a. Of Gallic Acid.—Gallic Acid is obtained from Gall Nuts, which are peculiar excrescences formed upon the branches and shoots of the Quercus infectoria by the puncture of a species of insect. The best kind is imported from Turkey, and sold in commerce as Aleppo Galls. Gall Nuts do not contain Gallic Acid ready formed, but an analogous chemical principle termed Tannic Acid, well known for its astringent properties and employment in the process of tanning raw hides.
Gallic Acid is produced by the decomposition and oxidation of Tannic Acid when powdered galls are exposed for a long time in a moist state to the action of the air. By boiling the mass with water and filtering whilst hot, the acid is extracted, and crystallizes on cooling, on account of its sparing solubility in cold water.
Gallic Acid occurs in the form of long silky needles, soluble in 100 parts of cold and 3 of boiling water; they are also readily soluble in Alcohol, but sparingly in Ether. The aqueous solution becomes mouldy on keeping, to obviate which, the addition of Acetic Acid or a drop or two of Oil of Cloves is recommended.
Gallic Acid is a feeble acid, scarcely reddening litmus; it forms salts with the alkaline and earthy bases, such as Potash, Lime, etc., but not with the oxides of the noble metals. When added to Oxide of Silver the metallic element is separated and the Oxygen absorbed.
b. Pyrogallic Acid.—The term pyro prefixed to Gallic Acid implies that the new substance is obtained by the action of heat upon that body. At a temperature of about 410° Fahr., Gallic Acid is decomposed, and a white sublimate forms, which condenses in lamellar crystals; this is Pyrogallic Acid.
Pyrogallic Acid is very soluble in cold water, and in Alcohol and Ether; the solution decomposes and becomes brown by exposure to the air. It gives an indigo blue colour with Protosulphate of Iron, which changes to dark green if any Persulphate be present.
Although termed an acid, this substance is strictly neutral; it does not redden litmus-paper, and forms no salts. The addition of Potash or Soda decomposes Pyrogallic Acid, at the same time increasing the attraction for Oxygen; hence this mixture may conveniently be employed for absorbing the Oxygen contained in atmospheric air. The compounds of Silver and Gold are reduced by Pyrogallic Acid even more rapidly than by Gallic Acid, the reducing agent absorbing the Oxygen, and becoming converted into Carbonic Acid and a brown matter insoluble in water.
Commercial Pyrogallic Acid is often contaminated with empyreumatic oil, and also with a black insoluble substance known as Metagallic Acid, which is formed when the heat is raised above the proper temperature in the process of manufacture.
CHEMISTRY OF THE PROTOSALTS OF IRON.