=579. Special Analysis.=—Aside from the examination of teas and coffees
for adulterants, the only special determinations which are required
in analyses are the estimation of the alkaloid (caffein) and of the
tannin contained therein. It is chiefly to the alkaloid that the
stimulating effects of the beverages made from tea and coffee are due.
The determination of the quantity of tannin contained in tea and coffee
is accomplished by the processes described under the chapter devoted to
that glucosid.
The general analysis, _viz._, the estimation of water, ether extract,
total nitrogen, fiber, carbohydrates and ash, with the exceptions noted
above, is conducted by the methods which have already been given.
For detailed instructions concerning the detection of adulterants of
tea and coffee the bulletins of the Chemical Division, Department of
Agriculture, may be consulted.[593]
=580. Estimation of Caffein= (=Thein=).—The method adopted by Spencer,
after a thorough trial of all the usual processes for estimating this
alkaloid, is as follows:[594] To three grams of the finely powdered tea
or coffee, in a 300 cubic centimeter flask, add about a quarter of a
liter of water, slowly heat to the boiling point, using a fragment of
tallow to prevent frothing, and boil gently for half an hour. When
boiling begins, the flask should be nearly filled with hot water
and more added from time to time to compensate for the loss due to
evaporation. After cooling, add a strong solution of basic lead acetate
until no further precipitation is produced, complete the volume to the
mark with water, mix and throw on a filter. Precipitate the lead from
the filtrate by hydrogen sulfid and filter. Boil a measured volume of
this filtrate to expel the excess of hydrogen sulfid, cool and add
sufficient water to compensate for the evaporation. Transfer fifty
cubic centimeters of this solution to a separatory funnel and shake
seven times with chloroform. Collect the chloroform solution in a tared
flask and remove the solvent by gentle distillation. A safety bulb,
such as is used in the kjeldahl nitrogen method, should be employed to
prevent entrainment of caffein with the chloroform vapors.
The extraction with chloroform is nearly complete after shaking out
four times; a delicate test, however, will usually reveal the presence
of caffein in the watery residue even after five or six extractions,
hence seven extractions are recommended for precautionary reasons. The
residual caffein is dried at 75° for two hours and weighed.
The principal objection which has been made to Spencer’s method is
that the boiling with water is not continued for a sufficient length
of time. For the water extraction, Allen prescribes at least six hours
cohobation.[595] In this method six grams of the powdered substance
are boiled with half a liter of water for six hours in a flask, with
a condenser, the decoction filtered, the volume of the filtrate
completed to 600 cubic centimeters with the wash water, heated to
boiling, and four cubic centimeters of strong lead acetate solution
added, the mixture boiled for ten minutes, filtered and half a liter
of the filtrate evaporated to fifty cubic centimeters. The excess of
lead is removed with sodium phosphate and the filtrate and washings
concentrated to about forty cubic centimeters. The caffein is removed
by shaking four times with chloroform. Older but less desirable
processes are fully described by Allen.[596]
In France this method is known as the process of Petit and Legrip, and
it has been worked out in great detail by Grandval and Lajoux and by
Petit and Terbat.[597]
=581. Estimation of Caffein by Precipitation with Iodin.=—The caffein
in this method is extracted, the extract clarified by lead acetate and
the excess of lead removed as in Spencer’s process described above. The
caffein is determined in the acidified aqueous solution thus prepared,
according to the plan proposed by Gomberg, as follows:[598]
Definite volumes of the aqueous solution of the caffein are acidulated
with sulfuric and the alkaloid precipitated by an excess of a set
solution of iodin in potassium iodid. After filtering, the excess of
iodin in an aliquot part of the filtrate is determined by titration
with a tenth normal solution of sodium thiosulfate. The filtration of
the iodin liquor is accomplished on glass wool or asbestos. The results
of the analyses are calculated from the composition of the precipitated
caffein periodid; _viz._, C₈H₁₀N₄O₂.HI.I₄. The weight of the alkaloid
is calculated from the amount of iodin required for the precipitation
by the equation 4I: C₈H₁₀N₄O₂ = 508: 194. From this equation it is
shown that one part of iodin is equivalent to 0.3819 part of caffein,
or one cubic centimeter of tenth normal iodin solution is equal to
0.00485 gram of iodin.
