Gallic Acid

gallic acid or 3,4,5-trihydroxybenzoic acid (trī'hīdrŏk`sēbĕnzō`ĭk), C₆H₂(OH)₃CO₂H, colorless crystalline organic acid found in gallnuts, sumach, tea leaves, oak bark, and many other plants, both in its free state and as part of the tannin molecule (see tannin). Since gallic acid has hydroxyl groups and a carboxylic acid group in the same molecule, two molecules of it can react with one another to form an ester, digallic acid. Gallic acid is obtained by the hydrolysis of tannic acid with sulfuric acid. When heated above 220°C;, gallic acid loses carbon dioxide to form pyrogallol, or 1,2,3-trihydroxybenzene, C₆H₃(OH)₃, which is used in the production of azo dyes and photographic developers and in laboratories for absorbing oxygen.

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gallic acid [′gal·ik ′as·əd]

(organic chemistry)

C₇H₆O₅A crystalline compound that forms needles from solutions of absolute methanol or chloroform, dissolves in water, alcohol, ether, and glycerol; obtained from nutgall tannins or fromPenicillium notatumfermentation; used to make antioxidants and ink dyes and in photography.

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Gallic Acid 

(also called 3,4,5-trihydroxybenzoic acid), an organic compound; forms a crystal hydrate with the first molecule of water (C₇H₆O₅- H₂O). Colorless crystals that darken on exposure to light. Readily soluble in boiling water and in alcohol but less soluble in ether and only slightly soluble in cold water. Dissociation constant K = 3.9 × 10^-5 (25° C). When heated to 100°-120° C, gallic acid loses water; the melting point of the anhydrous acid is 240° C (with decomposition). With ferric chloride it gives a bluish black color.

Free gallic acid is present in tea, tanbark, and tannin extracts;

it is produced by the hydrolysis of tannin. Gallic acid is used in chemical analysis, in the synthesis of dyes (anthragallol and others), in microscopy, and as a depolarizer in electrochemical analysis. Dry distillation of gallic acid yields pyrogallol.