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]
[Footnote 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.
These methods are applicable to the dried or highly concentrated
extracts of many tanning materials. Many tannins may be separated from
their strong aqueous solution in a state of considerable purity by
first agitating with ether to remove gallic acid, and then saturating
with common salt, and shaking well with acetic ether, which takes
up the tannin. Another method is to extract with alcohol, evaporate
to a small bulk at as low a temperature as possible, and treat at
once with a considerable quantity of cold water. The infusion is
then precipitated with successive small quantities of lead acetate;
the first and last portions of the precipitate are filtered off and
rejected as contaminated with colouring matters and other impurities,
while the remainder, after rapid washing, is suspended in water and
decomposed with sulphuretted hydrogen. The filtrate is shaken with
ether to remove gallic acid, and the aqueous portion is evaporated at
a low temperature in a partial vacuum to a thin syrup, and the drying
completed over sulphuric acid in vacuo.
General Chemistry.
The natural tannins are all compounds of carbon, hydrogen and oxygen
only. They all contain the benzene group of carbon atoms, but their
ultimate structure is, except in the case of gallotannic acid, very
imperfectly understood, and probably differs considerably in type in
different members of the family.
In order to make clear to those readers who have not studied modern
organic chemistry, what we do know on the subject, a few words of
introduction will be necessary. All organic compounds contain carbon,
in combination with hydrogen, and very frequently also with oxygen,
nitrogen, and other elements. A single atom of carbon is able to
combine with 4 atoms of hydrogen, as it does to form marsh gas, or
methyl hydride, CH_{4}. Other elements may be substituted for the
hydrogen; for instance, if we replace 3 of the hydrogen atoms with
chlorine, we obtain chloroform, CHCl_{3}. Again an atom of oxygen may
be inserted between the carbon atom and one of the hydrogen atoms,
producing methyl hydroxide or wood spirit. The group CH_{3} is called
methyl, and we may substitute in wood spirit this entire methyl group
for one of the atoms of hydrogen, when we shall have ordinary alcohol,
C_{2}H_{5}OH. This building-up process may be repeated almost _ad
infinitum_, producing a whole series of alcohols of higher and higher
boiling point as the atoms of carbon become more numerous. Again, if
in wood spirit we substitute an atom of oxygen for 2 of the remaining
atoms of hydrogen we obtain formic acid, CHO.OH, the first of a long
series of acids, of which the second, corresponding to ordinary
alcohol, is acetic acid, and the highest members, such as stearic acid,
C_{18}H_{35}O.OH, are solid fats. Hence the whole series are commonly
called the fatty acids. A few structural formulæ will serve to make
these points clearer, but it may be well to say that such formulæ must
be taken simply as indicating the order in which the different atoms
are united, and in no sense their actual position in space. The atoms
in a molecule are held together by attractions and are in continual
motion, so that they are more comparable to the planets of the solar
system than to a rigid shape.
_Methyl Hydride._ _Methyl Alcohol._ _Common Alcohol._
H H H H
│ │ │ │
H──C──H H──C──O──H H──C──C──O──H
│ │ │ │
H H H H
_Chloroform._ _Formic Acid._ _Acetic Acid._
Cl O H O
│ ║ │ ║
H──C──Cl H──C──O──H H──C──C──O──H
│ │
Cl H
In benzene, C_{6}H_{6} we have a compound of another type.
There is reason to think that the carbon atoms in this case
are united in a ring, as shown,
H H H
│ │ │
C──C══C
║ │
C──C══C
│ │ │
H H H
This benzene group forms the foundation of an immense number of bodies
known as the aromatic series, to which belong aniline, carbolic acid,
picric acid, gallic acid, and a host of other compounds important alike
in a scientific and commercial sense, and among which we may pretty
safely group the whole of the tannins. Commencing with benzene, we may,
by inserting atoms of oxygen, produce a series of alcohols or phenols,
of which common phenol (usually but incorrectly called carbolic acid)
is the first.
The following table gives a general view of some of these, so far as
they are known, with their corresponding acids:--
──────────────┬─────────────────┬────────────────────┬───────────────────
C_{6}H_{6} │ C_{6}H_{5}OH │ C_{6}H_{4}(OH)_{2} │ C_{6}H_{3}(OH)_{3}
Benzene. │ Phenol. │ Pyrocatechol │ Pyrogallol,
│ │ (or catechol), │ Phloroglucol.
│ │ Hydroquinol, │
│ │ Resorcinol. │
──────────────┼─────────────────┼────────────────────┼───────────────────
{H_{5} │ {H_{4} │ {H_{3} │ {H_{2}
C_{6}{CO.OH │ C_{6}{OH │ C_{6}{(OH)_{2} │ C_{6}{(OH)_{2}
│ {CO.OH │ {CO.OH │ {CO.OH
Benzoic acid.│ Salycylic acid, │ Protocatechuic acid│ Gallic acid,
│ Oxybenzoic acid.│ (and 5 other │ &c.
│ │ isomeric acids). │
──────────────┴─────────────────┴────────────────────┴───────────────────
It will be noticed that a large proportion of the formulæ given above
represent several compounds identical in composition, but frequently
very distinct in their properties. The explanation of these differences
lies in the different relative position of the OH and CO.OH groups
round the benzene ring. Thus the following diagram represents the
relative positions of the pyrocatechol series. It may be noted that
each phenol yields two isomeric[E] acids. Miller (C. S. Jour., xli.
