Given a weak acid AH, its dissolution into water is subject to the equilibrium :

AH + H2O ↔ A- + H3O+

The acidity constant Ka of the couple AH/A- is defined by:

the pKA is defined by:

([AH] indicates the concentration of AH in the solution).

By analogy, one can define the basicity constant of a weak base:

the pKB is defined by:

There is a relation between KA and KB:

where KE is the dissociation constant of water.