In practice, it is recommended to divide the aqueous extract of the
alkaloid, prepared as directed above, into two portions, one of which
is treated with the iodin reagent without further preparation, and
the other after acidulation with sulfuric. After ten minutes, the
residual iodin is estimated in each of the solutions as indicated
above. The one portion, containing only the acetic acid resulting from
the decomposition of the lead acetate, serves to indicate whether
the aqueous solution of the caffein contains other bodies than that
alkaloid capable of forming a precipitate with the reagent, since the
caffein itself is not precipitated even in presence of strong acetic
acid.
In the solution acidulated with sulfuric, the caffein, together with
the other bodies capable of combining with iodin, is precipitated. The
residual iodin is determined in each case, and thus the quantity which
is united with the caffein is easily ascertained. The weight of iodin
which has entered into the precipitated caffein periodid multiplied by
0.3819 gives the weight of the caffein in the solution.
Gomberg’s method has been subjected to a careful comparative study by
Spencer and has been much improved by him in important particulars.[599]
It is especially necessary to secure the complete expulsion of
the hydrogen sulfid and to observe certain precautions in the
addition of the iodin reagent. The precipitation should be made in a
glass-stoppered flask, shaking thoroughly after the addition of the
iodin and collecting the precipitate on a gooch. As thus modified, the
iodin process gives results comparable with those obtained by Spencer’s
method, and it can also be used to advantage in estimating caffein in
headache tablets in the presence of acetanilid.
=582. Freeing Caffein of Chlorophyll.=—Any chlorophyll which may pass
into solution and be found in the caffein may be removed by dissolving
the caffein in ten per cent sulfuric acid, filtering, neutralizing
with ammonia and evaporating to dryness. The residue is taken up with
chloroform, the chloroform removed at a low temperature and the pure
caffein thus obtained.[600]
=583. Proteid Nitrogen.=—The proteid nitrogen in tea and coffee may be
determined in the residue after extraction of the alkaloid by boiling
water as described above. More easily it is secured by determining
the total nitrogen in the sample and deducting therefrom the nitrogen
present as caffein. The remainder, multiplied by 6.25, will give the
quantity of proteid matter.
=584. Carbohydrates of the Coffee Bean.=—The carbohydrates of the
coffee bean include those common to vegetable substances; _viz._,
cellulose, pentosan bodies (xylan, araban), fiber etc., together
with certain sugars, of which sucrose is pointed out by Ewell as the
chief.[601] In smaller quantities are found a galactose yielding body
(galactan), as pointed out by Maxwell, a dextrinoid and a trace of a
sugar reducing alkaline copper solution.
The sucrose may be separated from the coffee bean by the following
process:[602] The finely ground flour is extracted with seventy per
cent alcohol, the extract clarified with lead acetate, filtered, the
lead removed from the filtrate with hydrogen sulfid, the excess of the
gas removed by boiling, the filtrate evaporated in a partial vacuum to
a sirup and the sucrose crystallized from a solution of the sirup in
alcohol.
For a quantitive determination, ten grams of the coffee flour are
extracted with ether and the residue with seventy-five per cent
alcohol. This process, conducted in a continuous extraction apparatus,
should be continued for at least twenty-four hours. The alcohol is
removed by evaporation, the residue dissolved in water, clarified with
basic lead acetate, filtered, the precipitate washed, the lead removed,
again filtered, the filtrate washed and wash water and filtrate
made to a definite volume. In an aliquot part of this solution the
sugars are determined by the alkaline copper method, both before and
after inversion. From the data obtained the percentage of sucrose is
calculated.
In a coffee examined by Ewell the percentage of sucrose was found to be
6.34. The pentose yielding constituents of the coffee bean amount to
from eight to ten per cent.