398), who has investigated these acids, remarks, "Of the 3 phenols
C_{6}H_{4}OH_{2}, catechol alone gives a precipitate with lead acetate,
and of the 6 acids, C_{6}N_{5}OH_{2}, CO.OH, none yields precipitates
with lead acetate, except the 2 which are obtained from catechol."
[Footnote E: _Isomeric_, of similar composition but different structure
and properties.]
Pyrocatechol 1-2. Resorcinol 1-3. Hydroquinol 1-4.
H──C══C──O──H H──C══C──O──H H──C══C──O──H
│ │ │ │ │ │
H──C C──O──H H──C C──O──H H──C C-H
║ ║ ║ ║ ║ ║
H──C──C──H H──C──C──O──H H──O──C──C-H
All the natural tannins with which we are acquainted, are derived
from, and yield on decomposition either catechol, phloroglucol, or
pyrogallol, and sometimes more than one of these. Artificial products,
however, with many of the reactions of tannins have been obtained from
other members of the group, and most phenols and their derived acids
give either purplish or greenish black with ferric salts.
Several classifications of the tannins have been suggested. The
division most obvious to the tanner is into those tannins which yield
the whitish deposit in the surface of the leather, called "bloom," and
those which do not. Stenhouse, some years since, divided tannins into
2 classes, one of which gives a bluish, and the other a greenish-black
with ferric salts. In the main these 2 classes correspond to the 2
former, as most tannins which yield a blue-black with iron acetate
also give bloom to the leather. In some cases, however, the difference
of tint is due to accidental impurities, and even gallotannic acid
will give a decided green with strongly acid ferric chloride. These
classifications both correspond to well-defined differences of
constitution, and it is obviously more scientific to arrange tannins
according to the products which they yield on decomposition, and which
indicate their ultimate structure, rather than on any less essential
point.
If those tannins which give bloom to leather are cautiously heated
to about 392° F. (200° C.), they are decomposed, and a substance
is volatilised which condenses in feathery crystals, and which on
examination turns out to be pyrogallol. Those tannins, on the other
hand, which yield no bloom, but red deposits, produce a somewhat
similar sublimate of catechol. From oak-bark and valonia, which yield
both bloom and red colouring matters, both catechol and pyrogallol
have been obtained. We may, therefore, divide tannins broadly into
derivatives of catechol, which yield no bloom, and usually give
greenish-blacks with iron acetate, and which include hemlock, mimosa,
cutch, gambier, quebracho, &c; derivatives of pyrogallol, which give
bluish-blacks with iron, deposit bloom in leather, and embrace galls,
sumach, divi-divi, myrobalans, pomegranate rind, &c., and tannins which
contain both pyrogallol and catechol, such as oak-bark and valonia, and
which, as is well known, yield bloom, and give blue-blacks with iron.
If tannins are boiled with dilute sulphuric or hydrochloric acids, and
allowed to ferment under the influence of pectose and other natural
ferments, which are always present in vegetable tanning materials, a
different series of decompositions takes place. Many tannins yield
glucose, or starch sugar, as one of their products, or as that of
closely associated impurities. Of this more must be said later. In
addition it will be found that the catechol tannins invariably yield
insoluble reddish-brown bodies which have been called phlobaphenes,
and which differ from the original tannins in containing one or more
molecules less water, and which, in chemical language, are anhydrides
of their respective tannic acids. The pyrogallol tannins, on the other
hand, yield gallic acid, or ellagic acid (the deposit forming bloom)
either alone or in mixture. Oak-bark and valonia give both bloom and
insoluble reds, and by digestion with acids in sealed tubes also gallic
acid.
If the red anhydrides, which are produced from the catechol tannins, be
fused with caustic potash, or in many cases, if they be simply boiled
with concentrated potash solution, they are broken up still further,
and from the fused mass, protocatechuic acid (which bears the same
relation to catechol that gallic acid does to pyrogallol) may always
be obtained. This is in many cases accompanied by phloroglucol, a
phenol isomeric with pyrogallol, as may be seen by the table on p. 61,
but which tastes sweet like a sugar. Cutch, gambier, mimosa, quebracho,
and probably many others, are phloroglucide tannins. The tannins which
do not yield phloroglucol frequently give acetic acid, and other acids
of the "fatty" group, along with protocatechuic acid. We may summarise
this classification in the following table:--
Tannins boiled with dilute sulphuric acid
yield (frequently glucose, and)
/─────────────────────^───────────────────────\
_Insoluble Reds_, which fused with potash
yield _protocatechuic acid_, and,----
/────────────────────────^───────────────────────────────────\
_Phloroglucol_, as chestnut, gambier, kino, cutch, quebracho,
rhatany, fustic, horse-chestnut, tormentil.
_Acetic acid_, coffee, Peruvian bark, male-fern.
_Reds and gallic }
and ellagic acids; } Oak-bark and valonia tannins.
no glucose_ }
_No reds, but gallic { Galls, myrobalans, sumach, divi-divi, pomegranate
and ellagic acids_ { rind.
{ _These are probably mixtures of two tannins
which yield_
_Gallic acid only_ Digallic, or pure gallotannic acid.
