pK_a

When a weak acid , denoted by HA, is put into water it dissociates into hydrogen ions and base ions A^- according to the equilibrium

HA + H₂O = H₃O⁺ + A^-

This equilibrium is governed by an equilibrium constant K_a . By analogy with pH, this equilibrium constant is defined by the relationship

pK_a = - log10 Ka ([ HA ] / [ A^- ])

For a strong acid which is highly dissociated the value of pK_a is small whereas for a weak acid which is only partially dissiated the pKa is large.

Table of contents