When coffee meal is extracted with a five per cent solution of sodium
carbonate, a gummy substance is obtained, which is precipitable by
alcohol. This gum, after washing with hydrochloric acid containing
alcohol, gives a gray, translucent, hard mass on drying. On hydrolysis
it yielded 75.2 per cent of dextrose, on distillation with hydrochloric
acid, thirteen per cent of furfuraldehyd and, on oxidation with nitric
acid, 18.7 per cent of mucic acid. This gum, therefore, consists
chiefly of a mixture of galactan, xylan and araban.
=585. Estimation of Galactan.=—From three to five grams of the
substance supposed to contain galactan are placed in a beaker with
sixty cubic centimeters of nitric acid of 1.15 specific gravity. The
mixture is evaporated on a steam bath until it is reduced to one-third
of its original volume, allowed to stand for twenty-four hours, ten
cubic centimeters of water added, well stirred and again allowed
to stand for twenty-four hours, until the mucic acid is separated
in a crystalline form. To remove impurities from the mucic acid it
is separated by filtration, washed with not to exceed twenty cubic
centimeters of water, placed together with the filter in the beaker,
from twenty-five to thirty cubic centimeters of ammonium carbonate
solution, containing one part of dry ammonium carbonate, nineteen
parts of water and one part of ammonium hydroxid, added and heated to
near the boiling point. The mucic acid is dissolved by the ammonium
carbonate solution and any insoluble impurity separated by filtration,
the filtrate being received in a platinum dish, the residue well washed
and the entire filtrate and wash water evaporated to dryness on a steam
bath acidified with dilute nitric, well stirred and allowed to stand
until the mucic acid separates in a crystalline form. The separation
is usually accomplished in half an hour, after which time the crystals
of mucic acid are collected on a tared filter, or gooch, and washed
with not to exceed fifteen cubic centimeters of water followed with
sixty cubic centimeters of alcohol, then with ether, dried at 100°
and weighed. For computing the amount of galactose, one gram of the
mucic acid is equal to 1.333 of galactose and one gram of galactose is
equal to nine-tenths gram of galactan. Before the commencement of the
operation, the material should be freed of fatty matters in the case of
oily seeds and other substances similar thereto.[603]
=586. Revised Factors for Pentosans.=—The factors given in paragraph
=154= have lately been recalculated by Mann, Kruger and Tollens, and
as a result of their investigations the following factors are now
recommended.[604] The quantity of furfurol is derived from the weight of
furfurolhydrazone obtained by the formula:
1. Furfurolhydrazone × 0.516 + 0.0104 = furfurol.
2. Furfurol × 1.84 = pentosans.
3. Furfurol × 1.64 = xylan.
4. Furfurol × 2.02 = araban.
The pentoses (xylose, arabinose) may be calculated from the pentosans
(xylan, araban) by dividing by 0.88.
The method of procedure preferred for the estimation of the pentosans
is that described in paragraph =157=. The phloroglucin is dissolved in
hydrochloric acid of 1.06 specific gravity before it is added to the
furfurol distillate. The latest factor for converting the phloroglucid
obtained into furfurol is to divide by 1.82 for small quantities and
1.93 for large quantities. After the furfurol is obtained, the factors
given above are applied.
=587. Application of Roentgen Rays to Analysis.=—The detection of
mineral matters in vegetable substances by roentgen photography has
been proposed by Ranvez.[605] This process will prove extremely valuable
in detecting the lacing of teas with mineral substances. Practically,
it has been applied by Ranvez in the detection of mineral substances
mixed with saffron with fraudulent intent.
Barium sulfate is often mixed with saffron for the purpose of
increasing its weight. Pure saffron and adulterated samples are
enclosed in capsules of black paper and exposed on the same sensitive
plate for a definite time to the rays emanating from a crookes tube.
In this case the pure saffron forms only a very faint shadow in the
developed negative, while the parts to which barium sulfate are
attached produce strong shadows. The principle involved is applicable
to a wide range of analytical research.