_Ellagic acid only_ Pure ellagitannic acid.
This classification is as yet very incomplete, and there are many
tannins of which the decomposition products have not been examined,
while our knowledge of the differences between the tannins which are
classed together is extremely limited. In order to make the information
which has been given practically available for further research, the
characteristics and mode of recognition of the different products
will be given, and as simple a scheme as possible of treatment of the
tannin to be examined will be described; but the recognition of such
products in a state of mixture presents great practical difficulties,
and the tanner will usually be compelled to confine his attention to
simpler, though less conclusive tests, based on the work of chemical
specialists. Such tests will be described later (p. 111).
[Illustration:
Pl. III.
_E & F N. Spon, London & New York_ "MARS PHOTO" SPRAGUE & CO. LONDON
ARTIFICIAL FERMENTATION.]
General Methods of Examination of Tannins.
_Decomposition by Heat._--The ordinary method is to distil the tannin
or dried extract in a small retort, and examine the distillate for
catechol and pyrogallol. Unless the heat be very carefully regulated,
much loss is caused by the destruction of the catechol and pyrogallol
with formation of metagallic acid, &c., and their detection is greatly
complicated by the presence of secondary products. This difficulty is
somewhat lessened by passing a stream of carbon dioxide through the
retort, which carries the products quickly out of the heated portion.
A better method is to heat the tannin in glycerin (Thorpe, Chem. Soc.
Abstr., 1881, 663; Allen, 'Commercial Organic Analysis,' 2nd ed.).
About 1 _grm._ of the sample is heated with 5 _c.c._ of pure glycerin
to 392°-410° F. (200°-210° C.) for 20 minutes. After cooling, about 20
_c.c._ of water is added, and the liquid is shaken with an equal volume
of ether, without previous filtration. The ethereous layer, which
contains the pyrogallol and catechol, is separated from the aqueous
portion, evaporated to dryness, and dissolved in 50 _c.c._ of water.
The filtered solution is divided into several portions and tested with
lime-water, ferric chloride, and ferric acetate (see pp. 66-7); by
these means it is easy to distinguish between catechol and pyrogallol;
and either may be detected in presence of a small portion of the other;
but if in nearly equal quantities, their recognition is difficult.
Catechol may be derived from catechin, &c., and pyrogallol from gallic
acid, and it is therefore necessary in some cases to remove these
bodies from the tannin before treatment. As a general rule, however,
catechins and catechol derivatives are only present in any quantity
with catechol-tannins, and the same is true of gallic acid with regard
to pyrogallol. (For methods of separation see pp. 69, 71, 80). Catechol
has been formed by long continued heating of cellulose, starch, and
other carbohydrates with water under pressure (see p. 67).
_Products of the Decomposition of Tannins by Heat._--Pyrogallol,
_pyrogallic acid_, C_{6}H_{6}O_{3}, has a bitter, but not sour taste,
and feebly reddens litmus, but the addition of the smallest trace of
alkali gives it an alkaline reaction. It is poisonous, 2 gr. having
killed a dog. It is soluble in less than 3 parts of cold water, and
still more freely in hot. It is also soluble in alcohol, ether,
acetone, ethyl acetate, and glycerin, but not in absolute chloroform,
or petroleum spirit. It fuses at 268° F. (131° C.) (_Etti_), and
sublimes at about 410° F. (210° C.).
With pure ferrous sulphate it gives a white precipitate, which
redissolves to a fine blue liquid in presence of the least trace of
ferric salt. Mineral acids change this to red, and the blue tint
is restored by cautious neutralisation with ammonia, and is not
destroyed, but sometimes rendered greenish by excess of acetic, and
other organic acids. Any excess of ammonia produces an amethyst-red,
and acetic acid restores the blue. Its solution is turned brown by
traces of nitrous acid. With lime-water it produces a beautiful but
evanescent purple, rapidly turning brown. In presence of alkalies it
absorbs oxygen from the air with great avidity, turning orange, brown,
and black. Pyrogallol does not precipitate gelatin. Its solution
rapidly reduces permanganate, Fehling's solution, and salts of gold,
silver,[F] mercury, and platinum. It precipitates copper and lead
acetates, and with ammoniacal cupric sulphate it gives an intense
purple-brown coloration. Gum arabic, saliva, and various other organic
matters cause solutions of pyrogallol exposed to the air to absorb
oxygen, by which purpurogallin is formed and separates in small yellow
capillary crystals. If 0·2 per cent. of pyrogallol be added to a 1
per cent. solution of gum arabic, it becomes yellow in a few hours,
and purpurogallin separates in hairlike crystals, which continue to
increase for some months. It these crystals are freed from pyrogallol
by washing with water, and a trace of alkali is added, they dissolve
with an intense blue colour. Purpurogallol is also formed by oxidation
with silver nitrate, potash permanganate, and many other reagents.
Pyrogallol forms compounds with aldehydes; with formaldehyde, a body
which reacts like a tannin and precipitates gelatin is produced.
If pyrogallol be heated with hydrochloric acid, and aldehyde,
chloral, or acetone, a red substance is produced. The less volatile
portions of crude beech-tar creasote contains ethers of pyrogallol,
methyl-pyrogallol, and propyl-pyrogallol, from which these bodies
may be obtained by the action of hydrochloric acid under pressure.
