TANNIN (from tan., OF., Fr. tan, oak-bark for tanning, from OHG. fauna, Ger. Tanne, fir, oak; probably connected with Skt. dhanran, how), TANNIC ACID, GALLOTANNIC ACID, or DI GALLIC Acm, An odorless and almost colorless substance, sol uble in water. alcohol, acetone, glycerin, and various oils, occurring in Turkish and Chinese gall-nuts and in various other plants. It has a bitter, astringent taste, and assumes a yellow coloration if exposed for some time to the action of light. It is used as an astringent in medicine and as a mordant in dyeing, and also finds em ployment in the manufacture of ink and of gallic and pyrogallic acids. Solutions of tannin reduce .
silver carbonate to metallic silver; they also re duce Fehling's solution (q.v.), and certain other substances, and precipitate gelatin and albumen from their solutions. On the other hand, solu tions of tannin may be precipitated by the addi tion of various salts and acids, including common salt, sal ammoniac, the acetate as well as the sul phate of potassium, hydrochloric and sulphuric acids, etc. Tannin is made by extracting the
powdered gall-nuts, which contain it in larger quantities, with a mixture of alcohol, ether and water; the layer of water then contains practically all the tannin; it is separated, purified by shaking with an excess of ether, and evaporated at a gentle heat. The residue thus obtained may be purified by dissolving in a dilute solution of common salt, and reprecipitating from this by adding an excess of salt in the solid state. With ferric salts tannin produces the well-known precipitate which is the coloring substance of ordinary black ink.
The names tannin and tannic acid are also applied to a variety of other substances of vege table origin and having properties more or less similar to those of gallotannic acid: they are all astringent, transform hide into leather, give a blue or green coloration with salts of iron, etc. Some of the more important of these substances may be briefly noticed here under their special names.