Science

pKa Definitions

Acid with one pKa

For an acidic solute with one pKa, the pKa is the number that denotes the pH at which half the molecules of the acid in aqueous solution exist in protonated form and are neutral, and half exist in deprotonated form and carry a negative charge.
 

   

Base with one pKa

For an basic solute with one pKa, the pKa is the number that denotes the pH at which half the molecules of the base in aqueous solution exist in deprotonated form and are neutral, and half exist in protonated form and carry a positive charge.

Substances with more than one pKa

When molecules have more than one pKa, the definition of pKa cannot be via a simple statement that “half are protonated and half are unprotonated”. Even so, it’s possible to visualise the pKa values from the appearance of the graph of Percent species vs. pH.