=33. Object Of Treatment.=—The next step, in the analytical work, after
sampling, drying, and incinerating, is the treatment of the sample with
solvents. The object of this work is to separate the material under
examination into distinct classes of bodies distinguished from each
other by their solubilities. It is not the purpose of this section
to describe the various bodies which may be separated in this way,
especially from vegetable products. For this description the reader may
consult the standard works on plant analysis.[17]
The chief object of a strictly agricultural examination of a field
or garden product is to determine its food value. This purpose can
be accomplished without entering into a minute separation of nearly
allied bodies. For example, in the case of carbohydrates it will be
sufficient as a rule, to separate them into four classes. In the first
class will be found those soluble in water as the ordinary sugars. In
the second group will be found those which, while not easily soluble in
water, are readily rendered so by treatment with certain ferments or by
hydrolysis with an acid. The starches are types of this class. In the
third place are found those bodies which resist the usual processes of
hydrolysis either with an acid or alkali, and therefore remain in the
residue as fiber. Cellulose is a type of these bodies. In the fourth
class are included those bodies which on hydrolysis with an acid yield
furfurol on distillation, and therefore belong to the type containing
five atoms of carbon or some multiple thereof in their molecule. For
ordinary agricultural purpose the separation is not even as complete as
is represented above.
What is true of the carbohydrates applies equally well to the fats and
to other groups. Especially in the analysis of cereals and of cattle
foods, the treatment with solvents is confined to the use in successive
order of ether or petroleum, alcohol, dilute acids, and alkalies, the
latter at a boiling temperature. The general method of treatment with
these solvents will be the subject of the following paragraphs.
=34. Extraction of the Fats and Oils.=—Two solvents are in general use
for the extraction of fats and oils; _viz._, ethylic ether and a light
petroleum. The former is the more common reagent. Before use it should
be made as pure as possible by washing first with water, afterwards
removing the water by lime or calcium chlorid, and then completing
the drying by treatment with metallic sodium. The petroleum spirit
used should be purified by several fractional distillations until it
has nearly a constant boiling-point of from 45° to 50°. The detailed
methods of preparing these reagents will be given in another place. For
rigid scientific determinations the petroleum is to be preferred to the
ether. It is equally as good a solvent for fats and oils and is almost
inert in respect to other vegetable constituents. Ether, on the other
hand, dissolves chlorophyll and its partial oxidation products, resins,
alkaloids and the like. The extract obtained by ether is therefore less
likely to be a pure fat than that secured by petroleum. For purposes of
comparison, however, the ether should be employed, inasmuch as it has
been used almost exclusively in analytical operations in the past.
=35. Methods Of Extraction.=—The simplest method for accomplishing
the extraction of fat from a sample consists in treating it with
successive portions of the solvent in an open dish or a closed flask.
This process is actually employed in some analytical operations, as,
for instance, in the determination of fat in milk. Experience has
shown, however, that a portion of the substance soluble, for instance,
in ether, passes very slowly into solution, so that a treatment such as
that just described would have to be long continued to secure maximum
results. The quantity of solvent required would thus become very large
and in the case of ether would entail a great expense. For the greater
number of analytical operations, therefore, some device is employed for
using the same solvent continually. The methods of extraction therefore
fall into two general classes; _viz._, extraction by digestion and
extraction by percolation. This classification holds good also for
other solvents besides ether and petroleum. In general, the principles
and practice of extraction described for ether may serve equally well
for alcohol, acetone and other common solvents.
=36. Extraction by Digestion.=—In the use of ether or petroleum the
sample is covered with an excess of the solvent and allowed to remain
for some time in contact therewith. The soluble portions of the
sample diffuse into the reagent. The speed of diffusion is promoted
by stirring the mixtures. The operation may be conducted in an open
dish or a flask. Inasmuch as the residue is, as a rule, to be dried
and weighed, an open dish is to be preferred. To avoid loss of reagent
and to prevent filling a working room with very dangerous gases, the
temperature of digestion should be kept below the boiling-point of the
solvent. The greater part of the soluble matter will be extracted with
three or four successive applications of the reagent, but, as intimated
above, the last portions of the soluble material are extracted with
difficulty by this process. In pouring off the solvent care must be
exercised to avoid loss of particles of the sample suspended therein.
