Encyclopædia Britannica, Ninth Edition/Tannin
TANNIN, a generic name for a class of vegetable substances which, as the name indicates, are all available for tanning, meaning the conversion of animal hide into leather. Tannin is widely diffused throughout the vegetable kingdom. An enumeration of the principal materials which form the commercial sources of the substance will be found under Leather, vol. xiv. p. 381, and in various special articles referred to from that heading.
Our chemical knowledge on the subject is very limited; and, as long as we know no better, each of the various tanning materials must be viewed as containing a "tannin" of its own kind. [1] Only a few have as yet been obtained in a state approximating chemical purity. The following characters are common to them all:—
(1) All are colourless or little-coloured non-volatile solids, soluble in water and in alcohol; the solution has an astringent taste.
(2) They colour blue litmus paper feebly red, yet all unite with the alkalies into soluble salts; the solutions of these eagerly absorb oxygen from the air, with formation of dark-coloured products.
(3) They form insoluble salts with the oxides of lead, zinc, copper, producible by addition of solution of the tannin to one of the respective acetate.
(4) They form very dark-coloured (green or blue) compounds with ferric oxide, conveniently producible by addition of the tannin to ferric or ferroso-ferric acetate. Ordinary old-fashioned black (gall-nut) ink may be quoted as an illustration.
(5) Tannin solutions precipitate gelatine as an insoluble compound, generally assumed to be chemically similar to the substance of leather.
(6) If a piece of raw hide be placed in a solution of any tannin, it imbibes the latter with formation of Leather (q.v.).
(7) Aqueous tannin-solutions, if mixed with dilute sulphuric acid, are readily oxidized by solution of permanganate of potash, which, being reduced to manganous salt, loses its intense violet colour.
Upon the last two propositions Löwenthal has based a convenient method for the assaying of tannin materials. A known weight of the substance to be analysed (say sumach) is extracted with water, and the extract diluted to a known volume. An aliquot part of the extract is then mixed with a certain proportion of a standard solution of indigo-carmin and of sulphuric acid, and, after large dilution with water, standard permanganate is dropped in from a burette (graduated glass tube) until the colour of the indigo is completely discharged. After deducting the volume of reagent which would have been taken up by the indigo alone, the rest is put down as corresponding to the “permanganate reducers generally.” Another measured volume of the extract is then poured over a sufficient weight of dry shavings of raw hide, after having been suitably diluted, and the whole is allowed to stand until the tannin has all passed into the hide. The liquid is then filtered, and a measured volume, corresponding to exactly the quantity of extract used for the assay, tested with permanganate. The volume of reagent used this time is deducted from that used in the assay as a correction. From the net permanganate the weight of pure gallotannic acid which it would oxidize is calculated on the basis of standard experiments, and from this weight the "percentage of tannin" is deduced. The method is purely empirical, and the results are of no value unless obtained according to a rigorously prescribed mode of procedure. Of individual tannins that of the gall-nuts, known as gallotannic acid, is best known. For its preparation (according to Pélouze) powdered gall-nuts are placed in an apparatus for extraction "by displacement," and in it soaked in a mixture of 9 parts of ether and 1 part of water for twenty-four hours. The liquid is then allowed to drain off, and the residue washed with aqueous ether. The liquid on standing separates into two layers,—a lower heavy layer, which contains the tannin, and an upper more purely ethereal layer, which contains gallic acid and other impurities. The lower layer is drawn off, washed once or twice with ether, and then evaporated to dryness at a gentle heat; the tannin remains as a porous friable mass of a slightly greyish-yellow colour. This is the tannin of the pharmaceutist.
Such tannin is not by any means an absolutely unitary substance. Its solution, if allowed to stand in the presence of a ferment which is naturally present in gall-nut extract, or more readily if boiled with sulphuric acid, yields a large proportion of gallic acid, which is easily obtained in pure crystals. According to Strecker, glucose is formed at the same time, whence he viewed tannin as a glucoside (see Sugar). But this is now recognized as a mistake, since Hugo Schiff showed that pure tannin is only digallic acid, C14H1009 = 2C7H605 (gallic acid) minus 1H20. Pure tannin, according to Schiff, can be obtained by dehydrating pure gallic acid by means of chloride of acetyl. The tannin of the Chinese gall-nuts seems to be identical with gallotannic acid.
Quercitannic Acid.—The tannin of oak bark is certainly different from gallotannic acid, because it yields no gallic acid when boiled with dilute vitriol. Etti Jahresb. über die Fortschr. der Chemie for 1880 , p. 898) prepares it by extracting the powdered bark with dilute alcohol at a gentle heat, adding ordinary ether to the alcoholic extract, and shaking out the tannin with acetic ether. The acetic ether extract is distilled to recover the solvent, the residue filtered, and the filtrate evaporated to dryness to obtain the pure (?) tannin as reddish- white powder of the composition C17H1609. At 130–140° C. it loses water and forms phlobaphen, C34H30017, a brown solid insoluble in water but soluble in solution of the tannin. Quercitannic acid forms quite a series of such anhydrides: C34H30O17; C34H28O16; C34H26O15; C34H24O14. Some, if not all, of these are contained in aqueous oak-bark extract, and they play an important part in its application for tanning. According to Etti, quercitannic acid is a tri-methyl substitution-product of digallic acid, C14H10O9 minus 3H plus 3CH3 = C17H16O9.
Besides these two tannins, those of coffee and cachou are the only ones which have been obtained in a relatively definite form.
- ↑ Coffee beans and tea leaves contain peculiar tannins.