[CHAPTER IV.]
THE CHEMISTRY OF TANNINS.
The essential constituents of tanning materials are various members of a large group of organic compounds called tannins or tannic acids.[D]
[D] Ger. gerbsäure; in German the word tannin denotes usually gallotannic acid only.
These bodies often differ widely both in chemical constitution and reaction, but have the common property of precipitating gelatin from solution, and forming insoluble compounds with gelatin-yielding tissues. By virtue of this power, they convert animal hide into the insoluble and imputrescible material called "leather." They are mostly uncrystallisable; and all form blackish-blue or blackish-green compounds with ferric salts, and in common with many other organic substances are precipitated by lead and copper acetates, stannous chloride, and many other metallic salts, and those of organic bases, such as quinine. In some cases, the tannin combines with the base only, liberating the acid; but frequently the salt as a whole enters into combination. This is the case with the precipitates formed with lead and copper acetates. With alkalies, the tannins and many of their derivatives give solutions which oxidise and darken rapidly, usually becoming successively orange, brown, and black. A. H. Allen has shown that these bodies also give instantaneously a deep-red coloration with a solution of potassium ferricyanide and ammonia. The reaction is one of considerable delicacy.
Tannins are more or less soluble in water; and freely so in alcohol, mixtures of alcohol and ether, and ethyl acetate, but scarcely in dry ether alone, nor in dilute sulphuric acid; and insoluble in carbon disulphide, petroleum spirit, benzene, and chloroform.
From their amorphous character, tannins are extremely difficult to purify; and when, as is frequently the case, two or more tannins occur in the same plant, it is often quite impossible completely to separate them. Owing to their considerable differences in character, no general method of purification can be given, but the following processes will be found in many cases to give good results. For the special methods adopted by different investigators, the original memoirs must be consulted, references to many of which will be found in the following pages.
Preparation and Purification of Tannins.
The oldest method of separating tannins from other constituents is that applied by Pelouze to the preparation of commercial gallotannic acid from gall-nuts. The finely pulverised material is placed in a percolator and exhausted with commercial ether containing water and alcohol. The liquid separates, on standing, into 2 layers of which the lower contains most of the tannin in a tolerably pure form, dissolved in water and alcohol with a little ether, while the upper mainly ethereous layer contains the gallic acid. Gall-nuts thus treated yield 35-40 per cent. of tannin. If equal parts of ether and 90 per cent. alcohol are used, a larger yield is obtained, but the liquid does not separate into 2 layers, and it is questionable if the product is so pure. For Chinese galls, washed ether acts better than ether alcohol. The tannin may be still further purified by dissolving in a mixture of 1 part water with 2 of ether, when 3 layers are formed, of which the lowest contains nearly pure tannin.