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Phenolphthalein

Phenolphthalein also abbreviated as phph is an organic compound used in acid-base titration as an indicator. This is because of the variation in colours with respect to the change in pH.

phph as a pH indicator shows:

  • Pink to red color in alkaline medium, above 8.5 pH.
  • Colorless in acidic medium, below 8.5 pH.

It was discovered by Adolf von Baeyer in 1871. 

Structure, Formula & Properties

IUPAC Name – 3,3-bis(4-hydroxyphenyl)-2-benzofuran-1-one
Molecular Formula – \(C_{20}H_{14}O_4\)
Molecular Weight – \(318.32 g mol^{-1}\)
Density – \(1.277 g cm^{-3}\)
Melting Point – \(258 – 263^{\circ}C\).

Structure of Phenolphthalein
Structure of phph (Source)

Phenolphthalein is synthesized by phthalic anhydride condensation, with two phenol equivalents under acidic conditions.

Applications

  1. The compound is popularly used in acid-base titration as an indicator.
  2. It is also used in various medical tests.
  3. It is also used in Cement Carbonation.

Related Topic on pH indicator: Methyl Orange

FAQs

The elements whose oxides can turn phenolphthalein solution pink are?

Since Sodium(Na) and Potassium(K) are metals with basic oxides, which can turn phph solution pink.

Colour of Phenolphthalein in acid.

phph is colourless in acid.

Colour of Phenolphthalein in base.

phph colour varies from pink to red in an alkaline substance.

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