Buffer capacity and effectiveness: Which factors most strongly determine how effective a buffer is?

Introduction / Context:
Buffers are indispensable in biological experiments. Their “capacity” is the amount of strong acid or base that can be added before pH changes appreciably. Choosing the right buffer conditions ensures reliable enzymatic activity and stability of macromolecules.

Given Data / Assumptions:

• Weak acid/conjugate base pair in water.
• Henderson–Hasselbalch relationship applies.
• Constant temperature for routine use.

Concept / Approach:
Buffer capacity increases with the total concentration of the conjugate pair and is maximal when pH ≈ pKa (that is, when the acid/base ratio is ~1). Very dilute buffers or those far from pKa have poor capacity. Temperature affects pKa slightly, but core determinants are concentration and ratio.

Step-by-Step Solution:
Use pH = pKa + log([base]/[acid]).Maximize capacity by choosing pH near pKa and sufficient total concentration.Select the option combining both determinants.

Verification / Alternative check:
Titration curves are flattest (best buffering) near pKa and become steeper (poor buffering) far from pKa or at very low concentration.

Why Other Options Are Wrong:

• Temperature only: Can shift pKa modestly but is not the primary determinant of capacity.
• Glassware: Not a chemical factor for capacity.

Common Pitfalls:
Using extremely dilute buffers or targeting pH more than 1 unit away from pKa, which yields unstable pH.

Final Answer:
Both (a) and (b).