GALLIC ACID, trioxybenzoic acid, or dioxysalicylic acid, C 7 H 6 O 5 + H 2 O or C 6 H 2 (OH) 3 .CO(OH) + H 2 O, the acidum gallicum of pharmacy, is a substance discovered by Scheele, which exists as such in the leaves of the bearberry, in pomegranate root-bark, and iu tea, in gall-nuts to the extent of about 3 per cent., 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 for twenty minutes to obtain a solution of the gallic acid; this is filtered through calico, and the deposit of acid it affords on cooling is drained by pressure between folds of bibulous paper, and purified by dissolving in boiling water, by 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. This is not contained in Chinese gall-nuts, which consequently require the addition of yeast or of common galls to determine the decomposition of their tannin necessary for the formation of gallic acid (see C. H. Viedt, Dingler ’ s Polyt. Journ., ccxvi., 1875, p. 454). Powdered gall-nuts, containing 43 per cent, of tannic acid, were found by Μ. Sacc to yield 50 ’ 4 per cent, of pure dry gallic acid (Compt. Rend., lxxii., 1871, p. 766). Gallic acid is most readily obtained by boiling with weak solution of acids the tannin procured from oak-galls by means of alcohol and ether. The changes which take place in this, as in the first described mode of preparation, apparently consist in the splitting up of tannin, or gallo-tannic acid, which, ...according to some experiments, is a glucoside of tannic acid, of the formula C 34 H 28 O 22 , to give with two molecules of water two molecules of digallic or tannic acid, C 14 H 10 O 9 , and glucose, C 6 H 12 O 6 . The former body, which may be represented as an etheric anhydride of gallic acid, C 6 H 2 (OH) 2 .CO(OH).O.CO.(OH) 3 C 6 H 2 , by the assimilation of a molecule of water is then converted into two molecules of gallic acid (see Armstrong, Organic Chemistry, p. 304, 1874). Gallic acid may be produced by heating an aqueous solution of diiodosalicylic acid with excess of alkaline carbonate, by acting on dibromosalicylic acid with moist silver oxide, and by other chemical methods. It crystallizes in white or pale fawn-coloured acicular prisms or silky needles, and is soluble in alcohol and ether, and in 100 parts of cold, and 3 of boiling water, is without odour, and has an astringent, acidulous taste, and an acid reaction. Dried at 100° C. it loses 9 ’ 5 per cent, of its weight of water; at about 200° C. it melts; and at 210 to 215° it is resolved into carbon dioxide and pyrogallol, C 6 H 3 (OH) 3 . With ferric salts its solution gives a deep blue colour, and with ferrous salts, after exposure to the air, an insoluble, 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 acid calcium carbonate 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 warm solutions, hence its employment in photography. With phosphorus oxychloride at 120° C. gallic acid yields tannic acid, and with concentrated sulphuric acid at 100°, rufigallic acid, C 14 H 8 O 8 . Phosphorus perchloride, and also, after several hours, solution of arsenic acid near the boiling point (J. Löwe, quoted Chem. News, xix., 1869, p. 41), convert it into ellagic acid, C 14 H 8 O 9 + H 2 O, a substance which occurs in gall-nuts, in the external membrane of the episperm of the walnut (T. L. Phipson, Chem. News, xx. p. 116), and probably many plants, and composes the “bezoar stones” found in the intestines of Persian wild goats. Gallic acid may be estimated, after removal of tannic acid by gelatin, by means of permanganate of potassium, with which it evolves carbon dioxide, and (F. Jean, Compt. Rend., lxxxii., 1876, pp. 982-4) by means of iodine in the presence of an alkaline carbonate.
Gallic acid has been advantageously employed as an internal medicinal agent in scarlatinal albuminuria, in which its effect appears to be due to an astringent and tonic action on the inflamed capillaries of the kidneys; T in other forms of albuminuria; in caseβ of chyluria, in which, as not causing nausea and headache, it is preferable to tannic acid; and in pyrosis, diarrhoea, some forms of dysentery, and tabes and rickets, and atonic states of the alimentary canal and of the body generally. In checking the night-sweats of phthisis it has been found of especial service. As a haemostatic, when administered internally, it has proved of value in haematemesis, epistaxis, fungus haematodes, menorrhagia, and more particularly in haematuria. Gallic acid has been highly recommended in haemoptysis, in which, however, ergot, from its not occasioning griping and constipation, or interfering with a liberal use of milk, has been found a better remedy. It may be given in considerable quantities without any evil consequences. The effects of too large a dose are to render the pulse hard and wiry, and to produce pallor, a whizzing sound in the ears, dizziness, and faintness. Its administration in a case of Bright’s disease described by Dr Bence Jones waβ followed by epilepsy (see Med. Times, 1853, ii. 495). As a topical styptic application, gallic is inferior to tannic acid. With glycerin it is combined to form the pharmaceutical preparation glycerinum acidi gallici.
