity and is not specified by this equilibrium ratio alone.