GAL′LIC ACID

H 3 C 7 H 3 O 5 .Aq. Syn. Acidum gallicum (B. P.), L. “A crystalline acid prepared from galls.” (B. P. L.) It may be also obtained from other vegetable substances. It appears to be a product of the oxidation of tannic acid, and probably does not exist ready formed in recent vegetables. Prep. 1. (Dumas.) Nut-galls, reduced to powder, are moistened with water, and exposed to the action of the air, in a warm situation (say 70° to 80° Fahr.), for two or three months, adding more water, from time to time, to make up for that lost by evaporation. At the end of the above period the mouldy, dark-coloured ma is strongly pre ed in a cloth, and the solid portion boiled in a considerable quantity of water. The solution (filtered whilst hot) deposits, on cooling, crystals of gallic acid, which, after being thoroughly drained and pre ed dry between bibulous paper, are purified by boiling them along with about 1 ⁄ 6 th of their weight of prepared animal charcoal in 8 parts of water, and filtering, ., as before. 2. (Graham.) A strong infusion or decoction of galls is precipitated with sulphuric acid in the cold; the resulting thick ma is mixed with dilute sulphuric acid (cold), and the liquid expre ed; the ‘marc’ is next treated with sulphuric acid diluted with twice its weight of water, and after boiling the mixture for some minutes the whole is allowed to cool; the resulting crystals are purified as before. 3. (Liebig.) A strong aqueous solution of tannic acid (tannin) is added to sulphuric acid as long as a precipitate falls; the powder is collected, washed, and di olved by the aid of heat in dilute sulphuric acid; the solution, after being boiled for a few minutes, deposits, on cooling, crystals of gallic acid in considerable quantity. 4. (Scheele.) A filtered decoction of galls is exposed for some months in an open ve el; after a time it grows mouldy, and becomes covered with a thick, glutinous pellicle; in two or three months the sides of the ve el and the under portion of the pellicle are found to be covered with small yellow crystals of gallic acid, which are purified as directed above. (See No. 1.) 5. (Ph. D., B. P.) The Dublin contains two formulæ for gallic acid, the one being based on that of Dumas or Scheele, the other on that of Graham or Liebig.— a. From galls (in coarse powder), 1 lb.; water, q. s. to make a stiff paste; a porcelain dish is ordered, and the exposure in the moistened condition is to be continued for 6 weeks; the solution of the first crop of crystals is to be made in 10 fl. oz. of boiling water, and when the filtrate has cooled to 80° Fahr., it is to be poured off from the crystals which have formed, which are then to be washed with ice-cold water, 3 fl. oz., and dried—first in blotting paper, and finally by a steam or water heat. By boiling the undi olved portion of the galls with 45 fl. oz. of fresh water, more crystals may be obtained. b. Powdered gall-nuts, 1 lb., are steeped for 24 hours in water, 1 pint, and after being placed in a porcelain displacement apparatus, are treated with water, 1 1 ⁄ 2 pint, added in succe ive portions; oil of vitriol, 5 fl. oz., diluted with an equal volume of water, and allowed to cool, is now added to the percolated infusion, and after thorough admixture the liquid is filtered from the viscid precipitate which forms; oil of vitriol, 5 fl. oz. (diluted as before), is then added to the filtrate, the precipitates, enveloped in calico, are submitted to powerful pre ure, and subsequently di olved in oil of vitriol, 16 fl. oz., previously diluted with water, 56 fl. oz.; the solution is boiled for 20 minutes, and set aside for a week; at the end of this time the deposit which forms is di olved in three times its weight of boiling water, and the solution treated as before. Prop. Gallic acid forms small, feathery, and nearly colourle crystals, which have a beautiful silky lustre; that of commerce is usually of a pale-yellow colour; it is soluble in 100 parts of cold water, and in 3 parts of boiling water; it is also soluble in alcohol, and slightly so in ether; the aqueous solution is decomposed by exposure to the air; di olved in hot oil of vitriol, it forms a deep, rich, red solution, which, when thrown into water, drops the gallic acid, deprived of some of its water. This substance is soluble in the alkalies, and dyes cloth like madder. When strongly heated, gallic acid is converted into meta gallic acid, or into pyrogallic acid, according to the manner in which the heat is applied. Tests. Gallic acid is distinguished from tannic acid by not affecting solutions of gelatin, the pro to salts of iron, or the salts of the alkaloids, and by giving a deep bluish-black precipitate with the sesquisalts of iron, which disappears when the liquid is heated. It is distinguished from pyrogallic acid by its inferior insolubility in water, and by its not affecting the solutions of the pro to salts of iron. To detect gallic acid mixed with tannic acid, the latter should be removed, either by digesting the substance in ether, or by immersing for some time in its solution a piece of skin depilated by lime, previously to applying the tests. Pur. Free from colour; decomposed by heat; soluble in water and in rectified spirit. It turns preparations of the sesquioxide of iron, di olved in water, of a bluish black colour, but throws down nothing from a solution of isingla . Uses, . The principal use of pure gallic acid is in the art of photography . It has recently been employed in medicine , as an internal astringent, in doses of 3 to 10 gr., thrice a day, or oftener; in hæmorrhage and fluxes, as well as for checking the night sweats in phthisis. Dr Todd says, that in all cases of internal hæmorrhage, or hæmorrhagic tendency, it is the best astringent or styptic we po e . As an external astringent, it is greatly inferior to tannic acid. It has been given in doses of 15 to 30 gr. in tape-worm, “but without any benefit.” (Pereira.) Purification. Gallic acid, as obtained by either of the above forms, is never quite pure; but it may be rendered absolutely pure by combining it with oxide of lead, and decomposing the compound (gallate of lead) by sulphuretted hydrogen. The sulphuret of lead acts like animal charcoal in removing the colour. (Liebig.) Commercial gallic acid “may be rendered nearly white by di olving it in 20 times its weight of boiling distilled water, and causing the solution to traverse a stratum of prepared animal charcoal, spread upon a calico filter. When the liquid pa es through colourle , it should be evaporated to 1-6th its volume, and then suffered to cool, in order to the separation of the crystallised acid.” (Ph. D.)