halfequivalence

Halfequivalence, or half-equivalence, is a point in an acid–base titration where exactly half of the titrant needed to reach the equivalence point has been added. At this stage the solution contains equal amounts of the acid form and the conjugate base (for a weak acid HA titrated with a strong base, [HA] = [A−]); similarly, for a weak base titrated with a strong acid, the concentrations of the base and its conjugate acid are equal.

The half-equivalence point is significant because the solution behaves as a buffer. By the Henderson–Hasselbalch relation, the pH at halfequivalence equals the pKa of the acid being titrated (or the pKa of the conjugate acid in the case of a base being titrated). This makes halfequivalence a useful reference for estimating pKa values from titration data and for identifying the buffer region of the titration curve.

Practically, halfequivalence is found by locating the volume of titrant that is half of the volume required to reach the equivalence point. In a titration with known concentrations, this volume is Veq/2, where Veq is the equivalence volume determined from stoichiometry or the titration plot.

Example: titrating a weak acid such as acetic acid with a strong base like NaOH. The equivalence point occurs at a certain added base volume Veq; half-equivalence occurs at Veq/2, and the pH at that point equals the pKa of acetic acid (about 4.76).

Halfequivalence thus provides a practical link between titration measurements and acid–base dissociation constants, helping to characterize buffering behavior and extract pKa values from experimental data.