Final answer:
Adjust both equations (4) and (5) to take into account the addition of a strong base to a diprotic acid solution when forming a buffer, as it affects both the concentrations of the acid and its conjugate bases, and the ionic strength of the solution.
Step-by-step explanation:
The modification suggested for an exact calculation of [H+] (or pH) in a diprotic acid solution when adding a strong base to create a buffer is to adjust both equations (4) and (5) to account for the strong base.
This is because when a strong base is added to a buffer solution containing a diprotic acid, the base will react with the hydrogen ions (H+) as well as the acid itself (H2A). The reaction between the strong base and acid will form the conjugate base (HA-), altering the equilibrium of the diprotic acid dissociation and thus affecting the pH of the solution.
Therefore, both the concentration of the diprotic acid (H2A), its first conjugate base (HA-), and its second conjugate base (A2-) need to be adjusted to reflect the addition of the strong base. Moreover, the counterion of the strong base should be included in the calculation as it also affects the ionic strength of the solution which can impact the activity coefficients of the species in solution and the overall pH.