Final answer:
The formula given is oversimplified for polyprotic acids like phosphoric acid. Calculating the buffer capacity of a sodium phosphate buffer should consider each dissociation step's pKa values and the buffer capacity should be determined empirically.
Step-by-step explanation:
The formula b = amount of OH-/H3O+ divided by volume of buffer x change in pH is a simplification and is typically used for monoprotic acids where the buffer components have a 1:1 ratio. When dealing with polyprotic acids, like phosphoric acid, this formula may not directly apply due to the multiple dissociation steps and the different buffering regions for each dissociation. Each dissociation step has its own Ka or Kb value, which affects the pH buffering differently.
Hence, for sodium phosphate buffers composed of NaH2PO4 and Na2HPO4, it's important to consider the relevant pKa values for the dissociation steps when calculating the buffer capacity. The Henderson-Hasselbalch equation is generally more useful to describe the pH in polyprotic systems, and the buffer capacity can be determined empirically by measuring the amount of strong acid/base added to cause a unit change in pH in the buffering zone.