TANNIC ACID, TANNIN. One or other of these bodies is found, to a greater or less extent, in most vegetable substances. The name tannin is derived from the fact that it is the only active constitu ent in the various barks, dfc., used in the familiar operation of making leather, or tanning.
Tannin is an acid body ; and inasmuch as researches upon the tannin obtained from various sources have 'proved that it is not in all cases identical, but that several modifications exist, the names gallotannic acid (from nut-galls), cafotannic acid (from coffee), quinotannic add (from cinchona bark), quercitannic acid (from oak-bark), kc., have been introduced.
Tannic acid is a powerful astringent, and hence has long been used in medicine. For this purpose it is always extracted from nut-galls. The galls are reduced to coarse powder, and digested in a percolator with ether which has been previously mixed and shaken with water. In the lower part of the vessel two strata of liquid appear; the heavier is a strong solution of tannic acid, by evaporating which, the acid is obtained, as a colourless or slightly yellowish friable mass, which does not crystallise, but somewhat resembles dried gum.
The yallotannic acid thus obtained is readily soluble in water : the solution has an astringent but not a bitter taste : it reddens vegetable blues, and decomposes alkaline carbonates with effervescence ; weak alcohol dissolves it, but ether only slightly. When the aqueous solu tion is exposed to the air, especially if the temperature be high, oxygen gas is absorbed, and an equal volume of carbonic acid gas evolved, while the gallotannic acid is converted into gallic and ellagic acid. Gallotannic acid precipitates gelatin from solution ; the compound has been called tannogdatin, and when the acid is in excess a viscid elastic mass is formed, which contains about half its weight of gallotannic acid. When the liquid from which the gelatin is precipitated is heated to the tannogelatin is re-dissolved. Gallotannic acid also precipitates albumen and starch. Boiled with dilute sulphuric or hydrochloric acids, gallotannie acid splits up into gallic acid and glucose.
When dried at 212° gallotannic acid consists of C„II„0..
Gallotannic acid combines with the alkalies to form salts, which are called gallotannates, and it precipitates most metallic oxides and organic bases from solution. The salts of protoxide of iron suffer no change
when a solution of gallotannic acid is added to them ; but by exposure to the air a deep bluish-black precipitate is formed. Gallotannate of peroxide of iron, formed by the action of the acid on a persalt of the metal, is the basis of writing-ink, and is a black pulverulent precipitate.
Gallamic or tanningcnamic add (C„II,NO, +3 Aq) is a product of the action of ammonia on gallotannic acid. It crystallises from alcohol in beautiful rectangular plates.
Tannomelanic acid is a dark ulmindike substance formed when gallotannic acid is boiled with a concentrated solution of potash in a vessel open to the air. If the potash be dilute, a portion of the acid is oxidised to tanno.cylic acid. The constitution of these acids has not been satisfactorily ascertained.
Suntach-tannic acid, from various species of sumach, is identical with gallotannic acid.
Cachoutannic acid or ntimotannic acid. Obtained from the well-known astringent substances Kul° and CATECEW by the displacement process previously described. It differs from gallotannic acid in not giving a precipitate with solution of tartar emetic, and in giving with persalts of iron a grayish-green precipitate. Moreover, it does not yield pyre gallie acid when heated. Its composition is said to be In other respects it resembles gallotannic acid.
Tanningenic or catechucic acid, called also catechize is another tannic acid contained in catechu. It is part of the insoluble portion which is left on digesting catechu in cold water. It may be dissolved in boiling water, and decolorised by animal charcoal : on cooling, it deposits in colourless granular crystals. It is tolerably soluble in alcohol ; less so iu ether. Heated to 422° Fahr., it fuses, and at a higher temperature is decomposed, yielding pyrocatechin or ory phenic acid Dilute acids dissolve it ; concentrated acids decompose it ; hot nitric acid converts it into oxalic acid. It does not form definite compounds with bases; gives a deep green colour to per salts of iron, but precipitates neither lime-water, baryta-water, tartar emetic, starch, gelatin, or the alkaloids.