Tannin or Tannic Acid

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, and their solutions which have an acid reaction and an astringent taste, are coloured dark blue or green by ferrous salts, a property utilized in the manufacture of ink (q.v.). Some tannins 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 phloroglucinol, e.g., moringa-tannin ; com mon tannin, however, is a digallic acid.

Common tannin, or tannic acid, 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. When pure the acid forms a colour less, amorphous mass, very soluble in water, less so in alcohol, and practically insoluble in ether. It may be obtained artificially by heating gallic acid with phosphorus oxychloride or dilute arsenic acid; and conversely on boiling with dilute acids or alkalis it takes up a molecule of water and yields two molecules of gallic acid (HO The tannin of oak, C191116010, 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. The tannin of coffee, found in coffee beans, is not precipi tated from its solutions by gelatin. Hydrolysis by alkaline solu tions gives a sugar and caffeic acid ; whilst fusion with potassium hydroxide gives protocatechuic acid. Moringa-tannin or maclurin,

found in clilorophora tinctoria, hydrolyses on fu sion with caustic potash to phloroglucinol 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 em ployed either alone or in conjunction with antimony salts as mordant in dyeing cotton with basic colouring matter.

Medicine.

Tannic acid is officinal 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 (1) glyceri num, acidi tannici; (2) suppositoria acidi tannici; (3) trochiscus acidi tannici. The United States also has a collodium stypticum and an ointment. In the intestine tannic acid controls intestinal bleeding, acting as a powerful astringent and causing constipation; it has been recommended to check diarrhoea.

Tannic acid is used in the treatment of various ulcers, sores and moist eruptions. The glycerin solution is used in tonsilitis 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.