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Phenolphthalein Chemical Structure

What is Phenolphthalein?

Phenolphthalein is a member of phenols and is functionally related to a 2-benzofuran-1(3H)-one. It is a chemical compound commonly used as a pH indicator in various applications. It is colorless in acidic solutions (below pH 8.3) and turns pink or magenta in alkaline/basic solutions (above pH 10). This characteristic color change makes it valuable in determining the acidity or alkalinity (pH) of a solution, particularly in laboratory settings, educational demonstrations, and various industrial processes.

Macsen Laboratories is the leading manufacturer and supplier of Phenolphthalein with ISO certification.
Name of Product Phenolphthalein
IUPAC Name 3,3-bis(4-hydroxyphenyl)-2-benzofuran-1-one
Synonyms Phthalimetten; Euchessina; Phthalin; Espotabs; Phenolax; Purgophen; Koprol; Laxogen; Fenolftaleina; fenolftaleína
CAS No 77-09-8
Molecular Formula C20H14O4
Molecular Weight 318.3 g/mol
Pubchem CID 4764


SR. No Criteria Limit/Specification
1 Appearance (Form) Solid Powder
2 Appearance (Color) White to Off-white
3 pH range (8.2 - 10.0) Colorless to Red-violet
4 Absorption maxima (buffer pH 9.8) 551 - 554 nm
5 Sulphated Ash NMT 0.05%
6 Chloride (Cl) NMT 0.01%
7 Sulphate (SO4) NMT 0.01%
8 Loss on drying (at 110°C) NMT 1%


  • Phenolphthalein is widely used in chemical laboratories to determine the pH of a solution during acid-base titrations and other experiments. Additionally, it functions as a constituent of universal indicator when used together with thymol blue, bromothymol blue, and methyl red.
  • It can be used to test the pH of water in swimming pools, aquariums, and other water treatment applications to ensure water quality and safety.
  • Phenolphthalein finds application in toy manufacturing, such as in disappearing ink components and disappearing pigments utilized in the creation of “Hollywood Hair” Barbie hair.
  • It is used in the carbonation of concrete.
  • Phenolphthalein, in its colorless reduced form, is employed in the Kastle–Meyer test, which is a physiological procedure utilized to detect substances suspected of containing blood.
  • It is also used in educational settings for chemistry demonstrations and engaging experiments, showcasing the concept of pH changes.
  • Additionally, it has been used as a laxative in the past, but it has now largely been replaced by safer alternatives due to concerns about its safety.


Q. What color is phenolphthalein when the solution is acidic ?

Phenolphthalein is colorless in acidic solutions.

Q. What is the pKa of phenolphthalein?

Phenolphthalein has a pKa value of 9.7.

Q. Is phenolphthalein an acid?

Phenolphthalein is a weak acid.

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