Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V26, Page 400 of the 1911 Encyclopedia Britannica.
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TANNIN, or TANNIC ACID, the generic name for a widely disseminated group of vegetable products, so named from their property of converting raw hide into leather (q.v.). They are soluble in water, their solutions having an acid reaction and an astringent taste; the solutions are coloured dark a.ue or green by ferrous salts, a property utilized in the manufacture of ink (q.v.). Their chemistry is little known. Some appear to be glucosides of gallic acid, since they yield this acid and a sugar on hydrolysis, e.g. oak tannin; whilst others yield protocatechuic acid and phioroglucin, e.g. moringa-tannin; common tannin, however, is a digallic acid. Common tannin, or tannic acid, C14I-i1509.2H20, occurs to the extent of 50% in gall-nuts, and also in tea, sumach and in other plants. It may be obtained by extracting powdered gall-nuts with a mixture of ether and alcohol, whereupon the tannin is taken up in the lower layer, which on separation and evaporation yields the acid. When pure the acid forms a colourless, amorphous mass, very soluble in water, less so in alcohol, and practically insoluble in ether. Common salt precipitates it from aqueous solutions. It forms a penta-acetate. It may be obtained artificially by heating gallic acid with phosphorus oxychloride or dilute arsenic acid (cf. P. Biginelli, Gazetta, 1909, 39, ii. pp. 268 et seq.); and conversely on boiling with dilute acids or alkalis it takes up a molecule of water and yields two molecules of gallic acid, C7H605. It is optically active—a fact taken account of in J. Dekker's formula (Ber., 1906, 39, p. 2497). The chemistry has also been investigated by M. Nierenstein and L. F. Iljin (see papers in the Ber., 1908, et seq.). The tannin of oak, C19H1601o, which is found, mixed with gallic acid, ellagic acid and quercite, in oak bark, is a red powder; its aqueous solution is coloured dark blue by ferric chloride, and boiling with dilute sulphuric acid gives oak red or phlobaphene. The tannin of coffee, C30H18016, found in coffee beans, is not precipitated from its solutions by gelatin. Hydrolysis by alkaline solutions gives a sugar and caffeic acid; whilst fusion with potassium hydroxide gives protocatechuic acid. Moringa-tannin or maclurin, C13H1o06'H20, found in Morus tinctoria, hydrolyses on fusion with caustic potash to phioroglucin and protocatechuic acid. Catechu-tannin occurs in the extract of Mimosa catechu; and kino-tannin is the chief ingredient of kino (q.v.).
End of Article: TANNIN, or TANNIC ACID

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