Acidity constant

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 pK_A 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 pK_B is defined by:

There is a relation between K_A and K_B:

and

where K_E is the dissociation constant of water.