Phthaleins

Phthaleins are a class of synthetic dyes that also function as acid–base indicators. They are prepared by condensing phthalic anhydride with two equivalents of phenol or substituted phenols, yielding a colorless closed lactone form (the spirolactone). In basic conditions, the lactone ring opens to produce a highly conjugated dye with vivid colors.

In acidic solutions the phthalein framework remains in the closed form and is typically colorless. Upon deprotonation and ring opening in alkaline solutions, the molecule adopts a quinoid structure that absorbs visible light, giving colors that range from pink or red to blue, depending on the substituents on the phenol rings. This color change makes phthaleins useful as pH indicators.

Common representatives include phenolphthalein, which changes from colorless to pink/red around pH 8.2–10.0 and is widely used in acid–base titrations. Thymolphthalein is another well-known example, offering a higher-pH transition and producing a blue color in basic solutions. The phthalein family also comprises various substituted derivatives such as cresolphthalein, among others, each with its own color range.

Applications of phthaleins are primarily in analytical chemistry as pH indicators for titrations and buffer studies. They are valued for sharp color transitions and broad working ranges. Synthesis generally involves condensation of phthalic anhydride with two phenolic components under acidic conditions, followed by dehydration to furnish the dye.