Phenolphthalein

Phenolphthalein is an organic compound used as an acid–base indicator. It is colorless in acidic and near-neutral solutions and pink in alkaline solutions.

Acid–base indicator: A substance whose observable properties change over a particular range of acidity or alkalinity. Phenolphthalein is one indicator whose color changes across a characteristic pH range.
Acid–base titration: A quantitative method that determines an acid or base concentration through a neutralization reaction. Phenolphthalein signals the endpoint of suitable titrations through a persistent pink color.
Methyl orange: An acid–base indicator that changes from red to yellow over an acidic pH range. Its lower transition range suits strong-acid titrations where phenolphthalein may not.
Adolf von Baeyer: A German chemist who investigated organic dyes and synthesized phenolphthalein in 1871. Baeyer first prepared phenolphthalein while studying phthalic anhydride and phenols.
Brønsted–Lowry acid–base theory: A theory defining acids as proton donors and bases as proton acceptors. It describes the proton transfers that alter phenolphthalein's molecular forms.
Acid–base equilibrium: The balance between proton-donating and proton-accepting forms in a chemical system. Changes in proton concentration shift the forms of phenolphthalein present in solution.
Endpoint (chemistry): The observed point in a titration at which an indicator changes color or another signal appears. A faint, lasting pink marks the endpoint in many titrations using phenolphthalein.
Bromothymol blue: An acid–base indicator that shifts between yellow and blue near neutral pH. Its transition near neutrality contrasts with phenolphthalein's alkaline color change.
Dye: A colored substance that binds to or is incorporated into a material to impart color. Phenolphthalein emerged from nineteenth-century research into synthetic organic dyes.
Acid dissociation constant: An equilibrium constant that quantifies the strength of an acid in solution. The indicator's transition range depends on the equilibria between its protonated forms.

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