Synthetic Tannins | Page 4

tannin can be
detected; and the yield (of leather, based on the pelt employed), which,
from a practical standpoint, is so important, is so very low that it is
hardly possible to speak of it as a tannin in the ordinary sense of the
word. Formaldehyde must, therefore, be termed a pseudo-tannin.
The tanning effect of formaldehyde is, according to Thuau, [Footnote:
Collegium, 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. Paternò [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