8.12 Titration of Weak Acids and Bases

Shape of a Weak Acid/Base Titration Curve

Titrating a weak acid with a strong base gives an S-shaped curve with a gradual initial rise, a buffer region (flat, gently sloping), a steep equivalence point, and then leveling off. Unlike strong acid/base titrations, the equivalence point pH is not 7 — it's basic (pH > 7) for weak acid + strong base, and acidic (pH < 7) for weak base + strong acid, due to the conjugate species produced.

Half-Equivalence Point and pKa

At the half-equivalence point, exactly half the weak acid has been converted to its conjugate base, so [HA] = [A⁻]. Plugging into Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]) = pKa + log(1) = pKa. This lets you read pKa directly off the curve, which also identifies the weak acid's Ka.

Equivalence Point Chemistry

At the equivalence point, moles acid = moles base added, but the solution isn't neutral. For weak acid + strong base, the resulting conjugate base (A⁻) hydrolyzes water, making the solution basic. Use stoichiometry to find moles of A⁻, then a Kb equilibrium calculation (Kb = Kw/Ka) to find the equivalence point pH.

Key terms

Half-equivalence point

The point where half the weak acid/base has reacted; [HA] = [A⁻], so pH = pKa.

Equivalence point pH (weak acid + strong base)

Greater than 7; the conjugate base formed hydrolyzes water to produce OH⁻.

Equivalence point pH (weak base + strong acid)

Less than 7; the conjugate acid formed hydrolyzes water to produce H3O+.

Buffer region

The flat portion of the titration curve where both weak acid and conjugate base are present, resisting pH change.

How to find Ka from a titration curve

Read the pH at the half-equivalence point; since pH = pKa there, Ka = 10^(-pH).

Try an AP-style question

Why is the equivalence-point pH above 7 when a weak acid is titrated with NaOH?

1. Na⁺ ions react with water to make the solution basic
2. Excess NaOH is always present at equivalence
3. The conjugate base A⁻ reacts with water to make OH⁻
4. The weak acid is still mostly un-ionized there