=588. Occurrence and Composition.=—The tannins and allied bodies,
which are of importance in this connection, are those which occur in
food products and beverages and also those made use of in the leather
industry. The term tannin is applied to a large class of astringent
substances, many of which are glucosids. Tannic acid is the chief
constituent of the tannins, and is found in a state of comparative
purity in nutgalls. The source from which the tannic acid is derived is
indicated by a prefix to the name, _e. g._, gallotannic, from nutgalls,
and caffetannic, from coffee etc. The tannins have lately been the
theme of a critical study by Trimble, and the reader is referred to
his work for an exhaustive study of the subject.[606] Tannin is one of
the most widely diffused compounds, occurring in hundreds of plants.
Commercially, the oaks and hemlocks are the most important plants
containing tannin. The sumach, mangrove, canaigre, palmetto and many
others have also been utilized as commercial sources of tannin. The
tannins as a class are amorphous and odorless. They are slightly acid
and strongly astringent. Their colors vary from dark brown to pure
white. They are soluble in water, alcohol, ether and glycerol and
insoluble in chloroform, benzol, petroleum ether, carbon bisulfid and
the oils. The tannins give blue or green precipitates with iron salts
and most of them brown precipitates with potassium bichromate. They are
all precipitated by gelatin or albumin. Tannins are not only generally
of a glucosidal nature, but are found quite constantly associated with
reducing sugars, or in unstable combination therewith.
The reducing sugars may be separated from the tannin by precipitating
the latter with lead acetate and determining the glucose in the
filtrate after the removal of the lead. A separate portion of the
tannin is hydrolyzed with sulfuric or hydrochloric acid and the
reducing sugars again determined. Any excess of sugars over the first
determination is due to the hydrolysis of the tannin glucosid.
=589. Detection and Estimation of Tannins.=—The qualitive reactions
above mentioned serve to detect the presence of a tannin. Of the iron
salts ferric acetate or chlorid is preferred. Ferrous salts do not
give any reaction with dilute tannin solutions. An ammoniacal solution
of potassium ferricyanid forms with tannins a deep red color changing
to brown. In quantitive work the tannins are mostly determined by
precipitation with metallic salts, by treatment with gelatin or hide
powder, or by oxidation with potassium permanganate. Directions for the
estimation of glucosids in general are found in Dragendorff’s book.[607]
=590. Precipitation with Metallic Salts.=—The methods depending on
precipitation of the tannins with metallic salts are but little used
and only one of them will be mentioned here. A full description of
the others is contained in Trimble’s book.[608] A method for the
determination of caffetannic acid in coffee has been worked out by Krug
and used with some satisfaction.[609]
In this method two grams of the coffee meal are digested for thirty-six
hours with ten cubic centimeters of water, a little more than twice
that volume of ninety-five per cent alcohol added and the digestion
continued for a day. The contents of the flask are poured on a filter
and the residue washed with alcohol. The filtrate contains tannin,
caffetannic acid and traces of coloring matter and fat. It is heated
to the boiling point and clarified with a solution of lead acetate.
A caffetannate of lead containing forty-nine per cent of the metal
is precipitated. As soon as the precipitate has become flocculent it
is collected on a filter, washed with ninety per cent alcohol until
the soluble lead salts are all removed, then with ether and dried.
The composition of the precipitate is represented by the formula
Pb₃(C₁₅H₁₅O₈)₂. The caffetannic acid is calculated by the equation:
Weight of precipitate: weight of caffetannic acid = 1267: 652.
