Explain buffer action of sodium acetate-acetic acid buffer.

Answer: In the buffer, acetic acid is weakly ionised while sodium acetate is completely ionised:

CH₃COOH ⇌ CH₃COO⁻ + H⁺ (small extent)
CH₃COONa → CH₃COO⁻ + Na⁺ (completely)

So the solution contains large amounts of both CH₃COOH and CH₃COO⁻.

1. On adding a little acid (H⁺): the added H⁺ combines with acetate ions to form weakly ionised acetic acid: CH₃COO⁻ + H⁺ → CH₃COOH. The H⁺ concentration hardly changes.
2. On adding a little base (OH⁻): the added OH⁻ is neutralised by acetic acid: CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O.

Hence the pH of the buffer remains almost constant; this resistance to change in pH is called buffer action.