FORMULA: HC7H5O5.H2O. MOLECULAR WEIGHT: 187.55.
SYNONYMS: Trioxybenzoic acid, Dioxysalicylic acid.
"An organic acid, usually prepared from tannic acid"—(U. S. P.).
Source, History, and Formation.—Though existing in a number of astringent plants, the greater portion of commercial gallic acid is derived from nutgalls. Scheele (1785), who first obtained it pure, established its non-identity with tannic acid. The manner of formation of gallic acid from nutgalls has been a subject of much discussion and experimentation. Before investigations were begun it was believed to exist ready-formed in galls, but in 1833 Pelouze showed that the larger portion of it was derived from the tannin of the galls, and advanced the theory that this conversion was accomplished by oxidization by the atmospheric oxygen, by which carbon dioxide was driven off. The elder Robiquet (1837) showed that its conversion could be accomplished without the aid of oxygen and without evolving carbon dioxide, but that it resulted from a ferment called pectase. Wetherill (1847), and subsequently, Mulder (1848), attempted to show that tannic acid differed from gallic acid only in the possession of a larger amount of water of crystallization. Liebig believed the change to be due to the liberation of a carbohydrate. In 1854 Strecker came to the conclusion that tannin was a glucosid, for by boiling it with diluted mineral acid he obtained a large amount of gallic acid and considerable glucose. This view was generally accepted for a long time, though opposed by the younger Robiquet (1854) and Hlasiwetz (1867), who advanced different theories regarding the supposed glucosid. The present theory is that advanced by Schiff (1871) and supported by others, that pure tannic acid be viewed as digallic acid (this being the first anhydrid of gallic acid), and that natural tannin is the glucosid of pure tannic, or digallic acid, for by the action of hot diluted mineral acids or a nitrogenous ferment upon it, digallic acid and glucose are evolved.
Preparation.—Considerable gallic acid is now made by the sulphuric acid process. The British Pharmacopoeia directs that 1 part of coarsely powdered galls be boiled for one-half hour with 4 parts (fluid measure) of diluted sulphuric acid and strained through cloth while hot. Crystals are deposited when cool, and are to be collected, treated with animal charcoal, and repeatedly crystallized.
The method more generally employed is as follows: Take of finely-powdered nutgalls, 3 Troy pounds. Mix the galls with enough distilled water to form a thin paste and expose the mixture to the atmosphere by allowing the vessel to stand in a warm place for about 4 weeks, taking the precaution to see that it has at all times sufficient distilled water to maintain a thin, pasty consistence, and to stir the mass occasionally with a glass rod. A porcelain or glass vessel should be used for this operation, and in no instance should an iron dish be employed, as iron or the presence of iron salts give to the product a color very difficult of removal. After exposure of the mixture for the required length of time the pasty mass is to be expressed and the residue is added to 8 pints of distilled water and boiled for a short time, and filtered through pure animal charcoal while hot. Upon cooling crystals of gallic acid are deposited, which, if further purification is necessary may be again dissolved, treated with charcoal, and recrystallized.
Description.—Gallic acid forms delicate "white, or pale fawn-colored, silky, interlaced needles, or triclinic prisms, odorless, having an astringent or slightly acidulous taste; permanent in the air. Soluble at 15° C. (59° F.), in 100 parts of water, and in 5 parts of alcohol; in 3 parts of boiling water, and in 1 part of boiling alcohol. Also soluble in 40 parts of ether, and in 12 parts of glycerin. Very slightly soluble in chloroform, benzol, or benzin. When heated at 100° C. (212° F.), the acid loses its water of crystallization (nearly 9.6 per cent). At about 222° C. (431.6° F.), it begins to melt, and at a higher temperature it is gradually decomposed. At a low, red heat it is consumed without leaving a residue. Gallic acid has an acid reaction"—(U. S. P.).
Gallic acid is entirely sublimed when heated, yielding carbonic acid gas and pyrogallol (C6H6O3).
Tests.—"If 5 Cc. of a cold, saturated, aqueous solution of the acid be treated in a watch-glass, with 6 drops of sodium hydrate T.S., the liquid will gradually acquire a deep-green color, which is changed to reddish or brownish-red by acids. Gallic acid neither colors nor precipitates pure ferrous salts, but forms a bluish-black precipitate with ferric salts. On adding to a cold, saturated, aqueous solution of gallic acid some calcium hydrate T.S., a bluish-white precipitate will form, where the test solution is temporarily in excess, and will disappear on shaking. When the test solution has been added in excess, the precipitate no longer dissolves, and the liquid acquires a tint which is blue by reflected and green by transmitted light, and becomes pink on the addition of a large excess of calcium hydrate T.S. (distinction from tannic acid). An aqueous solution of the acid should not precipitate alkaloids, gelatin, albumen, or starch T.S. (difference from and absence of tannic acid)"—(U. S. P.).
Solutions of gold, platinum, and silver salts, as well as those of the allied metals, are reduced by gallic acid or its salts, the solution of argentic nitrate converting gallic into tannic acid.
Action, Medical Uses, and Dosage.—Gallic acid does not coagulate albumen, and when ingested is quickly absorbed, and rapidly discharged by the kidneys, over the secretions of which, as well as of the skin, it has a marked control. Gallic acid is much inferior to tannic acid as a topical astringent; but administered internally, it is more powerful as a remote astringent. Indeed, tannic acid, in its passage through the system, becomes changed into gallic acid. As a remote astringent, gallic acid has been found very beneficial in uterine, pulmonary, and nephritic hemorrhages, and all hemorrhages of a passive character. Menorrhagia has promptly ceased under its use. Give 5 grains in pill-form 3 or 4 times a day during the flow, as well as during the intermenstrual period. It is best adapted to chronic passive cases. It has also been found useful in night sweats, pyrosis, chronic mucous discharges from the bowels or bladder, and has some reputation in arresting the excretion of albumen in Bright's disease of the kidney, and assists in maintaining the patient's strength. In hemoptysis give 3 grains each of gallic acid and Dover's powder every 2 hours, and at the same time administer ergot by the mouth or hypodermatically (Locke). From 2 to 5 grains every 3 hours controls bleeding from the nose and bowels during typhoid fever. As a remedy in diabetes insipidus it is asserted to arrest the polyuria by promptly constringing the relaxed renal capillaries. From 10 to 30 minims of the glycerole should be administered 4 times a day (Webster). Some cases of old, purulent conjunctivitis are cured by it, and it is of value in trachoma with soft, pasty granulations. One part of gallic to 3 parts of tannic acid should be insufflated upon the parts twice daily (Foltz). It has given benefit in purpura. Costiveness is not produced by its use. Its dose is from 3 to 20 grains 3 times a day, or oftener; it may be used in the same form as the tannic acid; of the glycerole 5 to 60 minims. Its hydro-glycerin solution may be employed as a wash, gargle, or injection.
Specific Indications and Uses.—Passive hemorrhages with pulse feeble and extremities cold, skin inelastic and capillaries relaxed; hematuria with nausea, vertigo, headache, and dull aching in the region of the kidney; soft, pasty, granular conjunctivitis.
King's American Dispensatory, 1898, was written by Harvey Wickes Felter, M.D., and John Uri Lloyd, Phr. M., Ph. D.