To this end it is best to pour the solvent through a filter. For the
extraction of large quantities of material for the purpose of securing
the extract for future examination, or simply to remove it, the
digestion process is usually employed. This excess of solvent required
is easily recovered by subsequent distillation and used again. The
method is rarely used for the quantitive estimation of the extract,
the process of continuous percolation being more convenient and more
exact.
=37. Extraction by Percolation.=—In this method the solvent employed
is poured on the top of the material to be extracted and allowed to
pass through it usually by gravitation alone, sometimes with the help
of a filter-pump. The principle of the process is essentially that of
washing precipitates.
Two distinct forms of apparatus are in use for this process. In
the first kind the solvent is poured over the material and after
percolation is secured by distillation in another apparatus. In the
second kind the solvent is secured after percolation in a flask where
it is at once subjected to distillation. The vapors of the solvent
are conducted by appropriate means to a condenser placed above the
sample. After condensation the solvent is returned to the upper part of
the sample. The percolation thus becomes continuous and a very small
quantity of the solvent may thus be made to extract a comparatively
large amount of material. This process is particularly applicable
to the quantitive determination of the extract. After distillation
and drying the latter may be weighed in the flask in which it was
received or the sample may be dried and weighed in the vessel in which
it is held both before and after extraction. One great advantage of
the continuous extraction method lies in the fact that when it is
once properly started it goes on without further attention from the
analyst save an occasional examination of the flow of water through
the condenser and of the rate of the distillation. For this reason the
process may be continued for many hours without any notable loss of
time. The vapor of the solvent in passing to the condenser may pass
through a tube out of contact with the material to be extracted or it
may pass directly around the tube holding the sample. In the former
case the advantage is secured of conducting the extraction at a higher
temperature, but there is danger of boiling the solvent in contact with
the material and thus permitting the loss of a portion of the sample.
=38. Apparatus Used for Extractions.=—For extraction by digestion, as
has already been said, an open dish may be used. When large quantities
of material are under treatment, heavy flasks, holding from five to
ten liters, will be found convenient. In these cases a condenser can
be attached to the flask and the extraction conducted at the boiling
temperature of the solvent. During the process of extraction it is
advisable to shake the flask frequently. By proceeding in this way the
greater part of the solvent matter will be removed after three or four
successive treatments.
In extraction by percolation various forms of apparatus are employed.
The ordinary percolators of the manufacturing pharmacist may be used
for the larger operations, while the more elaborate forms of continuous
extractors will be found most convenient for quantitive work. In each
case the analyst must choose that process and form of apparatus best
suited to the purpose in view. In the next paragraphs will be described
some of the more common forms of apparatus in use.
=39. Knorr’s Extraction Apparatus.=—The apparatus which has been
chiefly used in this laboratory for the past few years is shown in
the accompanying figure.[18] The principle of the construction of the
apparatus lies in the complete suppression of stoppers and in sealing
the only joint of the device with mercury.
The construction and operation of the apparatus will be understood by a
brief description of its parts.
A is the flask containing the solvent, W a steam bath made by cutting
off the top of a bottle, inverting it and conducting the steam into one
of the tubes shown in the stopper while the condensed water runs out of
the other. The top of the bath is covered with a number of concentric
copper rings, so that the opening may be made of any desirable size. B
represents the condenser, which is a long glass tube on which a number
of bulbs has been blown, and which is attached to the hood for holding
the material to be extracted, as represented at Bʹ, making a solid
glass union. Before joining the tube at Bʹ the rubber stopper which is
to hold it into the outside condenser of B is slipped on, or the rubber
stopper may be cut into its center and slipped over the tube after the
union is made. In case alcohol is to be used for the solvent, requiring
a higher temperature, the flask holding the solvent is placed entirely
within the steam-bath, as represented at Aʹ.
