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Buffer action and acidic buffer mechanism

Short answer

Buffer action is the resistance of a solution to pH change upon adding small amounts of strong acid or base. In an acidic buffer like CH₃COOH/CH₃COONa, added H⁺ reacts with CH₃COO⁻ to form CH₃COOH, while added OH⁻ is neutralized by CH₃COOH, keeping pH nearly constant.

What is buffer action? Describe the mechanism of acidic buffer solution with a suitable example.

Buffer action

The ability of a buffer solution to resist a change in its pH on adding a small amount of a strong acid or a strong base (or on dilution) is called buffer action. A buffer is made of a weak acid and its salt with a strong base (acidic buffer, e.g. CH₃COOH + CH₃COONa), or a weak base and its salt with a strong acid (basic buffer, e.g. NH₄OH + NH₄Cl).

Mechanism of an acidic buffer (CH₃COOH / CH₃COONa)

In the solution:

CH₃COOH  ⇌  CH₃COO⁻ + H⁺      (weak acid, slightly ionised)
CH₃COONa →  CH₃COO⁻ + Na⁺     (salt, fully ionised)

The salt gives a large reserve of acetate ions (CH₃COO⁻), which suppresses the ionisation of acetic acid (common ion effect). So the solution contains a large amount of undissociated CH₃COOH (acid reserve) and CH₃COO⁻ (base reserve).

  • On adding a small amount of a strong acid (H⁺): the extra H⁺ combines with the acetate ions to form the weak acid. CH₃COO⁻ + H⁺ → CH₃COOH Free H⁺ is removed, so pH is almost unchanged.
  • On adding a small amount of a strong base (OH⁻): OH⁻ is neutralised by the acetic acid (acid reserve), and more acetic acid ionises to replace the H⁺ used up. CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O Free OH⁻ is removed, so pH is almost unchanged.

The pH of the buffer is given by Henderson's equation: pH = pKa + log([salt]/[acid]). Since the ratio [salt]/[acid] changes only slightly, the pH stays nearly constant.

✓ Answer checked by Claude Opus 5.5 (high effort)

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