Chemical Foundations of Life

Question. How does a mixture of acetic acid ($CH_3COOH$) and sodium acetate ($CH_3COONa$) resist pH change when a strong acid, a strong base, or water is added?

Solution.

1. Sodium acetate is a salt and dissociates completely: $CH_3COONa \rightarrow CH_3COO^- + Na^+$ — this supplies a large, standing reservoir of the conjugate base, $CH_3COO^-$.
2. Acetic acid is a weak acid, so it stays mostly undissociated: $CH_3COOH \rightleftharpoons CH_3COO^- + H^+$. The solution now holds both $CH_3COOH$ and $CH_3COO^-$ at reasonably similar concentrations — satisfying the conditions for a buffer.
3. Add a strong acid ($H^+$): the reservoir of $CH_3COO^-$ absorbs it — $CH_3COO^- + H^+ \rightarrow CH_3COOH$. $H^+$ is consumed converting conjugate base into acid, so pH barely changes.
4. Add a strong base ($OH^-$): the acid neutralizes it — $CH_3COOH + OH^- \rightarrow CH_3COO^- + H_2O$. The acid is consumed converting into conjugate base and water, so pH barely changes.
5. Dilute with water: $[CH_3COOH]$ and $[CH_3COO^-]$ both drop by the same factor, so their ratio is unchanged — by the Henderson-Hasselbalch equation, pH stays (nearly) the same.

Answer: Because $CH_3COOH$ and $CH_3COO^-$ are both present in reasonably similar amounts, any added $H^+$ or $OH^-$ is consumed converting one species into the other rather than accumulating freely in solution, so pH shifts only slightly.