What is phenolphthalein?

Phenolphthalein (molecular formula C20H14O4) is a pH indicator. In acidic solutions it remains colorless, but in the presence of basic solutions it takes on a pinkish color with a turning point between pH = 8.0 (colorless) to pH = 9.8 (magenta or pinkish). Its IUPAC name is 3,3-bis(4-hydroxyphenyl)-2-benzofuran-1-(3H)-one.

However, at extreme pH (very acidic or basic) it shows other changes in coloration: at strongly basic pH it becomes colorless, while at strongly acidic pH it changes to orange.

This color change is due to the conjugation of the π electrons of the phenolphthalein derivative which at different pH absorbs at visible light wavelengths. It is related to the different open and closed structure of the lactonic ring present in these compounds.

3D structure

The different forms of phenolphthalein are summarized in the following figures:

  • Cationic form of phenolphthalein.

fenolftaleina cationica

  • Anionic form of phenolphthalein (monoanion and dianion).

fenolftaleina anionica monoanion dianion

  • Neutral form of phenolphthalein (tautomers: lactonic, zwitterionic and quinonic ones).

fenolftaleina neutra forma lactonica quinonica zwitterionica

Phenolphthalein is insoluble in water and is usually dissolved in alcohols for use. It is a weak acid that can lose H protons in solution. Therefore, the main applications of phenolphthalein derivatives are as dyes and pH indicators.

Synthesis of phenolphthalein

Phenolphthalein identification reaction

Dissolve some crystals of phenolphthalein in 1 mL of ethanol, add 1 mL of distilled water, shake and add a few drops of soda solution. Note your observations.

Physico-chemical properties

Phenolphthalein is a white powder soluble in water (400 mg/L), and it has a density of 1.277 g/cm3 at 32 ºC. Its melting point is in the range of 259-261 ºC. Its molar mass is 318.32 g/mol.

International Chemical Identifier

In order to facilitate the nomenclature and formulation of chemical compounds and the search for information on the internet, the InChI key of the IUPAC identifier for phenolphthalein is: KJFMBFZCATUALV-UHFFFAOYSA-N