[Illustration: FIGURE 18. KNORR’S EXTRACTION APPARATUS.]
[Illustration: FIGURE 19. EXTRACTION FLASK.]
[Illustration: FIGURE 20. EXTRACTION TUBE.]
[Illustration: FIGURE 21. EXTRACTION SIPHON TUBE.]
A more detailed description of the different parts of the apparatus
can be seen by consulting Figs. 19, 20, and 21. In A, Fig. 19, is
represented a section of the flask which holds the solvent, showing
how the sides of the hood containing the matters to be extracted pass
over the neck of the flask, and showing at S a small siphon inserted
in the space between the neck of the flask and the walls of the hood
for the purpose of removing any solvent that may accumulate in this
space. A view of the flask itself is shown at Aʹ. It is made by taking
an ordinary flask, softening it about the neck and pressing the neck
in so as to form a cup, as indicated at Aʹ, to hold the mercury which
seals the union of the flask with the condenser. The flask is held in
position by passing a rubber band below it, which is attached to two
glass nipples, _b_, blown onto the containing vessel, as shown in Fig.
18. The material to be extracted may be contained in an ordinary tube,
as shown in Fig. 20, which may be made from a test tube drawn out, as
indicated in the figure, having a perforated platinum disk sealed in
at D. The containing tube rests upon the edges of the flask containing
the solvent by means of nipples shown at _t_. If a siphon tube is to
be used, one of the most convenient forms is shown in Fig. 21, in
which the siphon lies entirely within the extracting tube, thus being
protected from breakage. By means of this apparatus the extractions
can be carried on with a very small quantity of solvent, there being
scarcely any leakage, even with the most volatile solvents, such as
ether and petroleum. The apparatus is always ready for use, no corks
are to be extracted, and no ground glass joints to be fitted.
=40. Soxhlet’s Extraction Apparatus.=—A form of continuous extraction
apparatus has been proposed by Soxhlet which permits the passage of
the vapors of the solvent into the condenser by a separate tube and
the return of the condensed solvent after having stood in contact
with the sample, to the evaporating flask by a siphon. The advantage
of this process lies in freeing the sample entirely from the rise of
temperature due to contact with the vapors of the solvent, and in the
second place in the complete saturation of the sample with the solvent
before siphoning. The sample is conveniently held in a cylinder of
extracted filter-paper open above and closed below. This is placed
in the large tube between the evaporating flask and the condenser.
The sample should not fill the paper holder, and if disposed to float
in the solvent, should be held down with a plug of asbestos fiber
or of glass wool. The extract may be transferred, by dissolving in
the solvent, from the flask to a drying dish, or it may be dried and
weighed in the flask where first received.
[Illustration: FIGURE 22. SOXHLET EXTRACTION APPARATUS.]
There are many forms of apparatus of this kind, one of which is shown
in Fig. 22, but a more extended description of them is not necessary.
The disadvantages of this process as compared with Knorr’s, are quite
apparent. The connections with the evaporating flask and condenser are
made with cork stoppers, which must be previously thoroughly extracted
with ether and alcohol. These corks soon become dry and hard and
difficult to use. The joints are likely to leak, and grave dangers
of explosion arise from the vapors of the solvents escaping into the
working room. Moreover, it is an advantage to have the sample warmed
by the vapors of the solvent during the progress of the extraction,
provided the liquid in direct contact with the sample does not boil
with sufficient vigor to cause loss.
The use of extraction apparatus with ground glass joints is also
unsatisfactory. By reason of unequal expansion and contraction these
joints often are not tight. They are also liable to break and thus
bring danger and loss of time.
=41. Compact Extraction Apparatus.=—In order to bring the extraction
apparatus into a more compact form, the following described device has
been successfully used in this laboratory.[19] The condenser employed
is made of metal and is found entirely within the tube holding the
solvent.
