GALLOTANNIC ACID is the most important of the various forms of tannin or tannic acid. It usually occurs as a spongy, light, inodorous, colorless, or faintly yellow mass, which is easily reduced to a fine powder, which possesses a strongly astringent, but not a bitter taste. It is freely soluble in water, the solution reddening litmus paper, and dissolving the carbonates with. effervescence. With the persalts of iron, G. A. gives a blackish blue precipitate of gallotannate of iron, and even when the iron solution is extremely dilute, a violet tint is evolved. This gallotannate of iron is the basis of ordinary writing ink (q.v.); and the reaction that we have described is so sensi tive, that G. A. is employed in the laboratory as a test for the detection of the persalts of iron. G. A. likewise precipitates tartar emetic, nearly all the vegetable alkaloids (morphia, quinia, etc.), the albuminates, and gelatine. If a piece of raw hide, freed from hair, be immersed in a solution of G. A., the gelatigenons tissue and the acid com bine, and leather is formed; and if the skin be of sufficient size, all the G. A. is removed from the solution.
G. A. fuses when exposed to heat, and at a temperature of about 120° it is decom posed, and yields pyrogallic acid and metagallic acid (C.II404), while water and carbonic acid are expelled. When a watery solution of G. A. is excluded from the air, it remains unchanged; but if the air is allowed free access to it, a fungous or moldy growth is developed, oxygen is absorbed, carbonic acid is given off, and the G. A. becomes decomposed into gallic acid (q.v.) and sugar. The same decomposition is more rapidly induced by the action of dilute sulphuric acid, the reaction being exhibited in the following formula: Gallotannic Water. Gallic acid. Sugar.
. acid.
Cs4H29034 +8H0 = 3(C,41160i0)--E- Ci2lli2012 On boiling G. A. in a concentrated solution of potash, gate acid is also formed.
The composition of the salts of this acid is but imperfectly known, but the acid is • generally considered as tribasic. None of the salts crystallize, and when in solution or
in a moist state, they rapidly absorb oxygen, and become decomposed.
G. A. occurs in large quantity in the gall-nut, which contains, according to Pelouze, as much as 40 per cent of this acid, and 3.5 per cent of gallic acid (Guibourt has found that some nuts contain as much as 65 per cent of G. A.); it is likewise found in all • parts of the gall or dyer' s oak (quercus infectoria), in sumach (thus coriaria), and in green tea.
The best method of obtaining it is from powdered gall-nuts, by extraction with com mercial ether (which contains about 10 per cent of water), in the percolation or dis placement apparatus _ kg/ ( G. A. is employed in medicine, in chemistry, and in the arts. Its uses in medi cine are due to its powerful astringent action. It is employed topically as a styptic in wounds, bleeding gums, piles, etc., and internally as an astringent in hemorrhage from the lungs, stomach, bowels, etc.; as we know that it becomes converted into gallic acid in its passage through the system, it is probably the latter acid which acts on remote parts when G. A. is administered. Internally, it may be given in doses of from three to ten grains, three or four times a day, in pills or in solution. It may be used as an astringent gargle or lotion, in the form of a watery solution containing three or more grains to the ounce. The compound ointment of galls, which 'is the best topical remedy for piles without hemorrhage, owes its efficacy to the G. A. contained in the powdered galls.
In chemistry, it is used in solution as a test for gelatine, persalts of iron, etc.; and in the arts, it serves various useful processes, especially in relation to the preparation of Feather, and the manufacture of white wines.