8.5 Acid-Base Titrations

The titration setup

In a titration, a solution of known concentration (titrant) is added gradually to a solution of unknown concentration until the reaction reaches the equivalence point — where moles of acid equal moles of base (accounting for stoichiometry). A titration curve plots pH vs. volume of titrant added, showing distinct regions before, at, and after equivalence.

Strong vs. weak titration curves

Strong acid–strong base curves show a sharp, large pH jump near equivalence (pH = 7 at equivalence). Weak acid–strong base curves have a gradual rise with a buffer region (half-equivalence point where pH = pKa) and an equivalence point pH > 7, since the conjugate base makes the solution basic. Weak base–strong acid curves show equivalence pH < 7.

Indicators and half-equivalence

An indicator changes color near its own pKa, so it should be chosen so its color change occurs at the titration's equivalence point pH. At the half-equivalence point, exactly half the weak acid (or base) has reacted, so [HA] = [A⁻], making pH = pKa — a key point for identifying an unknown acid's Ka from a titration curve.

Key terms

Equivalence point

Point in titration where moles of acid equal moles of base, adjusted for stoichiometry.

Half-equivalence point

Point where half the acid (or base) has reacted; pH = pKa for a weak acid titration.

Strong acid–strong base equivalence pH

pH = 7, since neither the resulting cation nor anion hydrolyzes.

Weak acid–strong base equivalence pH

pH > 7, because the conjugate base formed makes the solution basic.

Choosing an indicator

Pick one whose color-change range matches the pH at the titration's equivalence point.

Try an AP-style question

What is the pH at the equivalence point of a strong acid–strong base titration?

1. Above 7
2. Equal to pKa
3. 7
4. Below 7