Equivalence Point
Definition and meaning of Equivalence Point in chemistry.
The equivalence point is the point during a titration at which the moles of titrant added are exactly stoichiometrically equivalent to the moles of analyte originally present in the solution.
In more detail
At this point, the reactants have combined in the exact ratio given by the balanced chemical equation, so neither the titrant nor the analyte is in excess. It is a calculated, theoretical value, distinct from the endpoint, which is the experimentally observed signal (such as an indicator color change or a sharp pH-meter inflection) used to approximate it. The pH at the equivalence point depends on what species form: it equals 7 for a strong acid-strong base reaction but shifts above or below 7 when the product ions hydrolyze water, as with a weak acid-strong base titration.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Also called | Stoichiometric point |
| Strong acid-strong base pH | 7 |
| Detection methods | pH meter, color indicator, or conductivity measurement |
Titrating 25.0 mL of 0.100 M HCl with 0.100 M NaOH reaches the equivalence point after exactly 25.0 mL of NaOH has been added, since moles of acid then equal moles of base, leaving only NaCl and water and giving a pH of 7.
Frequently asked questions
Is the equivalence point the same as the endpoint?
No. The equivalence point is the theoretical point where moles of titrant exactly equal moles of analyte; the endpoint is the observed signal, such as an indicator's color change, that approximates it and may differ slightly (indicator error).
Why isn't the equivalence point always at pH 7?
Because the salt formed by the reaction can hydrolyze. For example, titrating a weak acid with a strong base produces a conjugate base that reacts with water to raise the pH above 7 at the equivalence point.