If small amount of an acid or alkali is added to a buffer solution, it converts them into unionised acid or base. Thus, its pH remains unaffected or in other words its acidity/alkalinity remains constant. e.g., H3O++A−⇌H2O+HA−OH+HA⟶H2O+A− If acid is added, it reacts with A−to form undissociated HA. Similarly, if base/alkali is added, OH−combines with HA to give H2O and A−and thus, maintains the acidity/ alkalinity of buffer solution.