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Buffer capacity; benzoic acid-benzoate buffer

Short answer

Buffer capacity is the moles of strong acid or base needed to change 1 L of buffer pH by one unit. In benzoic acid/benzoate buffer, added H⁺ is consumed by C₆H₅COO⁻ to form C₆H₅COOH, and added OH⁻ reacts with C₆H₅COOH to give C₆H₅COO⁻ and water; the salt/acid ratio stays nearly constant, preserving pH.

What is meant by buffer capacity of a buffer solution? How does a solution containing a mixture of benzoic acid and sodium benzoate maintain its constant pH value even on the addition of small amount of strong acid or alkali? Explain.

Buffer capacity

The buffer capacity (buffer index) is the number of moles of strong acid or strong base needed to change the pH of one litre of the buffer by one unit. A buffer has maximum capacity when [salt] = [acid] (pH = pKa) and when the concentrations of its components are high.

Action of benzoic acid / sodium benzoate buffer

The solution contains the weak acid C₆H₅COOH (mostly undissociated) and a large concentration of benzoate ions, C₆H₅COO⁻, from the fully ionised salt C₆H₅COONa.

  • On adding a small amount of strong acid (H⁺): the benzoate ions combine with H⁺ to give unionised benzoic acid, so [H⁺] hardly increases. C₆H₅COO⁻ + H⁺ → C₆H₅COOH
  • On adding a small amount of strong alkali (OH⁻): OH⁻ reacts with the benzoic acid to give benzoate and water, so [OH⁻] hardly increases. C₆H₅COOH + OH⁻ → C₆H₅COO⁻ + H₂O

In both cases the ratio [C₆H₅COO⁻]/[C₆H₅COOH] changes only a little, and pH = pKa + log([salt]/[acid]) remains almost constant.

✓ Answer checked by Claude Opus 5.5 (high effort)

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