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TANNIN, or TANNIC ACID

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Originally appearing in Volume V26, Page 400 of the 1911 Encyclopedia Britannica.
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TANNIN, or TANNIC See also:ACID , the generic name for a widely disseminated See also:group of See also:vegetable products, so named from their See also:property of converting raw hide into See also:leather (q.v.). They are soluble in See also:water, their solutions having an acid reaction and an astringent See also:taste; the solutions are coloured dark a.ue or See also:green by ferrous salts, a property utilized in the manufacture of See also:ink (q.v.). Their See also:chemistry is little known. Some appear to be glucosides of gallic acid, since they yield this acid and a See also:sugar on See also:hydrolysis, e.g. See also:oak tannin; whilst others yield protocatechuic acid and phioroglucin, e.g. moringa-tannin; See also:common tannin, however, is a digallic acid. Common tannin, or tannic acid, C14I-i1509.2H20, occurs to the extent of 50% in See also:gall-nuts, and also in See also:tea, See also:sumach and in other See also:plants. It may be obtained by extracting powdered gall-nuts with a mixture of See also:ether and See also:alcohol, whereupon the tannin is taken up in the See also:lower layer, which on separation and evaporation yields the acid. When pure the acid forms a colourless, amorphous See also:mass, very soluble in water, less so in alcohol, and practically insoluble in ether. Common See also:salt precipitates it from aqueous solutions. It forms a penta-acetate. It may be obtained artificially by See also:heating gallic acid with See also:phosphorus oxychloride or dilute See also: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 See also:

molecule of water and yields two molecules of gallic acid, C7H605. It is optically active—a fact taken See also:account of in J. See also:Dekker's See also: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 See also:powder; its aqueous See also:solution is coloured dark See also:blue by ferric chloride, and boiling with dilute sulphuric acid gives oak red or phlobaphene. The tannin of See also:coffee, C30H18016, found in coffee beans, is not precipitated from its solutions by See also:gelatin. Hydrolysis by alkaline solutions gives a sugar and caffeic acid; whilst See also:fusion with See also:potassium hydroxide gives protocatechuic acid. Moringa-tannin or maclurin, C13H1o06'H20, found in Morus tinctoria, hydrolyses on fusion with See also:caustic potash to phioroglucin and protocatechuic acid.

See also:

Catechu-tannin occurs in the See also:extract of See also:Mimosa catechu; and kino-tannin is the See also:chief ingredient of kino (q.v.).

End of Article: TANNIN, or TANNIC ACID

Additional information and Comments

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