Buffering behavior of polyprotic acids: At which pH values can a polyprotic acid like H3PO4 act as an effective buffer?

Introduction / Context:
Buffers resist changes in pH when small amounts of acid or base are added. Polyprotic acids such as phosphoric acid (H3PO4) have multiple dissociation steps and therefore multiple pKa values, enabling useful buffering over several pH regions.

Given Data / Assumptions:

• Polyprotic acid with distinct pKa1, pKa2, pKa3.
• Conjugate acid/base pairs coexist near each pKa.
• Standard aqueous conditions.

Concept / Approach:
Buffers are most effective when pH ≈ pKa because both the acid and its conjugate base are present in comparable amounts. A polyprotic system provides several such pH plateaus. Phosphate buffering in biology illustrates this with a key pair around pKa2 ≈ 7.2 (H2PO4−/HPO4^2−).

Step-by-Step Solution:
Identify that each dissociation step has its own pKa.Recall buffer effectiveness is maximized near pH = pKa.Conclude buffering is possible near any of the acid’s pKa values.

Verification / Alternative check:
Henderson–Hasselbalch: pH = pKa + log([base]/[acid]). When the ratio is about 1, buffering is strongest.

Why Other Options Are Wrong:

• A/B: Buffering does not require combination with polyprotic bases nor low concentration; it requires appropriate acid/base ratio.
• C: Not limited to neutrality; any pKa region qualifies.
• E: Buffers work in water; nonaqueous media are not required.

Common Pitfalls:
Assuming buffers only work at pH 7; in reality, they work around their specific pKa values.

Final Answer:
At pH values near any of its pKa values.