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. They are soluble in water, their solutions having an acid reaction and an astringent taste; the solutions are coloured dark blue or green by ferrous salts, a property utilized in the manufacture of ink. 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 phloroglucin, e.g. moringa-tannin; common tannin, however, is a digallic acid.
Common tannin, or tannic acid, C, 4 H, 0 0 9.2H 2 O, 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, C 7 H 6 0 5. It is optically active - a fact taken account of in J. Dekker's formula (Ber., 1906, 39, p. 2 497). 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, C/9H16010, 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, C3,H,80,6, 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, C1,H1006 H20, found in Morus tinctoria, hydrolyses on fusion with caustic potash to phloroglucin and protocatechuic acid. Catechu-tannin occurs in the extract of Mimosa catechu; and kino-tannin is the chief ingredient of kino (q.v.).
Tannic acid is official in both the British and United States Pharmacopoeias. It is incompatible with mineral acids, alkalis, salts of iron, antimony, lead and silver, alkaloids and gelatin. The British pharmacopoeial preparations are (r) glycerinum acidi tannici; (2) suppositoria acidi tannici; (3) trochiscus acidi tannici. The United States also has a collodium stypticum and an ointment. From tannic acid is also made gallic acid, which resembles tannic acid but has no astringent taste. When applied to broken skin or exposed surfaces it coagulates the albumen in the discharges, forming a protecting layer or coat. It is moreover an astringent to the tissues, hindering the further discharge of fluid. It is a powerful local haemostatic, but it only checks haemorrhage when brought directly in contact with the bleeding point. It is used in the treatment of haemoptysis in the form of a fine spray, or taken internally it will check gastric haemorrhage. In large doses, however, it greatly disorders the digestion. In the intestine tannic acid controls intestinal bleeding, acting as a powerful astringent and causing constipation; for this reason it has been recommended to check diarrhoea.
Tannic acid is largely used in the treatment of various ulcers, sores and moist eruptions. The glycerin is used in tonsillitis and the lozenges in pharyngitis. For bleeding haemorrhoids tannic acid suppositories are useful, or tannic acid can be dusted on directly. The collodium stypticum is a valuable external remedy. Tannic acid is absorbed as gallic acid into the blood and eliminated as gallic and pyrogallic acids, darkening the urine. Gallic acid does not coagulate albumen when used externally. It has been used internally in haemoptysis and haematuria. Combined with opium it is an efficient remedy in diabetes insipidus.
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