Phenolphthaleinendpoint

Phenolphthaleinendpoint is a term used in analytical chemistry to denote the endpoint of an acid–base titration as indicated by the phenolphthalein indicator. It is not a distinct chemical compound or process itself, but rather a descriptor for the stage at which phenolphthalein changes color and is judged to have signaled completion of the reaction.

Phenolphthalein is a triphenylmethane-type pH indicator. In acidic solutions it is essentially colorless, and as the solution becomes basic it undergoes a structural change that produces a pink to fuchsia color. The observable color transition for phenolphthalein typically occurs over a pH range around 8.2 to 10.0, though the exact range can shift with concentration, solvent, and impurities. The endpoint is usually defined operationally as the point at which a faint pink color persists for a specified time (commonly around 30 seconds), indicating the reaction has reached completion under the chosen conditions.

In practice, the phenolphthaleinendpoint is well suited for titrations where the equivalence point lies above pH 7, such as weak-acid/strong-base titrations, and it is commonly used in standardizing base solutions and in water hardness determinations. It is less appropriate for titrations that reach the equivalence point near neutral pH or in strongly acidic conditions, where the color change can be transient or ambiguous due to buffering or carbon dioxide in the atmosphere.

See also: phenolphthalein, pH indicator, titration endpoint.