RHYS LEWIS, AHS, DECD, UNISA / JON BAUGH / SCIENCE PHOTO LIBRARY RHYS LEWIS, AHS, DECD, UNISA / JON BAUGH / SCIENCE PHOTO LIBRARY
Animation of a molecule of phenolphthalein, an acid-base indicator, changing shape and colour as the pH of its environment changes. At low pH (acid conditions), the molecule comprises three aromatic rings attached to a central tetrahedral carbon atom, which is also bonded to an oxygen-containing lactone ring. The surrounding water molecules are seen to be largely protonated at low pH, forming the hydronium ion H3O+. As the pH rises, the water molecules become predominantly normal H20. As the pH rises to more than 8, more water molecules become the hydroxide ion, OH-. This leads to the deprotonation of the phenolphthalein. The lactone ring breaks and forms a carboxylic acid group, and one of the aromatic rings is broken down and forms conjugated double bonds to its oxygen and central carbon, which becomes trigonal planar. This rearrangement changes the molecule's absorption of light, giving it a bright pink colour. Phenolphthalein is widely used in chemistry as an indicator of alkaline (high pH) conditions due to this marked colour change.
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