Phenolphthalein is an organic compound used as a pH indicator in acid–base titrations. It is colorless in acidic solutions and turns pink to fuchsia in basic solutions. It belongs to the phthalein dye family.
🔬 Properties
Phenolphthalein is a weak acid. Its colorless, non-ionized form becomes pink in basic environments due to double deprotonation. At very high pH, it may turn colorless again as its conjugated system breaks down.
It is slightly water-soluble and usually dissolved in alcohol for lab use. In concentrated sulfuric acid, it appears orange-red due to protonation and formation of a stabilized cation.
🏭⚗️ Synthesis
It is synthesized by condensing phthalic anhydride with phenol under acidic conditions. Discovered by Adolf von Baeyer in 1871.
🧪 Applications
• pH Indicator: Widely used in acid–base titrations.
• Concrete Testing: Detects carbonation—pink in alkaline areas, colorless in carbonated zones.
• Spackling Products: Turns colorless as drywall filler reacts with air.
• Educational Use: Demonstrates reaction kinetics and pH-dependent color changes.
