FORMULA: HC14H9O9. MOLECULAR WEIGHT: 321.22.
SYNONYMS: Gallotannic acid, Digallic acid, Tannin, Tanninum.
"An organic acid obtained from nutgall"—(U. S. P.).
Source.—Tannin is a name applied to vegetable substances possessing acid properties and having an astringent taste, and which produce with iron salts a dark precipitate or solution, and precipitate albumen and gelatin. The tannin under consideration is produced from nutgalls, and to distinguish it from other tannins, is known as gallotannic or digallic acid. Tannic acid may be obtained from nutgalls (excrescence on Quercus lusitanica, Lamarck, var. infectoria, Nat. Ord. Cupuliferae, U. S. P.), from the leaves of the Rhus Coriaria, Linné, from some kinds of acorn cups, and from Japanese and Chinese galls. Allied tannic acids are also found in catechu, coffee, fustic, quercitron, pomegranate, kino, cinchona, tea, the oak, willow, elm, horse-chestnut, plum, pear, sumach, whortleberry, etc., in each instance possessing nearly the same properties, though their chemical composition is different. Some of them form a dark-green color with the salts of iron, and a few form a gray color. Gallotannic acid produces a dark-blue, or bluish-black precipitate with ferric salts. Tannic acid was distinguished as an individual compound by Deyeux and Seguin, in 1793 and 1795, respectively. Gallotannic acid was found by Schiff, in 1871, to be an anhydrid of gallic acid, having the constitutional formula: CO2H.C6H2(OH)2O.COC6H2(OH)3.
Preparation.—Allow sulphuric ether to percolate through a suitable quantity of powdered galls contained in a glass percolator, the lower end of which is loosely closed with a pellet of cotton. The liquor obtained in the receiver separates into two parts, and the ether must be allowed to percolate through the galls until the lower stratum of liquid in the receiver no longer increases. Pour off the upper layer, and evaporate the lower portion with a moderate heat, to dryness.
It is stated that a much larger quantity of tannic acid may be obtained by employing a mixture of 16 parts of ether and 1 part of alcohol. The percolated liquid separates into two layers. The lower one contains the tannic acid, which may be obtained perfectly pure on evaporation; the upper layer contains the gallic acid, coloring matter, and some tannic acid. The tannic acid in the upper layer may be obtained by evaporating the liquid to dryness, treating the residue with pure ether, until the lower of the two layers, into which the liquid separates, no longer presents a green color; and then separating it, adding, if necessary, a little alcohol, and evaporating.
Description and Tests.—"A light-yellowish, amorphous powder, usually cohering in form of glistening scales or spongy masses; odorless, or having a faint, characteristic odor, and a strongly astringent taste; gradually turning darker when exposed to air and light. Soluble at 15° C. (59° F.) in about 1 part of water, and in 0.6 part of alcohol; very soluble in boiling water, and in boiling alcohol; also in about 1 part of glycerin, with the intervention of a moderate heat; freely soluble in diluted alcohol, sparingly in absolute alcohol; almost insoluble in absolute ether, chloroform, benzol, or benzin. When heated on platinum foil, the acid is gradually consumed without leaving more than 0.2 per cent of ash. Tannic acid has an acid reaction upon litmus paper"—(U. S. P.).