This form of condenser is shown in Fig. 23, in which the tube E serves
to introduce the cold water to the bottom of the condensing device.
The tube D serves to carry away the waste water. The tube F serves
for the introduction of the solvent by means of a small funnel. When
the solvent is introduced and has boiled for a short time, the tube
F should be closed. In each of the double conical sections of the
condenser a circular disk B is found, which causes the water flowing
from A upward to pass against the metallic surfaces of the condenser.
A section of the double conical condenser is shown in the upper right
hand corner. It is provided with two small hooks _hh_, soldered on the
lower surface, by means of which the crucible G can be hung with a
platinum wire. The condenser is best made smooth and circular in form.
The crucible G, which holds the material to be extracted, can be made
of platinum, but for sake of economy also of porcelain. The bottom
of the porcelain crucible is left open excepting a small shelf, as
indicated, which supports a perforated disk of platinum on which an
asbestos film is placed.
[Illustration: FIGURE 23. COMPACT CONDENSING APPARATUS.]
The whole apparatus is of such size as to be easily contained in the
large test-tube T.
The mouth of the test-tube is ground so as to fit as smoothly as
possible to the ground-brass plate of the metallic condenser P.
In case it is desired to weigh the extract it may be done directly by
weighing it in the test-tube T after drying in the usual way at the
end of the extraction; or a glass flask H, made to fit freely into the
test-tube, may be used, in which case a little mercury is poured into
the bottom of the tube to seal the space between H and T. To prevent
spirting of the substance in H, or projecting any of the extracted
material without or against the bottom of the crucible G, the funnel
represented by the dotted lines in the right hand section may be used.
Heat may be applied to the test-tube either by hot water, or steam, or
by a bunsen, which permits of the flame being turned down to minimum
proportions without danger of burning back. When the test-tube alone
is used it is advisable to first put into it some fragments of pumice
stone, particles of platinum foil, or a spoonful of shot, to prevent
bumping of the liquid when the lamp is used as the source of heat.
Any air which the apparatus contains is pushed out through F when the
boiling begins, the tube F not being closed until the vapor of the
liquid has reached its maximum height. With cold water in the condenser
the vapor of ether very rarely reaches above the lower compartment and
the vapor of alcohol rarely above the second.
When the plate P is accurately turned so as to fit the ground surface
of the mouth of T, it is found that ten cubic centimeters of anhydrous
ether or alcohol are sufficient to make a complete extraction, and
there is not much loss of solvent in six hours. The thickness of the
asbestos film in G, or its fineness, is so adjusted as to prevent too
rapid filtration so that the solvent may just cover the material to
be extracted, or, after the material is placed in a crucible, a plug
of extracted glass wool may be placed above it for the purpose of
distributing the solvent evenly over the surface of the material to be
extracted and of preventing the escape of fine particles.
[Illustration: FIGURE 24. IMPROVED COMPACT EXTRACTION APPARATUS.]
In very warm weather the apparatus may be arranged as shown in figure
24. The bath for holding the extraction tubes is made in two parts,
K and Kʹ. The bath K has a false bottom shown in the dotted line O,
perforated to receive the ends of the extraction tubes and which holds
them in place and prevents them from touching the true bottom, where
they might be unequally heated by the lamp. The upper bath Kʹ has a
perforated bottom, partly closed with rubber-cloth diaphragms Gʹ Nʹ Hʹ.
The extraction tubes passing through this bath, water-tight, permit
broken ice or ice-water to be held about their tops, and thus secure
a complete condensation of the vapors of the solvent which in warm
weather might escape the metal condenser. In practice care must be
taken to avoid enveloping too much of the upper part of the extraction
tube with the ice-water, otherwise the vapors of the solvent will be
chiefly condensed on the sides of the extraction tube and will not be
returned through the sample. It is not often that the upper bath is
needed, and then only with ether, never with alcohol. This apparatus
has proved especially useful with alcohol, using, as suggested,
glycerol in the bath. The details of its further construction and
arrangement are shown in the figure. The extraction tubes are most
conveniently arranged in a battery of four, one current of cold water
passing in at A and out at B, serving for all. The bath is supported
on legs long enough to allow the lamp plenty of room. The details of
the condenser M are shown in Bʹ, Aʹ, T, Fʹ, and Lʹ. Instead of a gooch
Lʹ for holding the sample a glass tube R, with a perforated platinum
disk Q, may be used. The water line in the bath is shown by W. This
apparatus may be made very cheaply and without greatly impairing its
efficiency by using a plain concentric condenser and leaving off the
upper bath Kʹ.
