Gallic Acid, , is an acid which exists in small quantity in gall-nuts, in valonia (the acorn-cup of Quercus aegilops), in divi-divi (the pod of Casalpinia coriaria), in sumach, and other vegetables. It is usually prepared from gall-nuts, which, in addition to gallic acid, contain a large proportion of tannin (tannic acid or gallo-tannic acid). When the gall-nuts are digested with water for some weeks fermentation takes place, and the tannic acid is gradually converted into gallic acid. The same result is obtained more quickly if sulphuric acid be present. To obtain pure gallic acid the gall-nuts are boiled with water, and the hot liquor separated. On cooling gallic acid crystallises out, and is further purified by solution in hot water and treatment with animal charcoal.
It forms delicate, silky, acicular crystals, nearly colourless, and having a sourish taste. It is soluble in 3 parts of boiling water, but only in 100 of cold water, and on this account it can be readily purified by recrystallisation. With solution of iron salts (ferric) it produces a blue-black colour, and finally yields a black precipitate on exposure to the air. Hence it may be used in the production of ink, for which purpose it has some advantages over tannin or gall-nuts. When the crystals are strongly heated pyrogallic acid is produced and sublimes over. Gallic acid is a useful astringent. As it does not coagulate albumen it is readily absorbed into the blood, and in this way it is efficacious in Bright's disease. Where a decided local astringent effect is desired tannic acid is much more powerful.