• Dissociation constant

The pKa is the negative base-10 logarithm of the acid dissociation constant (Ka) of a solution. Under the pKa definition, the lower the pKa, the stronger the acid. This quality is described based on the fact that strong acids completely dissociate in water while weak acids only do so partially, forming a dissociated state (A-) and an undissociated state (AH) that coexist in solution:

AH ⇆ A- + H+

The concentration of both sides is constant in given conditions and thus, a defined condition can be defined as

Ka = [A-] [H+] / [AH]

The equation above indicates the concentration of released protons (acid) in relation to the molecule concentration in the solution. Hence, Ka represents the strength of an acid and as this value can be in the form of 10-3 to 10-6, the addition of a negative base logarithm allows for this value to be more intuitive.1

In DrugBank, it is possible to find the experimental and predicted value of pKa.


  1. Shimadzu [Link]