TANNIN (Fa., Aratiores tannantes; GER., Gerbstoffe).
The word " tannin" doea not, as formerly supposed, denote a single definite compound, but is a generic nsme applied to a large class of organic bodies, moatly uncrystallizable, which often differ widely both in chemical constitution and reaction, but have the common property of precipitating gelatine from its solution, and forming insoluble compounds with gelatine-yielding tissues. By virtue of this power, they convert animal hide into the insoluble and imputreseiblo material called " leather " (see pp. 121:3-40). They all form blackIsh-blue or blackish-green compounds with ferric aalts, and are precipitated by acetate of lead and of copper ; but these properties are common to many other organic substances. They give insoluble precipitates with many organic bsses, and with a large number of metallic salts. In some cases, the tmuin combines with the base only, liberating the acid ; but frequently the salt as a whole enters into combination. This ia the case with the precipitates formed with acetates of lead and copper. With alkaliea, the tannins and many of their derivatives give solutions which oxidize and darken rapidly, usually becoming successively orange, brown, and black. A. H. Allen has shown that theao bodies also give inatantancously a deep-red coloration with a solution of potassium ferricyanide and aiumonia. The reaction is one of considerable delicacy.
The tannins aro very widely distributed through the vegaable kingdom. They arc probably to be regarded in most casea rather as w nste.products eliminated by the plant, than as bodies stored up for future nutrition, like starch and augar, and are enclosed in the tissue-cells.
General Chemistry.—A large number of plillIts, from every part of the world, contain principles varying in properties and constitution, but having tho common characteristics of an astringent taste, of precipitating gelatine from solution, and of giving a blackish coloration with persalts Of iron. The whole class are denominated " tannins; " but it is obvious that the different tannins are often widely distinct, and that the general staternents frequently mado are usually true only of certain individual members of the group, On the same grounds, it may also bo concluded that any genelal method of analysis can only give arbitrary percentages, which afford no safe rueans of comparing tannins of different species, although it may be of considerable use in deciding the relative values of different samples of We same nntterial.
Frorn the tanner's point of view, tannins may be divided into two principal classes, viz. those which produce a light fawn-coloured deposit on leather (technically known as " bloom "), and those whieh do not. To the first of these, belong the tannins of gall-nuts, valonia, oak-bark, myrabolans, sumach, aud divi-divi, w hile the second includes cutch, gambler, henulock, larch, rhatany, and mangrove, and all the varieties of mimosa. Bearing in mind that the same plant frequently contains more than one species of tannin (of which the different characters of the tannins of gall-nuts, valouia, and oak-bark, all produced by oaks, is a atriking exaznple), it will appear that this classification is generally coincident with an important difference of chemical con stitution ; most tannins which give " bloom " are derivatives of gallic acid (see Acid—Gallic, pp. 50-1), while those which deposit insoluble red and brown colouring matters are derived frorn protocate chuic acid,—oak-bark and valonia excepted. To the chemical student, it may be interesting to note that both these bodies are benzene derivatives, belonging to the " aromatic" group of carbon com pounds, of which benzene is the simplest type. Their relation is shotvn by the following formula :— Stenhouse attempted some years since to separate tannins into t wo classes, according to the bluish or greenish-black which they gave with persalts of iron. It has been shown that this is dependent in some oases on associated impurities, and it is influenced by the amount of acidity of the non solution used, gallic and gallots,nnic acids giving a di_ ep green coloration with strongly acid ferric chloride, while with ferric acetate they give a blue- or purple-black. As a general rule, however, it may be stated that the tannins which give a blue- or purple-black with ferric acvtate ale gallic acid derivatives, and likely to give bloom to leather ; while those which give a green-blaek are derived from protocatechuic acid, and will probably yield no bloom. Gallie and pyrogallic acids give with ferric acetate a purple-black ; while protoeatechuic and pyrocatechuic acids give a dark-green.