Weak Acid or Base Titration
Titrate vinegar with sodium hydroxide, or ammonia with hydrochloric acid. The curve has a flat buffer region, a point where the pH equals the pKa, and an equivalence point that is not at pH 7.
Why a Weak Acid Curve Looks Different
A weak acid is only partly dissociated, so it starts at a higher pH than a strong acid of the same concentration. As base is added, the acid HA turns into its conjugate base A−. With both present the solution is a buffer: the pH changes only slowly, the flat middle part of the curve.
Half-way to equivalence there is exactly as much A− as HA, so the pH equals the pKa — a classic way to measure it. At the equivalence point only A− is left, which is a weak base, so the pH is above 7. For a weak base titrated with acid everything is mirrored and the equivalence point is below 7.
Titration Curve
pHWeak Acids and Bases
25 °C| Substance | K | pK |
|---|---|---|
| Acetic acid (vinegar) | Ka = 1.8×10−5 | 4.74 |
| Formic acid | Ka = 1.8×10−4 | 3.74 |
| Hydrofluoric acid | Ka = 6.8×10−4 | 3.17 |
| Ammonium ion | Ka = 5.6×10−10 | 9.25 |
| Ammonia | Kb = 1.8×10−5 | 4.74 |
Worked Techniques
Between about 10% and 90% of the way, the Henderson–Hasselbalch equation gives a good estimate.
Read the pH half-way to the equivalence point and you have measured the pKa of the acid.
Only the conjugate base is left, which reacts with water and makes the solution basic.
Sources & Verification
- S. S. Zumdahl & S. A. Zumdahl, Chemistry, 10th ed., Cengage, 2018 — Ch. 14 (Acids and Bases) and Ch. 15 (Acid–Base Equilibria).