gallic acid or 3,4,5-trihydroxybenzoic acid trī˝hīdrŏk´sēbĕnzō´ĭk [key], C 6H 2(OH) 3CO 2H, 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, C 6H 3(OH) 3, which is used in the production of azo dyes and photographic developers and in laboratories for absorbing oxygen.
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