gallic acid

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gallic acid


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 tannintannin,
 tannic acid,
or gallotannic acid,
astringent vegetable product found in a wide variety of plants. Sources include the bark of oak, hemlock, chestnut, and mangrove; the leaves of certain sumacs; and plant galls.
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). 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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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

gallic acid

[′gal·ik ′as·əd]
(organic chemistry)
C7H6O5 A crystalline compound that forms needles from solutions of absolute methanol or chloroform, dissolves in water, alcohol, ether, and glycerol; obtained from nutgall tannins or from Penicillium notatum fermentation; used to make antioxidants and ink dyes and in photography.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.