GALLIC

ACID  , trioxybenzoic acid(HO) (HO)3(3 .) C 6H2CO2H•H2O, the acidum gallicum of pharmacy, a substance discovered by K . W . Scheele; it occurs in the Leaves of the bearberry, in pomegranate root-bark, in tea, in gall-nuts to the extent of about 3 %, and in other vegetable productions . It may be prepared by keeping moist and exposed to the air for from four to six weeks, at a temperature of 20° to 25° C., a paste of powdered gall-nuts and water, and removing from time to time the mould which forms on its surface; the paste is then boiled with water, the hot solution filtered, allowed to cool, the separated gallic acid drained, and purified by dissolving in boiling water, recrystallization at about 27° C., and washing of the crystals with ice-cold water . The production of the acid appears to be due to the presence in the galls of a ferment . Gallic acid is most readily obtained by boiling the tannin procured from oak-galls by means of alcohol and ether with weak solution of acids . It may also be produced by heating an aqueous solution of di-iodosalicylic acid with excess of alkaline carbonate, by acting on dibromosalicylic acid with moist silver oxide, and by other methods . It crystallizes in white or pale fawn-coloured acicular prisms or silky needles, and is soluble in alcohol and ether, and in too parts of cold and 3 of boiling water; it is without odour and has an astringent and an acid taste and reaction . It melts at about 200° C., and at 2100 to 215° it is resolved into carbon dioxide and pyrogallol, C6H3(OH)3 . With ferric salts its solution gives a deep blue colour, and with ferrous salts, after exposure to the air, an in-soluble, blue-black, ferroso-ferric gallate . Bases of the alkali metals give with it four series of salts; these are stable except in alkaline solutions, in which they absorb oxygen and turn brown . Solution of calcium bicarbonate becomes with gallic acid, on exposure to the air, of a dark blue colour .

Unlike tannic acid, gallic acid does not precipitate albumen or salts of the alkaloids, or, except when mixed with

gum, gelatin . Salts of gold and silver are reduced by it, slowly in cold, instantaneously in warns solutions, hence its employment in photography . With phosphorus oxychloride at 12o° C. gallic acid yields tannic acid, and with concentrated sulphuric acid at roe, rufigallic acid, C14H808, an anthracene derivative . Oxidizing agents, such as arsenic acid, convert it into ellagic acid, C14H809+H20, probably a fluorene derivative, a substance which occurs in gall-nuts, in the external membrane of the episperm of the walnut, and ;probably in many plants, and composes the " bezoar stones " found in the intestines of Persian wild goats . Medicinally, gallic acid has been, and is still, largely used as an astringent, styptic and haemostatic . Gallic acid, however, does not coagulate albumen and therefore possesses no local astringent action . So far is it from being an haemostatic that, if perfused through living blood-vessels, it actually dilates them . Its rapid neutralization in the intestine renders it equally devoid of any remote actions .