Chemistry / 01 · acid–base titration

What happens at equivalence?

Add a strong base to a strong acid. Watch the excess ions disappear, then follow the sharp turn on the pH curve.

Fig. 01 / A balance of amounts25 °C · ideal solution

Burette: 0.100 mol/L strong base.
Beaker: 25.0 mL strong monoprotic acid.
Symbols: + = excess H₃O⁺; − = excess OH⁻.
Each symbol ≈ 0.25 mmol; a partial symbol is fractional. Spectator ions and trace ions are omitted.

Acid in excess

1.95pH

Current stateEquivalence

Base added

Equivalence volume

Added base 20.0 mL

0 mL60 mL

Initial acid 0.100 mol/L

0.050 mol/L0.150 mol/L

Equivalence is about amount.

Neutral is about pH.

Here, equivalence is also neutral: pH 7.00. That coincidence needs a strong acid, a strong base and 25 °C. Weak-acid or weak-base titrations can reach equivalence at a different pH.

Model limits. Complete dissociation, a 1:1 reaction and additive volumes; no activity or temperature corrections. pH includes dilution and water autoionization (K_w = 10⁻¹⁴), so the curve remains finite at equivalence. Liquid height is schematic; ion symbols show net excess, not molecular scale. This is a conceptual model, not a laboratory procedure.