Methyl- and propyl-pyrogallols differ from ordinary pyrogallol in
having an atom of hydrogen replaced by the groups CH_{3} or C_{3}H_{7}
respectively; and it is very probable that some tannins are derivatives
of such modified pyrogallols.
[Footnote F: Hence its use as a "developer" in photography.]
If pyrogallol be heated rapidly to 482° F. (250° C.) it parts with
the elements of water, and is converted into metagallic acid,
C_{6}H_{4}O_{2}, a black amorphous body, insoluble in water, soluble in
alkalies. When pyrogallol is made in the ordinary way by heating gallic
or tannic acids to 410° F. (210° C.), much of this body is formed, even
if the process be conducted in a stream of carbonic acid, and the yield
of pyrogallol usually amounts to only about 5 per cent. of the gallic
acid employed (see p. 65).
Catechol.--_Pyrocatechol_, _pyrocatechin_, _oxyphenic acid_,
C_{6}H_{4}(OH)_{2}.
Sources.--Beside that of the decomposition of certain tannins by heat
(see p. 63), catechol is produced by the dry distillation of catechin
and some allied bodies which frequently accompany the tannins. It is
also formed together with pyrogallol and its homologues (see above) by
the dry distillation of wood, wood tar creasote consisting largely of
ethers of pyrocatechol and its homologues, methyl and pyrocatechol,
&c., and hence it is also found in crude pyroligneous acid. It has also
been produced by heating carbohydrates with water under pressure, and
is found ready formed in Virginia-creeper (_Ampelopsis hæderacea_), and
probably in other plants. It has also been formed synthetically.
Reactions.--Catechol melts at 232° F. (111° C.) and sublimes at about
the same temperature, condensing in brilliant laminæ like benzoic acid.
It is readily soluble in water, alcohol, and ether, and is extracted
from its aqueous solution when shaken with the latter. Its aqueous
solution precipitates lead acetate but not gelatin or alkaloids. With
lime-water or caustic soda solution it becomes reddish, but remains
clear for some time. It does not colour ferrous salts, but gives a
dark green with ferric (avoiding excess); and after some time a black
precipitate. The green is changed to a fine violet-red by alkalies and
hydric sodic carbonate, and restored by acids. To fir-wood moistened
with hydrochloric acid it gives, like phloroglucol, a violet coloration
by combination with the trace of vanillin which this wood contains.
This reaction does not seem to be given by pyrogallol or by common
phenol. Catechol gives a red coloration with citric acid, and after
standing, ceases to react with iron.
_Decomposition of Tannins by Dilute Acids._--It has been stated that
tannins when heated with dilute sulphuric or hydrochloric acids are
decomposed, yielding frequently glucose, and either gallic or ellagic
acids, or red anhydrides. To determine whether glucose is produced, the
tannin must first be carefully purified from glucose, gums, or other
bodies likely to interfere, by the methods mentioned on p. 58. Either
the tannin itself or its washed lead-salt may be used, and must be
heated to 212° F. (100° C.) for some hours in a sealed tube, or tightly
closed bottle with dilute hydrochloric acid. After cooling, the mixture
must be allowed to stand for some time to separate any sparingly
soluble products, which must be filtered off. The filtrate must be
shaken with ether and acetic ether to remove gallic acid (p. 59), the
aqueous solution must be boiled, neutralised with soda, precipitated
with basic lead acetate to remove any traces of tannin or colouring
matters, the liquid again filtered, and excess of lead removed with
dilute sulphuric acid, the mixture again neutralised with soda, and
heated to boiling with Fehling's copper-solution, when a yellow or red
precipitate of cuprous oxide will prove the formation of glucose. The
precipitate produced by cooling may consist (of lead chloride, if the
lead salt has been used,) of ellagic acid, or of red anhydrides or
phlobaphenes of the tannin. The lead chloride may be removed by washing
with boiling water. If the remaining precipitate has a pale yellow or
fawn colour it probably consists of ellagic acid (see p. 71), soluble
in ammonia and hot alcohol and dissolving freely in strong nitric acid,
forming an intense crimson liquid.
The ethereous layer will contain the gallic acid, if any has been
formed, and must be evaporated to dryness, and the residue taken up
with cold water, and filtered. Addition of a few drops of solution of
potassium cyanide will produce a fine red coloration if gallic acid be
present, which rapidly fades, but is restored by shaking. A solution
of picric acid, to which excess of ammonia has been added, gives a
red coloration rapidly changing to a fine green, even in very dilute
solutions of gallic acid.
It is not, however, generally necessary to resort to so elaborate a
process merely to distinguish the class to which tannins belong. The
tannin, or its infusion, may be simply boiled with dilute hydrochloric
acid for some time, replacing the acid lost by evaporation. The
solution is diluted to 50 _c.c._ and allowed to cool. Ellagic acid and
phlobaphenes may separate, and must be filtered off. If the precipitate
is pale, it is probably ellagic acid, and maybe recognised by the
nitric acid test. If red, it probably consists of phlobaphenes, and may
be treated with cold alcohol, in which phlobaphenes are freely soluble,
but ellagic acid very little. The ellagic acid will therefore be left
on the filter if present in any quantity, while the alcoholic solution
may be precipitated by the addition of water, and the phlobaphenes
further examined by treatment with potash.