The watery solution, exposed to the air, absorbs oxygen, and is transformed into carbonic acid gas, which escapes, leaving behind gallic and ellagic acids. Oils do not dissolve it. Tannic acid combines with a solution of animal gelatin, forming a white, curdy, insoluble substance, the tannate of gelatin; a piece of prepared skin introduced into a solution of tannic acid, absorbs the acid and is converted into leather. With the per-salts of iron, tannic acid and its salts strike a deep-blue, nearly black color, which is a tannate of iron, and the principal ingredient of ordinary ink. Ink stains are gallo-tannates of iron, and are readily removable by oxalic and citric acids, owing to the solubility of the iron basis. When potassium hydroxide is added in excess to a solution of tannic acid, tannoxylic or rubitannic acid is formed; if the mixture be boiled, instead of exposed to the air, tannomelanic or tannohumic acid is formed, a bibasic, dark, humus-like powder. Concentrated sulphuric acid dissolves the dry tannins, forming yellow solutions which, when heated, become deep-red, owing to the formation of rufi-gallic and meta-gallic acids. Potassium bichromate causes brown precipitates with the majority, if not all, of the tannins (Trimble, On Tannins). Tannic acid precipitates most metallic oxides from the solution of their salts; is more or less completely precipitated from its solution by mineral acids, and gives, with those acids, compounds soluble in pure water. If tannic acid be treated with oxidizing bodies, as with nitric acid, chromic acid, chlorine, bromine, or the higher oxides, it is completely destroyed, under production of formic and oxalic acids. Acetate of lead added to a solution of tannic acid, produces a white precipitate; tartar emetic gives a white precipitate, of a gelatinous character. When given internally, tannic acid will be found, when passed in the urine, to have changed into gallic acid. There is a substance formed in white wines, called glaïadine, which renders them turbid and disposed to mucous fermentation; a solution of tannic acid will arrest this by coagulating the above-named substance.
The U. S. P. gives the following tests: "The addition of a small quantity of ferric chloride T.S. to an aqueous solution of the acid, produces a bluish-black color or precipitate. On adding to an aqueous solution (1 in 100) of tannic acid a small quantity of calcium hydrate T.S., a pale, bluish-white, flocculent precipitate is produced which is not dissolved on shaking (difference from gallic acid), and which becomes more copious and of a deeper blue by the addition of a moderate excess of calcium hydrate T.S., while a large excess of the latter imparts a pale-pinkish tint to the solution. The aqueous solution of the acid imp produces precipitates with most alkaloids and bitter principles, and with test solutions of gelatin, albumen, and starch (distinction from gallic acid). On dissolving 2 Gm. of tannic acid in 10 Cc. of boiling water, and allowing the liquid to cool, no turbidity should be produced on diluting 5 Cc. of the solution with 10 Cc. of alcohol (absence of gum or dextrin), or with 10 Cc. of water (absence of resin)"—(U. S. P.).
Action, Medical Uses, and Dosage.—Tannic acid is a pure astringent. It has a bitter, astringent taste, and a constringing action upon mucous tissues. As a general rule it does not derange the stomach, yet it precipitates pepsin from the gastric secretions. It generally produces constipation, by contracting the intestinal vessels, thus diminishing the secretions and retarding peristaltic action. It sometimes, and especially when long given, occasions gastric and intestinal pain, febrile phenomena, with thirst and eructations of gas, while the tongue is coated, and defecation tenesmic. It powerfully coagulates blood and albumen, and enters into the blood in the form of gallic acid. It probably controls hemorrhage by acting upon the vascular coats. Erythema, dyspnoea, and a cyanotic condition have been produced by it.