=591. The Gelatin Method.=—The precipitation of tannin with gelatin is
the basis of a process for its quantitive estimation which, according
to Trimble, is conducted as follows:[610] Two and a half grams of
gelatin and ten grams of alum are dissolved in water and the volume of
the solution made up to one liter. The solution of gelatin and also
that of the tannin are heated to 70° and the tannin is precipitated
by adding the gelatin reagent slowly, with constant stirring, until
the precipitate coagulates, and, on settling, leaves a clear liquor in
which no further precipitate is produced on adding a few drops more
of the reagent. In case the clearing of the mixture do not take place
readily, the process should be repeated with a more dilute tannin
solution. The precipitate is collected on two counterpoised filter
papers one placed inside the other, dried at 110° and weighed, the
empty filter paper being placed on the pan with the weights. For pure
tannin (gallotannic acid) fifty-four per cent of the weight of the
precipitate are tannin. Ammonium chlorid and common salt have been used
in place of the alum in preparing the reagent, but if the proportion of
alum mentioned above be used, satisfactory results will be obtained in
most cases.
=592. The Hide Powder Method.=—The principle of this method is based on
the change in specific gravity, _i. e._, total solids, which a tannin
solution will undergo when brought into contact with raw hides in a
state of fine subdivision. The hide powder absorbs the tannin, and the
total solid content of the solution is correspondingly diminished. The
method is conducted according to the official directions as follows:[611]
_Preparation of the Sample._—The bark, wood, leaves or other materials
holding the tannins, are dried and ground to a fine powder and
thoroughly extracted with water as mentioned below. In each case the
solution or extraction is made as thorough as possible and the volume
of the extract is made up to a definite amount.
_Quantity of Tanning Material._—Use such an amount of the tanning
material as shall give in 100 cubic centimeters of the filtered
solution about one gram of dry solids. In the case of barks, woods,
leaves and similar materials, transfer to a half liter flask, fill
the flask with water at approximately 80° and let stand over night in
a bath which is kept at 80°, cool, fill to the mark, shake well and
filter. In the case of extracts and sweet liquors, wash the proper
quantity into a half liter flask with water at approximately 80°,
almost filling the flask, cool and fill to the mark.
_Determination of Moisture._—Dry five grams of the sample in a flat
bottom dish at the temperature of boiling water until the weight
becomes constant.
_Determination of Total Solids._—Shake the solution, which should
be at a temperature of about 20°, and immediately remove 100 cubic
centimeters with a pipette, evaporate in a weighed dish and dry to
constant weight at the temperature of boiling water.
_Determination of Soluble Solids._—Filter a portion of the solution
through a folded filter, returning the filtrate to the filter twice
and adding a teaspoonful of kaolin, if necessary. Evaporate 100 cubic
centimeters of the filtrate and dry as above.
_Determination of Tanning Substances._—Extract twenty grams of hide
powder by shaking for five minutes with 250 cubic centimeters of water,
filter through well washed muslin or linen, repeat the operation three
times and dry as much as possible in a suitable press. Weigh the wet
powder and determine the residual moisture in about one-fourth of the
whole by drying to constant weight at 100°. Shake 200 cubic centimeters
of the unfiltered solution of the tannin with the rest of the moist
hide powder for about five minutes, add five grams of barium sulfate,
shake for one minute and filter through a schleicher and schüll folded
filter, No. 590, fifteen centimeters in diameter, returning the first
twenty-five cubic centimeters of the filtrate. Evaporate 100 cubic
centimeters of the clear filtrate and dry the residue to constant
weight at a temperature of boiling water. The difference between the
soluble solids obtained in the filtered tannin solution and the residue
as obtained above is the amount of tanning material absorbed by the
hide powder. This weight must be corrected for the water retained by
the hide powder. The shaking must be conducted by means of a mechanical
shaker, in order to remove all the tannin substance from the solution.
The simple machine used by druggists, and known as the milkshake, is
recommended.
_Testing the Hide Powder._—Shake ten grams of the hide powder with 200
cubic centimeters of water for five minutes, filter through muslin
or linen, squeeze out thoroughly by hand, replace the residue in the
flask and repeat the operation twice with the same quantity of water.
Pass the last filtrate through paper until a perfectly clear liquid is
obtained. Evaporate 100 cubic centimeters of the final filtrate in a
weighed dish, dry at 100° until the weight is constant. If the residue
amount to more than ten milligrams the sample should be rejected. The
hide powder must be kept in a dry place and tested once a month.
