TANNINS, or TANNIC ACIDS, are com pounds of high molecular weight, widely dis tributed in the vegetable kingdom. Some varie ties are found in diseased vegetable tissues or in abnormal growths, while others are present in the different parts of the healthy plant. Gallotannic acid is present in gall-nuts, caf fetannic acid in coffee beans, morringa-tannin in yellow wood, quercitannic acid in oak-bark, ellagitannic acid in pomegranate rind, etc. These tannins are not by any means identical in all their physical properties and their chemi cal conduct; still they possess some characteris tics in common. They are usually amorphous, have an astringent taste, dissolve in hot water, convert animal hide into leather and yield dark blue or green precipitates with ferric salts. Of these' varieties gallotannic acid— the chief con stituent of the commercial product — has. more than any other tannin, received the attention of investigators. The statements in this article refer chiefly to this compound.
Gallotannic acid is extracted from powdered galls by treatment with aqueous or alcoholic ether. On standing the liquid separates into an upper layer containing gallic acid and other impurities and into a lower layer, from which crude tannic acid is obtained by spon taneous evaporation. From this product im purities may be further removed by treatment with dry ether in which the acid is insoluble. Ethyl acetate and acetone have also been used quite extensively for the extraction and puri fication of tannic acid. The compound hag further been purified by treating its water solution or suspension with lead acetate. This reagent precipitates a lead salt which is readily decomposed with hydrogen sulphide.
Gallotannic acid is an amorphous substance with an astringent taste, soluble in water, or dinary alcohol, glycerine, ethyl acetate and acetone; almost insoluble in dry ether, chloro form, benzene, carbon bisulphide and petroleum benzine. Its aqueous solution gives an acid reaction with litmus, although its acidity can not be determined by direct titration, an indi cator being practically valueless. With pure ferrous salts aqueous solutions of tannic acid give no coloration at first, but a color is soon developed by atmospheric oxidation and a blue black precipitate is ultimately formed. With ferric salts a bluish-black precipitate is at once formed; this reaction is taken as a basis for the use of the compound in the preparation of blue-black inks. Cold alkaline solutions of tannic acid absorb atmospheric oxygen very readily, forming highly-colored oxidation prod ucts. Tannic acid solutions form precipitates with many metallic salts, with a number of mineral acids, with nearly all alkaloids and glucosides and with gelatin, albumin and starch. Animal hide kept in a solution of the acid is gradually converted into leather. At 215° C. tannic acid decomposes into water, carbon dioxide, pyrogallol and metagallic acid and with boiling dilute sulphuric acid it is hydro lysed into gallic acid.
Tannic acid is extensively employed in the process of tanning, in the manufacture of gallic and pyrogallic acids, in the preparation of inks and as a 'mordant in dyeing. With formalde hyde it forms a condensation product which is used in the manufacture of hygienic fabrics. Tannic acid has also found extensive applica tion in medicine. It is recommended as an anti dote to metallic poisons with which it usually forms insoluble precipitates. It is employed as an astringent to prevent excessive secretion in ulcers and sores and to check bleeding. Tan nigen (tannyl triacetate), tannocol (gelatin tannate), tannalbin and bismuth tannate have been prepared and employed in intestinal ca tarrh. Mercurous tannate has been used in syphilis, tannochrom in skin diseases, tanno bromine in the treatment of scalp diseases.
Although oak-bark, gall-nuts and other natural sources of tannic acid had been used by the ancients, the compound was first isolated and described by Scheele in 1787. Berzilius (1W) assigned to it the formula C.His012. Liebig (1834) modified this to In 1850 Mulder called attention to the hydrolysis of the compound into gallic acid: Strecker (1852) claimed that tannic acid was a glu coside in which three molecules of gallic acid combined with one molecule of glucose; while others up to the present day have contended that the purified substance contains no sugar. Schiff claimed to have converted gallic acid into tannic acid by a process of condensa tion. He called his product digallic acid and assigned to it the formula Ci.}1100., which had also been adopted by same earlier investigators. Schiff's digallic acid formula remained unques tioned for many years, but in 1890 Ph. van Tieghem, C. Scheibler and others made the observation that tannic acid was optically active, a property that ) rendered Sehiff's digallic acid formula improbable. After several years of study Nierenstein (1908) announced the con viction that tannic acid was a mixture of Schiff's digallic acid and optically active leuco tannin. While Emil Fischer's researches with the carefully purified substance pointed to the conclusion that tannic acid might be considered as a compound of one molecule of glucose with 10 molecules of gallic acid. The hydrolysis of the compound into glucose and gallic acid, as well as its synthesis, are cited as possible confirmations of this view. At the same time it is frankly admitted that the question of struc ture has not received its final settlement.
Of late years compounds showing the prop erties of tannic acid have been prepared by synthetic processes. Of these so-called a Syn Neradol and Neradol D deserve men tion. They are obtained by heating phenol sul phonic acids with formaldehyde. It is claimed that the amount of Neradol required to tan skin is less than of any other vegetable tannin.