In view of its astringent power, tannic acid is very valuable in gastro-intestinal disorders, with undue acid, watery, or mucoid secretions, and accompanied with flatulence. In the various forms of non-irritative diarrhoea, without fever or inflammation, it is of marked utility. Chronic dysentery is asserted to be benefited by it. Tannic acid should never be given when there is fever or active inflammation. The diarrhoea and colliquative sweats of phthisis are controlled by it, the febrile condition here, if present, not contraindicating its use. It is effectual in uterine and other passive hemorrhages, and as a wash or injection to remove chronic mucous discharges, as in bronchial catarrh, gonorrhoea, gleet, leucorrhoea, etc." Use it in hemorrhage, from abortion, or any passive uterine hemorrhage, with pain and nervous disturbance. Take 30 grains each, of tannic acid and Dover's powder, and divide into 5 or 10 powders, and let 1 be taken every hour or two until the bleeding is arrested. This will cheek the flow, provided there is no organic lesion" (Locke, Syl. of Mat. Med.). In this manner it is of much value in menorrhagia. Hematuria, hematemesis, and hemoptysis (by spray) are benefited if the blood is small in amount. If the hemorrhage be active in the latter case, the agent is too slow in producing its effects. It has likewise been recommended in diabetes, combined with opium, and to arrest excessive perspiration; also, in conjunction with morphine, in Asiatic cholera. Externally, it has been successfully used in excoriations, prolapsus ani, piles, fissure of the anus or rectum, sore nipples, phagedenic ulcers, aphthous ulceration of the mouth, sore throat, severe salivation, and in toothache, in solution with ether. Applied to nasal polypi, it is stated to have produced a rapid disappearance of the abnormal growths. In the form of ointment, it will frequently prove effectual in curing vaginal leucorrhoea, being introduced into the vagina on lint or cotton, and allowed to remain there, changing it every 3, 4, or 5 hours. In solution or powder, in the form of spray passed upon the affected parts, it gradually overcomes chronic mucous irritation or congestion, and has been beneficially applied in chronic nasal, faucial, pharyngeal and laryngeal mucous affections. Dissolved in 3 parts of mucilage, it has effected cures in chronic granular conjunctivitis, corneal ulceration, and other affections of the eye. It may be used in ophthalmia neonatorum, with granular conjunctiva (1 part in 10 of water); and in purulent conjunctivitis, with but little swelling and small quantity of secretion, use a wash of from 2 to 10 grains to 1 ounce of distilled water. In the early stage of trachoma, when there is slight roughness of the conjunctiva, the beginning of granulation, with injected and clouded upper part of the cornea, a glycerole (5 grs. to 1 fl ounce) dropped into the eye in 2-drop doses will allay the gritty sensation. In advanced trachoma, with soft, pasty granulations, it may be used in connection with gallic or boric acids (Rx Boric acid drachm iij; tannic acid drachm i; Rx Gallic acid drachm i; tannic acid drachm iij). It is of little value to destroy aural polypi, and when used in suppurative otitis media, as it sometimes is, it is objectionable in that hardened masses, difficult of removal, are formed. Its solution in glycerin is a powerful styptic. It may be employed in the form of a wash, by adding 5 grains to a fluid ounce of water; or in ointment, 1 part of the acid to 10 or 15 of lard. It is a valuable remedy, the only disadvantage being its tendency to produce constipation, which may be avoided by the addition of a small quantity of podophyllum resin, in cases where this resin is not contraindicated.
The glycerole of tannin is very efficient in sores occuring from the use of false teeth; and in ingrowing toe-nail, with fungous granulations, the pure acid is especially useful. It forms a good dressing for burns. Several cases of cholera in the collapsed stage have been cured by our physicians, by doses of 10 or 15 grains of tannic acid, repeated every 10 or 15 minutes, until the discharges ceased; and continuing it afterward at longer intervals, with other appropriate treatment. Tannic acid, as an internal astringent, sometimes leaves the tissues upon which it acts harsh and dry. Dr. Chausarel has proved that tannic acid is the best antidote against poisonous fungi, or mushrooms, etc.; 30 or 40 grains of tannic acid, dissolved in a pint and a half of water, may be taken in small glassful doses every 5 minutes; if too much time has not elapsed an emetic may be first administered. Tannic acid is one of the best antidotes against poisoning by strychnine, forming an insoluble tannate of strychnine; it may be given freely. Dose of tannic acid, from half a grain to 10 grains. Suppositories, consisting each of 12 or 15 grains of butter of cacao, and 3 to 5 grains of tannic acid, are valuable in some rectal and vaginal diseases, as anal prolapsus, hemorrhoids, abrasion of the vaginal epithelium, leucorrhoea, etc.
Specific Indications and Uses.—Relaxed states of the gastro-intestinal tract, with excessive secretions, and no fever or inflammation; soft, pasty or fungoid granulations; passive hemorrhages; leucorrhoea with vaginal relaxation.