Final answer:
To determine a buffer's effective pH range, assess the titration curve around the pKa value, where the pH change per added base is minimal. Phosphoric acid has two main equivalence points at pH ≈ 4.6 and pH ≈ 9.8, corresponding to its first two dissociation events, and buffer zones around the half equivalence points.
Step-by-step explanation:
To determine the pH range over which a particular buffer system works best, look for the range around the pKa value at the midpoint of the titration curve, where the pH change is minimal. For phosphoric acid (H3PO4), a polyprotic acid, there are three pKa values corresponding to its three dissociable protons.
The values are approximately pKa1 ≈ 2.14, pKa2 ≈ 7.20, and pKa3 ≈ 12.35. The buffer regions are within ±1 pH unit from each pKa.
The titration curve for phosphoric acid with a strong base like NaOH shows two distinct equivalence points, one for each protonation event that occurs under normal laboratory conditions, at pH ≈ 4.6 and pH ≈ 9.8, respectively. The third point for the last proton is less defined because HPO42- is a very weak acid.
The shape of the titration curve for a weak acid with a strong base displays a gradual pH change around the equivalence points, which contrast with the steeper curve seen in strong acid-strong base titrations.
The curve typically has a buffer zone around the half equivalence points, where the pH is equal to the pKa and the acid-to-conjugate base ratio is 1:1. Understanding the titration curve is essential to identify these points and to predict buffer capacity.