Gallic Acid.--_Dioxysalicylic acid_, C_{6}H_{2}(OH)_{3}CO.OH, exists
ready formed in some plants, and is a product of the fermentation
of gallotannic acid under the influence of the nitrogenous ferment,
pectase, or of its decomposition by boiling with acids or alkalies.
It crystallises in white, or yellowish white needles, containing 1
mol. (9·5 per cent.) of water, which it loses at 212° F. (100° C.).
It is soluble in 100 parts of cold or 3 of boiling water, in alcohol
or glycerin, and slightly so in ether, by agitation with which it may
however be removed from its aqueous solution. Gallic acid fuses at a
temperature of about 449° F. (232° C.) (Etti, Chem. Soc. Jour., xxxvi.
160), but at about 410° F. (210° C.) begins to lose carbonic dioxide,
and yields a crystalline sublimate of pyrogallol (see p. 66). If the
heat be raised suddenly to 482° F. (250° C.) a considerable quantity of
black shining metagallic acid is formed.
Aqueous solution of gallic acid gives the following
reactions:--Solution of ferric chloride gives a deep blue coloration
which is destroyed by boiling. Ferrous sulphate, if free from ferric
salt, gives no reaction in dilute solutions, but a white precipitate
in strong ones. The mixture rapidly darkens by oxidation. In alkaline
solution gallic acid absorbs oxygen from the air and darkens from the
formation of tannomelanic acid. Lime-water produces a white precipitate
which rapidly becomes blue from oxidation. The same reaction is
produced by baryta-water, or by the chlorides of barium or calcium on
addition of ammonia (distinction from pyrogallol). It is distinguished
from gallotannic acid by the following:--It does not precipitate
gelatin, except in the presence of gum. It does not precipitate tartar
emetic in presence of ammonic chloride, though both tannin and gallic
acid are precipitated by tartar emetic alone. It precipitates lead
acetate but not lead nitrate, while tannin precipitates both. A dilute
solution of potassium cyanide gives a red coloration which disappears
on standing, but is restored by shaking with air. If to even a very
dilute solution of gallic acid, sodic arsenate, or some other faintly
alkaline salts be added, the mixture absorbs oxygen and becomes a deep
green. Aqueous solution of picric acid to which excess of ammonia
has previously been added gives a red coloration, changing to green.
Tannic and pyrogallic acid produce no reaction with cyanide, and
with ammonic picrate a reddish coloration only. Gallic acid reduces
silver nitrate and gold chloride rapidly when hot, but not Fehling's
solution, and decolorises acidified potassic permanganate. If tannin
and other oxidisable bodies be removed from its solution it may be
estimated quantitatively by titration with permanganate in presence
of indigo (see p. 118). It may be separated from tannin by gelatin
or hide raspings (see pp. 121, 124). Gallotannic and quercitannic
acids may also be removed by precipitation with ammoniacal solution
of cupric sulphate, or by cupric acetate, in presence of excess of
ammonic carbonate (see also p. 125). Many other tannins, however,
give precipitates with cupric salts which are soluble in ammonia and
ammonic carbonate. In absence of such tannins it may be estimated
gravimetrically by precipitation with cupric acetate. The precipitate
is rapidly washed with water and digested with a solution of ammonic
carbonate, in which it dissolves; any insoluble cupric tannate is
filtered off, the solution is evaporated to dryness and the residue
moistened with nitric acid and ignited. The weight of the remaining
cupric oxide multiplied by 0·9 gives the weight of the gallic acid plus
a little tannin dissolved by the ammonic solution.
Gallic acid may also be separated from tannin by lead acetate strongly
acidified with acetic acid, by which tannic acid is precipitated, while
lead gallate is dissolved.
Ellagic acid C_{14}H_{8}O_{9}, when pure, is a sulphur-yellow
crystalline body almost insoluble even in boiling water, and only
slightly so in alcohol and ether, though by agitation with the latter,
small quantities may be completely removed from aqueous solution. In
hot alcohol it dissolves with a yellow colour, and crystallises on
cooling. Solid ellagic acid gives with ferric chloride at first a
greenish, and then a black coloration. In strong nitric acid it is
soluble with a deep crimson coloration: that from divi-divi gives a
crimson liquid on dilution with water, but from other sources it is
rather orange.
Ellagic acid may be obtained in considerable quantity by pouring
a concentrated alcoholic extract of divi-divi into water, when it
separates and may be filtered off and recrystallised from hot alcohol.
It may also be obtained by boiling the aqueous extracts of divi,
myrabolans, pomegranate rind, &c., with dilute hydrochloric acid, and
purified by the same means. It may be prepared from gallic acid by
heating the latter with dry arsenic acid to 320° F. (160° C.), but is
difficult to purify from traces of arsenic. Ellagic acid has not been
reconverted into gallic acid. Its constitutional formula is, according
to Schiff,
{CO.OH
{OH
C_{6}H_{2}{
{O────┐
{O } │
} │
{CO} │
{O────┘
C_{6}H_{2}{
{OH
{OH
differing from gallotannic acid only by the loss of two atoms of
hydrogen.
