GALLIC ACID. (FR. Acide gallique ; GER. Gallussaure.) Formula 07H805.
This acid exists in many plants and vegetables in small quantities, hut it is usually obtained from a subatance contained in nut-galla which is called "tannin." The meet simple method is to submit powdered nut-galls to the action of the atmosphere for about three months, placing it in a cool spot and keeping it moistened -with water ; after a short time the mass swells up and becomes mouldy. At the end of this time it is submitted to pressure, the escaping liquid beiog rejected so long as it runs out coloured ; the remainder is digested with boiling water, which, after being filtered and allowed to cool, deposits crystals of impure gallic acid. To purify these, they are re-dissolved in seven or eight times their weight of boiling water, a little animal black being added te decolorize the solution.
Another method of converting tannin into gallic acid is due to Liebig. It consists in extract ing the tannin from the pulverized gall-nuts with water, and precipitating the solution with sulphuric acid diluted with seven or eight times its weight of water. The whole is boiled for a short time, during which the conversion takes place. The liquor is concentrated at a gentle heat, and on cooling deposits nearly colourless crystals of gallic acid.
The firat of these methods is, however, the moat convenient, and it is in use at the present time, the manufacture being carried on as follows ;—The nut-galla are carefully chosen and sorted over, the most valued being the green ones, whigh furnish 40 per cent. of their weight of gallic acid ; the white variety is not nearly so rich, and is usually rejected. The nuts are reduced to powder and placed in a large, shallow leaden basin ; in this they are moistened continually for some days with warm water, which is added as long as it is absorbed by the nuts ; when this is complete the whole is placed in a compact heap, so as to retain as much warmth as possible. At the end of a few days fermentation sets in, and the surface of the heap becomes covered with a thick, grey mould. At the expiration of about three months the fermentation is complete ; during the whole of this time the mass should be moistened whenever it shows any signs of becoming dry. The nuts are next submitted to a heavy pressure, and for this purpose they are placed in a s 'lid wooden press, tied up in bags made of cloth. The expressed liquid runs out in a thick, black stream, and
contains much gummy and extractive matter which would seriously interfere with the subsequent crystallization. The nuts are next digested in boiling water in a wooden vessel lined out with lead and heated by a steam-pipe from below ; the whole should be constantly stirred with a wooden paddle during the process of ebullition. A small quantity of dilute sulphuric acid is added to the hot liquor, for the purpose of converting any unfermented tannin into gallic acid, and of facilitating the subsequent processes of filtration. An hour or two after the addition of the acid, the steam is turned off, and the contents are left to stand for twelve hours ; the clear liquor is then run off and filtered through cloths, while the residue is collected and boiled a second time in the same manner. The liquors are concentrated in leaden basins, into which steam is introduced by means of a leadon pipe ; as soon as a thin film appears on the surface the steam is withdrawn and the concentrated liquor is allowed to cool. At the end of twenty-four hours the crystallization is com plete, and the crystals of crude gallio acid are drained, and re-dissolved in a leaden vessel, and the hot solution is filtered through a strainer before being once more left to crystallize. The crystals thus obtained are still slightly coloured, and in order to purify them they are dissolved in water, and a. small quantity of animal black is added to the solution. After filtering, a few drops of hydrochloric acid are added for the purpose of dissolving truces of gallato of iron, which would impart a yellowish tinge to the crystals. A third and final crystallization affords beautiful white, silky needles of gallic acid, in which form it is sent into the market.
Gallia acid is inodorous, and has a slightly acid taste. It dissolves in 100 parts of cold water, and in 3 parts of boiling water ; also readily in both alcohol and ether. Gallic acid was at one time largely consumed in photography, indeed the demand for it seems to have grown with that art, but of late it has been abandoned iu favour of pyrogallic acid, a substance obtained by the decomposition of gallio acid by heat.