A buffer resists changes in pH. It is a mixture of a weak acid and its conjugate base, in comparable amounts: exactly the kind of ions you met in salt solutions. Blood, seawater and most lab reactions rely on buffers.
Key idea
The weak acid HA neutralizes added base, $\mathrm{HA + OH^- \to A^- + H_2O}$, and the conjugate base $\mathrm{A^-}$ neutralizes added acid, $\mathrm{A^- + H_3O^+ \to HA + H_2O}$. Both reactions go essentially to completion, so the strong acid or base is replaced by a weak one and the pH changes only a little.
Why it works
The pH of a buffer depends on the ratio $[\mathrm{A^-}]/[\mathrm{HA}]$ (as the next lesson shows). Adding a little strong acid turns a little $\mathrm{A^-}$ into HA: the ratio changes slightly, so the pH does too, but only slightly. In plain water, the same acid has nothing to react with, so it all stays as $\mathrm{H_3O^+}$ and the pH crashes. A strong acid with its own salt (HCl with NaCl) cannot buffer: $\mathrm{Cl^-}$ is too weak a base to take up added acid.
Worked examples
Worked example: Adding acid to a buffer and to water
A liter holds $0.10\ \text{mol}$ $\mathrm{CH_3COOH}$ and $0.10\ \text{mol}$ $\mathrm{CH_3COO^-}$ (pH 4.74). Add $0.010\ \text{mol}$ HCl. Find the new pH, and compare with adding the same HCl to a liter of pure water.
- Neutralization: $\mathrm{CH_3COO^- + H_3O^+ \to CH_3COOH + H_2O}$. Acetate: $0.10 - 0.010 = 0.09\ \text{mol}$; acetic acid: $0.11\ \text{mol}$.
- $\text{pH} = 4.74 + \log\dfrac{0.09}{0.11} = 4.74 - 0.09 = 4.65$. A change of 0.09.
- In water: $[\mathrm{H_3O^+}] = 0.010\ \text{M}$, pH 2.00. A change of 5 units.
Answer: The buffer goes to 4.65; water goes to 2.00.
Worked example: Which mixtures are buffers?
Which of these make a buffer: (a) HCl and NaCl; (b) $\mathrm{NH_3}$ and $\mathrm{NH_4Cl}$; (c) HF and NaF; (d) 0.20 mol $\mathrm{CH_3COOH}$ with 0.10 mol NaOH?
- (a) No: HCl is strong, and $\mathrm{Cl^-}$ cannot take up acid.
- (b) Yes: weak base plus its conjugate acid. (c) Yes: weak acid plus its conjugate base.
- (d) Yes: the NaOH turns 0.10 mol of the acid into acetate, leaving 0.10 mol of each. A buffer can be made by half-neutralizing a weak acid.
Answer: (b), (c) and (d).
Common mistake
Thinking a buffer keeps the pH from changing at all, or that a strong acid plus its salt makes a buffer. A buffer slows the change; enough acid or base will still overwhelm it. And it needs a weak acid with its conjugate base, so both partners can react.
Blood is held near pH 7.4 mainly by the carbonic acid / hydrogen carbonate pair, with the lungs adjusting the amount of $\mathrm{CO_2}$.