Prepare a solution of pure gallotannic acid by dissolving five grams in
one liter of water. Determine the total solids by evaporating 100 cubic
centimeters of this solution and drying to constant weight. Treat 200
cubic centimeters of the solution with hide powder exactly as described
above. The hide powder must absorb at least ninety-five per cent of
the total solids present. The gallotannic acid used must be completely
soluble in water, alcohol, acetone and acetic ether and should contain
not more than one per cent of substances not removed by digesting with
excess of yellow mercuric oxid on the steam bath for two hours.
_Testing the Non-Tannin Filtrate. For Tannin_:—Test a small portion
of the clear non-tannin filtrate with a few drops of a ten per cent
solution of gelatin. A cloudiness indicates the presence of tannin, in
which case the determination must be repeated, using twenty-five grams
of hide powder instead of twenty grams.
_For Soluble Hide_:—To a small portion of the clear non-tannin
filtrate, add a few drops of the original solution, previously filtered
to remove reds. A cloudiness indicates the presence of soluble hide
due to incomplete washing of the hide powder. In this case, repeat the
determination with perfectly washed hide powder.
=593. The Permanganate Gelatin Method.=—This process, which is
essentially the method of Löwenthal, as improved by Councler, Schroeder
and Proctor and as used by Spencer for the determination of tannin in
teas, is conducted as described below.[612] The principle of the process
is based on the oxidation of all bodies in solution oxidizable by
potassium permanganate, the subsequent precipitation of the tannin by
a gelatin solution, and the final oxidation, by means of permanganate,
of the remaining organic bodies. The difference between the total
oxidizable matter and that left after the precipitation of the tannin
represents the tannin originally in solution.
_Reagents Required._—The following reagents are necessary to the proper
conduct of the potassium permanganate process:
(1). Potassium permanganate solution containing about one and a third
grams of the salt in a liter:
The potassium permanganate solution is set by titration against the
decinormal oxalic acid solution mentioned below. The end reaction with
the indicator must be of the same tint in all the titrations, _i. e._,
either golden yellow or pink.
(2). Tenth-normal oxalic acid solution for determining the exact titer
of the permanganate solution:
(3). Indigo-carmin solution to be used as an indicator and containing
six grams of indigo-carmin and fifty cubic centimeters of sulfuric
acid in a liter. The indigo-carmin must be very pure and quite free of
indigo-blue.
(4). Gelatin solution, prepared by digesting twenty-five grams of
gelatin at room temperature for one hour in a saturated solution of
sodium chlorid, then heating until solution is complete, cooling and
making the volume up to one liter:
(5). A salt acid solution, made by adding to 975 cubic centimeters of
a saturated solution of sodium chlorid, enough strong sulfuric acid to
bring the volume of the mixture to one liter:
(6). Powdered kaolin for promoting filtration.
_The Process._—Five grams of the finely powdered tea (or other
vegetable substance containing tannin) are boiled with distilled water
in a flask of half a liter capacity for half an hour. The distilled
water should be at room temperature when poured over the powdered tea.
After cooling, the volume of the decoction is completed to half a
liter, and the contents of the flask poured on a filter. To ten cubic
centimeters of the filtered tea infusion are added two and a half times
as much of the indigo-carmin solution and about three-quarters of a
liter of distilled water.
The permanganate solution is run in from a burette, a little at a time,
with vigorous stirring, until the color changes to a light green, and
then drop by drop until the final color selected for the end of the
reaction, golden yellow or faint pink, is obtained. The number of cubic
centimeters of permanganate required is noted and represented by a in
the formula below. The titration should be made in triplicate and the
mean of the two more nearly agreeing readings taken as the correct one.
