Tannin is any of a group of complex organic substances, containing phenols, hydroxy acids, or glucosides, which occur occuring widely in plants, dissolved in cell-sap. Tannins are particularly common in the bark of oak, mangrove, and sumac, unripe fruits, leaves, and oak galls, and are extracted by boiling in water. They may be classified as hydrolyzable (yielding gallic acid) or condensed. Tannins are used for tanning, for making dyes and inks, and in medicine as an astringent.

 

Gallic acid

Gallic acid (HO)₃C₆H₂COOH, also known as 3,4,5-trihydroxybenzoic acid, is an acid occurring plants, including oak galls, sumach, and tea. It is obtained from tannins by hydrolysis. Gallic acid is used in ink, dyes, and as a mild antiseptic and astringent. On heating it gives pyrogallol, a photographic developer.