Gallotannic acid is met with, in commerce, in the form of light buff-coloured scales, with a faint peculiar odour and a powerfully astringent taste. It is soluble in 6 parts of cold water or glycerin, and very readily in hot. It is also very soluble in alcohol containing water, but much less so in absolute. It is moderately soluble in washed, but scarcely at all in anhydrous ether, chloroform, benzene, or petroleum spirit.
The commercial acid usually contains more or less of gallic acid, which may be detected by dissolving in water, shaking with ether, and after decanting and evaporating the ether, applying the tests described under gallic acid ([p. 70]). It may also be frequently distinguished in the simple aqueous solution of the tannic acid by the tests given. Its quantity may be determined (in the absence of other impurities) by the Löwenthal method ([p. 121]), the gallic acid forming the "not-tannin," by comparison with a solution of pure gallic acid.
Commercial tannic acid is sometimes adulterated with starch, which is left undissolved on treating the sample with ordinary alcohol.
For the estimation of gallotannic acid see [pp. 118] et seq. For its principal reactions, Table, [p. 113].
Ellagitannic acid, C14H10O10, is contained in divi-divi, myrabolans, and as a glucoside in pomegranate rind. When boiled with dilute acids, or treated with water at 230° F. (110° C.) in a sealed tube, it yields its anhydride, ellagic acid (see [p. 71]), C14H8O9. In its reactions ellagitannic acid closely resembles gallotannic acid.
Quercitannic acid. Oak-bark tannin.—This tannin may be prepared from oak bark by agitating an alcoholic extract with ethyl acetate, and separating and evaporating the ethereous layer. It is still contaminated with a brownish-green terpene resin, and with some of the higher anhydrides of the tannin. The resin may be removed by treating the dried extract with ether or benzene, in which it is readily soluble; and the phlobaphenes, or higher anhydrides, by dissolving the tannin in ether-alcohol, or probably to a great extent, by simple solution in cold water in which the phlobaphenes are scarcely soluble.
It may also be prepared by evaporating the alcoholic extract, and extracting with water, which leaves the phlobaphenes, or higher anhydrides undissolved. The first anhydride, which is partially soluble, may be precipitated by the addition of salt, and the quercitannic acid extracted by shaking the filtered solution with acetic ether. In very dilute alcohol it yields a pure yellow precipitate with lead acetate. In aqueous solution the precipitate is light-brown. It gives a blue-black with ferric salts. When pure, quercitannic acid yields nothing to pure ether or to benzene.
If quercitannic acid be heated to 266°-284° F. (130°-140° C.) it loses water, and yields a red anhydride slightly soluble in water, which constitutes the red colouring matter of oak bark, and which has also been called "difficultly soluble tannin." It precipitates gelatin and is one of the class which Eitner well designates "tanning colouring matters." It gives a brownish red with lead acetate, and a blue-black with ferric salts; it is difficultly soluble in water and ether, but readily so in alcohol of all strengths. Together with other anhydrides, it exists naturally formed in the bark. At higher temperatures, or by boiling with acids, a series of higher anhydrides may be obtained which are quite insoluble in water, but are soluble in alcohol and caustic alkalies. No glucose is produced by treatment of pure quercitannic acid with acids, that formed by so treating oak-bark extract being due to the alteration of the lævulose present. If oak-reds are fused with potash they yield, according to Johansen (Chem. Soc. Jour., xxxii. 721), protocatechuic and butyric acids. If heated in sealed tubes with dilute hydrochloric acid, gallic acid only is formed, with evolution of methyl chloride. The constitutional formula of quercitannic acid is as yet very uncertain: it is probably a methyl derivative of digallic acid. Its formula is variously given as C17H16O9 (Etti, Chem. Soc. Jour., xliv. 994), C28H26O15 (Ibid. xl. 901, Löwe), C19H16O10 (Böttinger, Berichte, xvi. 2710). For further details the original memoirs must be consulted.
For the reactions of oak-bark and valonia infusions the Table, [p. 113], may be consulted.
Before describing the catechol-tannins it will be necessary to speak of a group of compounds of which it is probable that these tannins are decomposition products. These are the catechins. It is as yet by no means certain how far they should be considered a group, some chemists holding the opinion that there is only one catechin, of which the rest are merely impure preparations.