Phenolphthalein
An acid–base indicator that is colourless in acid and pink in alkaline solution, changing between pH 8.3 and 10.0.
Formula / equation
C₂₀H₁₄O₄
Unit
Dimensionless
In depth
Its high transition range makes it the standard choice for titrating a weak acid with a strong base, where the equivalence point is alkaline. The colour arises from an extended conjugated system that forms when the molecule loses two protons and opens its lactone ring.
Examples in the real world
Titrating ethanoic acid with NaOH; the colour fades slowly in air as carbon dioxide dissolves.