(CO.OH). H2O; 188. - A crystalline acid prepared from galls; it may be considered as salicylic acid in which two atoms of hydrogen are replaced by two of hydroxyl (cf. p. 810).

Preparation. - From galls, by pulverising, moistening with water, and allowing them to ferment for six weeks in a temperature of 60° to 70° F. The tannin present in the galls is split up by the fermentation into gallic acid and glucose, the tannic acid, or digallic acid (p. 1,031) splitting up into gallic acid, C14H10O9 + H2O = 2C7H6O5. It is purified by solution, and re-solution in boiling water, filtering and crystallising.

Properties. - Crystalline, in acicular prisms or silky needles, nearly white or of a pale fawn-colour. Its taste is acidulous and astringent. Solubility. - It requires about 100 parts of cold water for its solution, but dissolves in 3 parts of boiling water. Soluble also in rectified spirit.

Reactions. - It gives a bluish-black precipitate with a persalt of iron. It leaves no residue when burned with free access of air. Its aqueous solution gives no precipitate with solution of isinglass, and is thus distinguished from tannic acid.

Impurity. - Tannic acid from imperfect fermentation.

Tests. - No precipitate with solution of isinglass (no tannic acid).

Dose. - 2 to 10 grains.

Preparations.

B.P.

Dose.

Glycerinum Acidi Gallici (1 in 6).......................................................

12-60 min.

U.S.P.

Unguentum Acidi Gallici (with benzoated lard, 1 part in 10)

Uses. - Gallic acid resembles tannic acid in its action, but does not coagulate albumin, and is used chiefly in cases of haemorrhages from the lungs, or kidneys, or where the affected part can only be reached through the circulation.

In haemoptysis it is useful in ten-grain doses every two hours. Like tannin it is said to lessen the amount of albumin in the urine in albuminuria.