Air-dried ellagic acid, C_{14}H_{8}O_{9} + OH_{2}, contains 1 mol.
of water, which it loses at 212° F. (100° C.) but reabsorbs in moist
air. When heated to 392°-410° F. (200°-210° C.) it forms an anhydride,
C_{14}H_{6}O_{8}, losing another molecule of water, which it does not
recover from moist air, but is slowly reconverted to ellagic acid by
boiling with water.
The phlobaphenes or reds are chemically the anhydrides of the different
tannic acids from which they are derived, or in other words they
are formed from the tannins by the loss of one or more molecules of
water. It is in this way that they are produced by the action of
acids, and similarly they are often formed when alcoholic or highly
concentrated aqueous extracts are poured into cold water, under which
circumstances a part of the tannin seems unable to take up water
again, and separates as a red precipitate. They exist ready formed in
most tanning materials capable of producing them. They are soluble in
alcohol, by which they may be extracted from tanning materials or dried
residues containing them. They are also dissolved by dilute alkalies
and alkaline carbonates, and by borax, which is said to be used in the
preparation of some extracts, and was suggested by Sadlon as a means of
making phlobaphenes available for tanning. Many of them are scarcely
soluble in water even at a boiling temperature, though they become
more so in presence of sugar, tannic acid, and some other substances.
Their solubility in water depends on their degree of hydration, many
tannins giving a series of anhydrides of which those containing only
one molecule of water less than the original tannin are quite soluble
in water, while the higher members of the series become less and less
soluble as they lose water. Those which are soluble form the colouring
matters of tanning materials, and generally are practically tannins,
precipitating gelatin and combining with hide to form leather. Hemlock
bark yields a series of such bodies, of which the lower members are
deep red soluble tannins, while the higher form the red sediment, so
well known to extract-tanners. Thus it is chemically impossible to
decolorise hemlock extract without at the same time greatly lessening
its tanning power, though by careful manufacturing and concentration
at low temperature, the proportion of the higher anhydrides formed may
be kept at a minimum. In many cases it is known, as in gambier, and
in others it is probable that the tannin itself is merely the first
anhydride of the series, and derived from a catechin which itself is a
white crystalline body destitute of tanning properties (see p. 79).
_Decomposition of the Phlobaphenes by Fusion with Caustic
Alkalies._--It has been mentioned (p. 64) that the reds of different
tannins yielded, in addition to protocatechuic acid, either
phloroglucol, or acetic acid, or some other member of the fatty acid
series. Some tannins, as those of alder and hop, give both phloroglucol
and acetic acid, but it is very possible that this arises from the
presence of two distinct tannins in these materials. It is stated that
all those tannins which yield acetic acid on fusion with potash, also
yield considerable quantities of glucose to dilute acids, while the
phloroglucide tannins do not do so. Gallic acid fused with caustic
soda has been found by Barth and Schroeder to produce a small quantity
of phloroglucol, and it is similarly formed by resorcinol and common
phenol (Chem. Soc. Jour., xliv. 60). It is therefore possible that in
some cases where phloroglucol is detected, it may have been formed by
the action of the alkali, and not have been originally a constituent of
the tannin.
The following is the best method in which to proceed to investigate
the products of the action of potash. 20 _grm._ of the red, or of the
tannin from which it is derived, or its lead salt, is boiled with 150
_c.c._ of solution of caustic potash of 1·20 sp. gr. for 3 hours, and
the liquid is then concentrated with constant stirring till it becomes
pasty. It is then cooled and treated with a volume of dilute sulphuric
acid slightly more than enough to neutralise the alkali employed. After
cooling it is filtered from potassium sulphate and other solid matters,
and the filtrate treated with sodium bicarbonate till its wine-red
reaction with litmus shows that the sulphuric acid is neutralised. The
liquid is then shaken with an equal measure of ether, the ethereous
layer drawn off and the treatment repeated several times. On distilling
off the ether, phloroglucol is left and may be purified by solution in
water, when protocatechuic acid and other products may be precipitated
by neutral lead acetate, and filtered off, and the phloroglucol again
extracted with ether, and recognised by its reaction with ferric
chloride and bromine water, and by its sweet taste.
Phloroglucol.--_Phloroglucin_ C_{6}H_{6}O_{3} is a phenol isomeric
with pyrogallol. It crystallises with 2 molecules of water, which
it loses at 212° F. (100° C.). It melts at about 428° F. (220° C.),
sublimes without odour, and solidifies again on cooling. It is soluble
in water, alcohol, and ether, and by agitation with the latter it
may be removed from its aqueous solution. It is not precipitated by
any metallic salt but basic lead acetate. It is coloured deep violet
red by ferric chloride. If bromine be added to its concentrated
solution in water, it absorbs 3 atoms, forming tribromophloroglucol,
C_{6}H_{6}Br_{3}O_{3}, which separates in crystalline needles, with
evolution of heat and a very irritating odour. If a deal shaving
be moistened with solution of phloroglucol, and then with strong
hydrochloric acid, it soon takes a deep violet colour, from the
formation of phloroglucol-vanillin with the trace of vanillin contained
in all coniferous woods. Pyrocatechol gives a similar reaction, and it
is stated by Etti (Chem. Soc. Jour., xliv. 60) that pyrogallol forms
a similar compound; it does not, however, give the colour reaction on
deal. It is extremely probable that this reaction may be used to detect
phloroglucide tannins without the troublesome fusion with potash, in
cases where pyrocatechol is absent. The reaction is strongly given
by gambier, which is known to contain phloroglucol, but not by oak
bark and valonia, though these contain protocatechuic acid. If to a
dilute solution of phloroglucol a solution of aniline or toluidine
nitrate be added, and then a trace of potassic nitrite, the liquid
gradually becomes yellow or orange, then turbid, and finally deposits
a cinnabar-red precipitate. This reaction is given by gambier, but is
also produced by oak bark infusion, which is not supposed to contain
phloroglucol; and gall tannin, pyrogallol, and other substances give
similar but browner precipitates.
