Acidity constant

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In chemistry, given a weak acid, AH, its dissolution into water is subject to the equilibrium :

AH + H₂O ↔ A^- + H₃O⁺

This is often written as:

AH ↔ A^- + H⁺

The acidity constant or acid dissociation constant, , of the couple AH/A^- is defined by:

the pK_a is defined by:

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

What does all this mean? One way to think of pK_a is this: the pK_a is the pH at which an ionizable group is 50% protonated. (Assumptions: perfect aqueous system, proton system, etc., etc.)

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.