Weak acid and weak base titration curves begin at less extreme pH values and have smaller vertical sections than strong acidandstrong base curves. Their equivalence-point pH is not automatically 7; it depends on the acidbase system.
How the curves differ
Weak acids and bases only partially ionize. As titrant is added, a weak species and its conjugate coexist, producing a buffer region where pH changes gradually. At the equivalence point, stoichiometrically equivalent amounts have reacted, but the conjugate product may undergo hydrolysis and shift the pH.
| Titration | Features |
|---|---|
| Weak acid + strong base | Higher initial pH than a strong acid; buffer region; smaller vertical rise; equivalence point above pH 7 because the conjugate base reacts with water. |
| Weak base + strong acid | Lower initial pH than a strong base; buffer region; smaller vertical fall; equivalence point below pH 7 because the conjugate acid reacts with water. |
| Weak acid + weak base | Shallow curve near equivalence, often without a clear vertical section; equivalence-point pH depends on relative and values. |
For a weak acid titrated with strong base, the half-equivalence point has equal concentrations of acid and conjugate base, so . For a weak base titrated with strong acid, at half-equivalence.
A common misconception is that every equivalence point occurs at pH 7. This is generally true for strong acidandstrong base titrations at 298 K, but not when the salt formed hydrolyses.
Exam technique
In an IB Chemistry HL sketch, label both axes, initial pH, buffer region, half-equivalence point and equivalence point. Match the curve’s direction to the titrant. For an indicator, state that its transition range must lie within the steep pH change. A weak acidweak base titration usually requires a pH probe because no indicator gives a sharp, reliable endpoint.