Protocatechuic Acid.--C_{6}H_{3}(OH)_{2}CO.OH, one of the six isomeric
dihydroxybenzoic acids of this formula (see Miller, Chem. Soc. Jour.,
xli. 198), crystallises in needles and plates with 1 mol. water, which
it loses at 212° F. (100° C.). It melts at 228° F. (109° C.) and on
further heating is decomposed into pyrocatechol and carbonic acid.
It is somewhat soluble in cold water, and readily so in hot water,
alcohol, and ether. It is coloured bluish green by ferric chloride,
which is changed to red by alkalies. Solutions of protocatechuates
give a violet coloration with ferric salts. It is precipitated by lead
acetate, and reduces silver ammonio-nitrate, but not Fehling's solution
(see also p. 107).
Constitution of Tannins.
Having described the products of decomposition, something must be
said of the way in which these constituents are combined to form the
unaltered tannins. The only tannin of which we have as yet anything
approaching complete knowledge is that obtained from galls, sumach, and
myrabolans, and which is thence called gallotannic acid.
_Gallotannic, or digallic acid_ exists as the principal tannic acid
of the galls of oak, tamarisk, &c.; and, in mixture with more or less
ellagitannic acid, in myrabolans, divi-divi, sumach, pomegranate rind,
and many other plant-products. It has also been formed by Schiff from
gallic acid, by mixing it, after drying at 230° F. (110° C.), with
phosphorus oxychloride to a thin paste, and heating, first to 212° F.
(100° C.) and then to 248° F. (120° C ). Much hydrochloric acid was
evolved, and the gallic acid was converted into a yellow powder, which
after purification by washing with ether, dissolving in water, allowing
the gallic acid to crystallise out, saturating with salt, washing the
precipitated tannin in salt solution, and redissolving in alcohol and
ether, had all the reactions of purified gall tannin, but was perfectly
reconverted into gallic acid on boiling with hydrochloric acid, without
the formation of any trace of ellagic acid, or glucose. By analysis of
the tannin and its acetyl compounds it was shown to be digallic acid,
and its constitutional formula is almost certainly as follows:--
{CO.OH
{OH
C_{6}H_{2}{
{OH
{O }
}
{CO}
{OH
C_{6}H_{2}{OH
{OH
By boiling gallic acid with solution of arsenic acid, Schiff obtained a
product which precipitated gelatin, and otherwise reacted like tannin,
and he regards this as digallic acid, but other experimenters have
failed to obtain digallic acid by this means, and have found that on
complete removal of arsenic, the compound was reconverted to gallic
acid. It therefore remains a moot point whether digallic acid is really
formed, and reconverted into gallic acid by the prolonged action of
hydric sulphide, which is necessary to remove the arsenic, or whether,
as seems more probable, the supposed tannin is merely an arsenical
compound of gallic acid.
Gallotannic acid as obtained from plants invariably yields traces of
glucose, as well as of ellagic acid, when boiled with dilute acids. It
is still an open question whether the glucose exists in the plant as a
glucoside of tannic acid or is always the product of some impurity (as
is shown by Etti to be the case with oak bark, where lævulin is always
present). It seems most probable however that natural gallotannic acid
is really a glucoside of digallic acid, or possibly, according to the
theory of Hlasiwetz, a gummide, or compound of dextrin, which, by the
action of acids, is easily converted into glucose. What has been said
of gallotannic acid in this respect, applies to many other tannins,
which like it give glucose by treatment with acids.
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_, C_{14}H_{10}O_{10}, 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),
C_{14}H_{8}O_{9}. 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 C_{17}H_{16}O_{9} (Etti, Chem.
Soc. Jour., xliv. 994), C_{28}H_{26}O_{15} (Ibid. xl. 901, Löwe),
C_{19}H_{16}O_{10} (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.
Catechin is a white crystalline substance, contained to the extent
of some 30 per cent. in cube gambier, and in smaller proportion in
block gambier and cutch, and very probably in all tanning materials
yielding catechol-tannins. It melts at 422-1/2° F. (217° C.) and
yields a sublimate of catechol on further heating. It is readily
soluble in alcohol and in boiling water, but requires 1133 parts of
cold water for its solution. Hence it separates on cooling from a hot
solution of gambier, and may be purified by redissolving in hot water
and treatment with animal charcoal, and subsequent crystallisation.
