Phenolphthalein << `fee` nohl THAL een >> is a chemical compound used as an indicator of alkalinity or acidity. It is also used in making dyes. It is prepared by heating phenol and phthalic anhydride with sulfuric acid. Pure phenolphthalein forms small white or yellowish-white crystals that dissolve in alcohol or ether. It turns red in the presence of an alkaline substance.
Phenolphthalein was once widely used as the active ingredient in many laxative products, but it was discovered to be a carcinogen (substance known to cause cancer). For this reason, it was phased out of over-the-counter laxatives by 2000.
