Equivalence point

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Equivalence point or stoichiometric point occurs during a chemical titration when the amount of titrant added is equivalent, or equal, to the amount of analyte present in the sample. In some cases there are multiple equivalence points which are multiples of the first equivalent point, such as in the titration of a diprotic acid. A graph of the titration curve exhibits an inflection point at the equivalence point. A striking fact about equivalence is that in a reaction the equivalence of the reactants as well as products is conserved.

[edit] Acid-base titration example

Equivalence point occurs during an acid-base titration when equal amounts of acid and base have been reacted. A graph of pH against concentration becomes almost vertical at the equivalence point. The equivalence point of a titration does not mean that the solution has reached pH 7; merely that all the initial reactants have been reacted. The actual pH of the solution at equivalence point is determined by considering the acidity or basicity of the aqueous product of the reaction, most commonly by the Brønsted-Lowry Theory of acids and bases.

When performing a manual titration, it may be difficult or impossible to detect when the equivalence point is reached. Often a pH indicator is added to the reaction vessel with an endpoint that is very close to the equivalence point. This causes a visible color change at the equivalence point and therefore at the point that no more titrant should be added.

Acid-base titrations are commonly taught in school and are the most familiar form of titration to most people but is only one form of titration. See titration for more detail.