It may also be extracted from its aqueous solution by agitation with
ether. It possesses no acid properties, though some writers have
incorrectly called it catechuic acid. The aqueous solution gives
precipitates with lead acetate and mercuric chloride, and reduces
ammonia-nitrate of silver; but, unlike tannins, it does not precipitate
gelatin, alkaloids, or tartar emetic. It is oxidised by permanganate
in presence of free acid, and hence, when in solution, is estimated
by the Löwenthal method as "not tannin." It dissolves in concentrated
sulphuric acid with a deep purple coloration. By fusion with caustic
alkalies it yields protocatechuic acid and phloroglucol together with
hydrogen. Its constitution is very uncertain, but it is probable
that the phloroglucol stands in a somewhat similar relation to the
protocatechuic acid that glycerin does to the fatty acids in natural
fats; and that its decomposition by fused alkalies is a process much
akin to saponification. This constitution is represented by the
following formula:--
{O.C_{6}H_{3}(OH)_{2}
C_{19}H_{18}O_{8} or C_{7}H_{8}O_{2}{
{O.C_{6}H_{3}(OH)_{2}
When acted on by heat and dilute acids the following anhydrides are
produced--the formulæ given must be taken as to some extent provisional.
Catechin 2C_{19}H_{18}O_{8} = C_{38}H_{36}O_{16} Not acid; does not
precipitate gelatin.
Catechutannic acid C_{38}H_{34}O_{15}}
} Acid, precipitate
Dianydride C_{38}H_{32}O_{14}} gelatin.
Trianydride C_{38}H_{30}O_{13}} Insoluble in water.
Catechuretin C_{38}H_{30}O_{12}} Soluble in alcohol and
alkalies.
The white deposit which occurs on pit sides and in the interior of
leather where gambier is largely used, and which is sometimes called
"the whites," consists of catechin. It may be decomposed by warm
sulphuric acid. This deposit is favoured by the use of _hot_ gambier
liquors. It is probable that by exposure to the air and by boiling,
catechin is gradually converted into catechutannic acid during the
tanning process, and hence the practical tanning value of cube gambier
is probably greater than analysis indicates. Hunt has, however, shown
(Jour. Soc. Chem. Ind., iv. 266) that, as estimated by the Löwenthal
process, the tannin is to some extent lessened by boiling.
_Kinoin_, C_{14}H_{12}O_{6} (Etti, Berl. Ber., xi. 1879), obtained from
green or malabar kino, a product very similar to cutch, by boiling
with dilute hydrochloric acid and extraction by agitation with ether,
is very similar in its properties to catechin. It does not itself
precipitate gelatin, but like catechin, yields a series of anhydrides
or reds, which do so. On dry distillation it yields catechol and
common phenol; and when heated with hydrochloric acid at 248°-266° F.
(120°-130° C.) methyl chloride, catechol and gallic acid. Hence its
constitution is probably that of methyl-catechol gallate.
_Quebracho-catechin_ was found by P. N. Arata (Chem. Soc. Jour., xl.
1152) in the wood of quebracho colorado (p. 40), but in too small
quantity for detailed investigation. It probably bears a similar
relation to quebrachitannic acid that ordinary catechin does to
catechutannic acid. It is insoluble in cold and only slightly soluble
in hot water, but very soluble in alcohol and ether. Its solution is
clouded by normal lead acetate, and gives rose-coloured precipitates
with basic lead acetate and mercurous nitrate, and blackish with ferric
acetate; it reduces silver-nitrate and gold chloride, and is coloured
yellow by nitric acid, red by sulphuric acid, yellowish by sodium
hypochlorite, and green by Fehling's solution. It does not precipitate
gelatin, or alkaloids.
_Catechutannic_ acid has been pretty fully described under catechin,
of which it is the first anhydride (p. 80). It is possibly identical
with mimo-tannic acid, the tannin of cutch and mimosa bark, which is
chemically very similar, but greatly differs in its practical effect
in tanning. For some further reactions of cutch and gambier infusions
see p. 113. It gives a greyish-green precipitate with ferric salts, and
(distinction from gallotannic acid) precipitates cupric sulphate but
not tartar emetic.
_Quebrachitannic_ acid is got from the wood of the quebracho colorado,
_Quebrachia lorentzii_ (formerly _Loxopterygium_) which must not be
confounded with the bark of the _Aspidospermum quebrachia_, which is
valuable, not for its tannin, but for an alkaloid, aspidospermin,
which is used for medical purposes. It has been pretty thoroughly
investigated by P. N. Arata (Chem. Soc. Jour., xxxiv. 986 and xl.
1152). It seems, however, a little doubtful to the writer whether the
substance investigated by Arata was not the anhydride of the tannin,
rather than the tannin itself, as it presents many points of analogy to
catechu-red and was less soluble in water than quebracho tannin appears
in practice to be.
According to Arata, quebrachitannic acid is a pale red amorphous mass,
having an astringent taste and yielding a light cinnamon coloured
powder. It is insoluble in carbon-bisulphide, turpentine oil, and
benzene. Its aqueous solution gives a white precipitate with both
normal and basic lead acetate, which when heated, acquires first a
rose, and then a chocolate colour; with ferric chloride a green liquid
is produced, which changes after a time to red and becomes black on
addition of sodium acetate. It forms white precipitates with gelatin,
albumen, and alkaloids. By dry distillation it yields catechol. By
fusion with potash or the action of sulphuric acid, phloroglucol and
protocatechuic acid, while nitric acid converts it into oxalic and
picric acids. While it shows great similarity in its reactions to
catechutannic acid, it differs materially in percentage composition,
containing only 52·5 as compared with 62·0 per cent. of carbon.