Synthetic Tannins | Page 4

1909, 363, 211.] increased by those salts which bring about colloidal polymerisation of the formaldehyde, the resultant compounds being absorbed by the hide fibre. Fahrion considers this to be a true tannage, and is supported by Nierenstein [Footnote: _Ibid._, 1905, 157, 159.]:--
R.NH_2 R.NH-| +O.C.H. = CH_2 + H_2O R.NH_2 | R.NH-| (Hide.) H (Leather.)
A peculiar combination between true tannage and pickling is to be found in the tawing process (tannage with potash, alum, and salt), whereby, firstly, the salt and the acid character of the alum produce a pickling effect, and secondly, the alum at the same time is hydrolysed, and its dissociation components partly adsorbed by the hide, thereby effecting true tannage. This double effect is still more pronounced in the synthetic tannins which contain colloidal bodies of pronounced tanning intensity on the one hand, inorganic and organic salts on the other, which then act as described above. Their real mode of action can only be explained with the aid of experimental data. The following chapters will deal with the different behaviour of the various groups of synthetic tannins.

PART I SECTION I
THE SYNTHESIS OF VEGETABLE TANNINS
1. TANNIN
The first investigations of gall-tannin date from the year 1770, at which time, however, no exact differentiation between tannin and gallic acid was made. The first step in this direction was made when Scheele,[Footnote: Grell's _Chem. Ann._, 1787, 3, I.] in 1787, discovered gallic acid in fermented gall extract, and in the same year Kunzemuller [Footnote:_Ibid._, 1787,3,413.] separated gallic acid (or pyrogallol) as a crystalline body from oak galls. Dize [Footnote: _Jour. Chim. et Phys._, 1791, 399.] continued the investigations, which were brought to a conclusion with Deyeux' work [Footnote: _Ann. Chim._, 1793, 17, I.]; both recognised that the substance isolated was not a single substance, but was a mixture of gallic acid, a green colouring matter, a rosin (tannin?), and extraneous matter. Proust [Footnote: _Ibid._, 1799, 25, 225.] was the first to differentiate the crystalline gallic acid from the amorphous, astringent substance, which latter he named "Tannin."
Amongst the numerous subsequent investigations of tannin must be especially noted the one by Berzelius [Footnote: Pogg,_Ann._, 1827, 10, 257.], who purified the potash salt and decomposed this with sulphuric acid. Pelouze [Footnote: Liebig's _Ann._, 1843, 47, 358.], later on, observed the formation of the crystalline gallic acid from tannin, when the latter is boiled with sulphuric acid; this had already been observed by J. Liebig.[Footnote: _Ibid._1843, 39, 100.] Both had noticed the absence of nitrogen. In addition to the methods of preparation of tannin then in vogue neutral solvents were mainly employed by subsequent investigators; Pelouze [Footnote: _Jour. Prakt. Chem._, 1834, 2, 301, and 328.] treated powdered galls with ether containing alcohol and water, and considered the upper layer to be a solution of gallic acid and impurities, the bottom layer to contain the pure tannin.
The EMPIRICAL FORMULA of tannin has also been the subject of much speculation by the different investigators, the difficulty here being that of obtaining a pure specimen of the substance free from sugars, and which could be submitted to elementary analysis. Whereas these early purified substances were thought to correspond to the formula of digallic acid (galloylgallic acid), C_14H_10O_9, Fischer and Freudenberg [Footnote: _Ber._, 1912, 915 and 2709.] were able to show, with approximate certainty, that the constitution of tannin is that of a pentadigalloyl glucose.
Early attempts at hydrolysing tannin gave varying results, some investigators claiming the presence, and others the absence of sugars. Here, again, E. Fischer and Freudenberg [Footnote: _Ibid._] were able to conclusively prove that on hydrolysing tannin with dilute acids, 7.9 per cent. glucose is dissociated, and that hence glucose forms part of the tannin molecule. Fischer and Freudenberg also determined the optical activity of pure tannin in water: [Greek: a]_D was found to lie between +58�� and +70��.
Graham found [Footnote: _Phil. Transact._, 1861, 183.] that the tannin molecule is of considerable size, since its diffusion velocity is 200 times less than that of common salt. Patern�� [Footnote: _Zeits. phys. Chem._, 1890, iv. 457.] was the first to determine the molecular weight of tannin, employing Raoult's method; he found that tannin in aqueous solution behaves like a colloid and that hence Raoult's method is not applicable. When, on the other hand, he dissolved tannin in acetic acid, results concordant with the formula of C_14H_10O_9, corresponding to a molecular weight of 322, were obtained. Sabanajew [Footnote: _Ibid._, 1890, v. 192.] later determined the molecular weight of tannin in aqueous solution as 1104, in acetic acid solution as 1113-1322, Krafft [Footnote: _Ber._, 1899, 32, 1613.] as 1587-1626 in aqueous solution. Walden [Footnote: _Ibid._, 1898, 3167.] determined the molecular weight of tannin-schuchardt as 1350-1560, tannin-merck as 753-763, digallic acid as 307-316 (calculated 322). Feist [Footnote: _Chem. Ztg._, 1908, 918.] determined the molecular weight of tannin as 615 and one