Phenolphthalein
coloured pH indicator used at high pH
Phenolphthalein ( feh-NOL(F)-thə-leen) is a chemical compound with the formula C20H14O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions. It belongs to the class of dyes known as phthalein dyes.
Phenolphthalein is slightly soluble in water and usually is dissolved in alcohols in experiments. It is a weak acid, which can lose H+ ions in solution. The nonionized phenolphthalein molecule is colorless and the double deprotonated phenolphthalein ion is fuchsia. Further addition of hydroxide in higher pH occurs slowly and leads to a colorless form, since the conjugated system is broken. Phenolphthalein in concentrated sulfuric acid is orange-red due to protonation and creation of a stabilised trityl cation.
Uses
pH indicator
Phenolphthalein's common use is as an indicator in acid-base titrations.
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