One hundred cubic centimeters of the filtered tea infusion, obtained
as directed above, are mixed with half that quantity of the gelatin
reagent, the first named quantity of the acid salt solution added,
together with ten grams of the powdered kaolin, the mixture well
shaken for several minutes and poured on a filter. Twenty-five cubic
centimeters of the filtrate, corresponding to ten of the original
tea solution are titrated with the permanganate reagent, under the
conditions given above, and the reading of the burette made and
represented by _b_. The quantity of permanganate solution, _viz._, _c_,
required to oxidize the tannin is calculated from the formula _a - b_ =
_c_. The relation between the permanganate, oxalic acid and tannin is
such that 0.04157 gram of gallotannic acid is equivalent to 0.063 gram
of oxalic acid. The relation between the oxalic acid solution and the
permanganate having been previously determined the data for calculating
the quantity of tannin, estimated as gallotannic acid, are at hand.
=594. The Permanganate Hide Powder Method.=—Instead of throwing out the
tannin with gelatin it may be absorbed by hide powder. The principle of
the process, save this modification, is the same as in the method just
described. As described by Trimble, the analysis is conducted according
to the following directions:[613]
_Reagents Required._—The reagents required for conducting the
permanganate hide powder process are as follows:
1. _Permanganate Solution._—Ten grams of pure potassium permanganate
are dissolved in six liters of water. The solution is standardized with
pure tannin. The moisture in the pure tannin is determined by drying at
100° to constant weight and then a quantity of the undried substance,
representing two grams of the dried material, is dissolved in one
liter of water. Ten cubic centimeters of this solution and double
that quantity of the indigo solution to be described below, are mixed
with three-quarters of a liter of water and the permanganate solution
added from a burette with constant stirring until the liquid assumes
a greenish color and then, drop by drop, until a pure yellow color
with a pinkish rim is obtained. Fifty cubic centimeters of the pure
tannin solution are digested, with frequent shaking, with three grams
of hide powder which has been previously well moistened and dried in a
press for eighteen or twenty hours, the contents of the flask thrown
on a filter and ten cubic centimeters of the filtrate titrated with
the permanganate solution as directed above. The difference between
the amount of permanganate solution required for the first and second
titrations represents the amount of pure tannin or oxidizable matter
removed by the hide powder.
2. _Indigo Solution._—The indicator which is used in the titrations
is prepared by dissolving thirty grams of sodium sulfindigotate in
three liters of dilute sulfuric acid made by adding one volume of the
strong acid to three volumes of water. The solution is shaken for a few
minutes, thrown upon a filter and the insoluble residue washed with
sufficient water to make the volume of the filtrate six liters.
3. _Hide Powder._—The hide powder used should be white, wooly in
character and sufficiently well extracted with water to afford no other
extract capable of oxidizing the permanganate solution.
_The Process._—The reagents having been prepared and tested as above,
the solution of the substance containing the tannin, prepared as
described further on, is titrated first with the permanganate solution
in the manner already given. Fifty cubic centimeters of the tannin
solution are then shaken, from, time to time for eighteen hours,
with three grams of hide powder, thrown upon a filter and ten cubic
centimeters of the filtrate titrated with the potassium permanganate as
above described. From the data obtained, the quantity of permanganate
solution corresponding to the tannin removed by the hide powder is
easily calculated. The value of the permanganate solution having been
previously set with a pure tannin, renders easy of calculation the
corresponding amount of pure tannin in the solution under examination.
=595. Preparation of the Tannin Infusion.=—A sample weighing about a
kilogram should be secured, representing as nearly as possible the
whole of the materials containing tannin in a given lot. The sample
is reduced to a fine powder and passed through a sieve containing
apertures about a millimeter in diameter. The quantity of the sample
used for the extraction depends largely upon its content of tannin.
Five grams of nutgalls, ten grams of sumach or twenty grams of oak
bark represent about the quantities necessary for these classes of
tannin-holding materials. The sample is boiled for half an hour with
half a liter of water, filtered through a linen bag into a liter flask
and washed and pressed with enough water to make the volume of the
filtrate equal to one liter. The proper quantities of this solution are
used for the analytical processes described above.