gallic acid

gallic acid, substance occurring in many plants, either in the free state or combined as gallotannin. It is present to the extent of 40–60 percent combined as gallotannic acid in tara (any of various plants of the genus Caesalpinia) and in Aleppo and Chinese galls (swellings of plant tissue), from which it is obtained commercially by the action of acids or alkalies. An Aleppo gall has a spherical shape, is hard and brittle, and is about the size of a hickory nut; it is produced on oak twigs by a gall wasp (Cynips tinctoria). A Chinese gall is produced by insects on Asian sumac (Rhus semialata).

When heated to 200°–250° C, gallic acid splits into carbon dioxide and pyrogallol (pyrogallic acid), the photographic developer. With iron salts it gives a deep blue-black colour, the basis of writing ink. It is used in the manufacture of a few dyes. As bismuth subgallate it has been employed in medicine as a mild skin antiseptic and astringent (q.v.; an agent that tends to shrink mucous membranes and raw surfaces and to dry up secretions). Propyl gallate is an important antioxidant for the prevention of rancidity in edible oils and fats. Gallic acid is 3,4,5-trihydroxybenzoic acid and has the formula (HO)₃C₆H₂·CO₂H.