Tannic Acid, Or Tannin. The astringent principles existing in a great variety of plants, which render them capable of combining with the skins of animals to form leather, of precipitating gelatine, of forming bluish black precipitates with the per-salts of iron (or if a free acid be present a dark green color), were formerly termed tannin. These substances, being found to possess acid properties, are now known as tannic acid, and various distinctive names are given to them as they are found of different chemical compositions, though agreeing in their essential properties. Thus the tannic acid derived from the gall nut is termed gallotannic acid; that of the oak, quercitannic acid; of the fustic (morus tinctoria), moritannic acid; of the cinchona, quinotannic acid, etc. The principal sources of tannin have been named in the article Leather, and the method of extracting it has been particularly described in the article on Galls, which are the most abundant source of it. Besides this variety, which is the same as that existing in the bark and leaves of many forest trees, fruit trees, and shrubs, and in some roots, as those of the tormentilla and bistort, there is another less known, as the tannin of the catechu and kino, which precipitates the salts of iron dark green instead of blue.
Gallotannic acid when pure is a whitish, uncrystallizable solid substance, without odor, intensely astringent to the taste; it dissolves freely in water, to a less extent in dilute alcohol, and sparingly in ether. The best solvent for medical uses is glycerine. It changes blue litmus paper to red, and expels carbonic acid from its compounds with effervescence. Its formula is C27H22O17. Its aqueous solution exposed to the air absorbs oxygen, and is converted into gallic acid. Besides its use in tanning, gallotannic acid is employed to produce with the salts of iron the gallotannate of iron, which is the basis of most of the writing inks. It is also employed in medicine for its astringent property, chiefly in checking haemorrhages, as a wash for ulcers, ophthalmic affections, etc. If taken internally in large quantities, it is an irritant; but in small doses it is absorbed and makes its appearance in the urine as gallic acid, having undergone a process of oxidation in the organism. The combinations of tannic acid with iron and with lead have been applied in the form of ointments to the dressing of ringworms, gangrenous sores, etc.