=42. Solvents Employed.=—It has already been intimated that the chief
solvents employed in the extraction of agricultural samples are ether
or petroleum and aqueous alcohol. The ether used should be free of
alcohol and water, the petroleum should be subjected to fractional
distillation to free it of the parts of very high and very low boiling
points, and the alcohol as a rule should contain about twenty per cent
of water.
There are many instances, however, where other solvents should be used.
The use of aqueous alcohol is sometimes preceded by that of alcohol of
greater strength or practically free of water. For the extraction of
soluble carbohydrates (sugars) cold or tepid water is employed, the
temperature of which is not allowed to rise high enough to act upon
starch granules. For the solution of the starch itself an acid solvent
is used at a boiling temperature, whereby the starch molecules undergo
hydrolysis and form dextrin or soluble sugars (maltose, dextrose). By
this process also the carbohydrates, whose molecules contain five, or
some multiple thereof, atoms of carbon form soluble sugars of which
xylose and arabinose are types. The solvent action of acids followed
by treatment with dilute alkalies at a boiling temperature, completes
practically the solution of all the carbohydrate bodies, save cellulose
and nearly related compounds. The starch carbohydrates are further
dissolved by the action of certain ferments such as diastase.
Dilute solutions of mineral salts exert a specific solvent action on
certain nitrogenous compounds and serve to help separate the albuminoid
bodies into definite groups.
Under the proper headings the uses of these principal solvents will be
described, but a complete discussion of their action, especially on
samples of a vegetable origin, should be looked for in works on plant
analysis.[20]
The application of acids and alkalies for the extraction of
carbohydrates, insoluble in water and alcohol, will be described,
in the paragraphs devoted to the analysis of fodders and cereals.
The extraction of these matters, made soluble by ferments, will be
discussed in the pages devoted to starch and artificial digestion.
It is thus seen that the general preliminary treatment of a sample
preparatory to specific methods of examination is confined to drying,
extraction with ether and alcohol, and incineration.
=43. Recovery of the Solvent.=—In using such solvents as ether,
chloroform, and others of high value, it is desirable often to recover
the solvent. Various forms of apparatus are employed for this purpose,
arranged in such a way as both to secure the solvent and to leave the
residue in an accessible condition, or in a form suited to weighing
in quantitive work. When the extractions are made according to the
improved method of Knorr, the flask containing the extract may be at
once connected with the apparatus shown in figure 25.[21] A represents
the flask containing the solvent to be recovered, W the steam-bath,
B the condenser sealed by mercury, M and R the flask receiving the
products of condensation. It will be found economical to save ether,
alcohol, and chloroform even when only a few cubic centimeters remain
after the extraction is complete. In the figure the neck of the flask
A is represented as narrower than it really is. The open end of the
connecting tube, which is sealed on A by mercury, should be the same
size as the tube connecting with the condenser in the extraction
apparatus.
[Illustration: FIGURE 25.—KNORR’S APPARATUS FOR RECEIVING SOLVENTS.]
[Illustration: FIGURE 26. APPARATUS FOR RECOVERING SOLVENTS FROM OPEN
DISHES.]
It often happens that materials which are dissolved by the ordinary
solvents in use are to be collected in open dishes in order that
their properties may be studied. At the same time large quantities
of solvents must be used, and it is desirable to have some method of
recovering them. The device shown in Fig. 26 has been found to work
excellently well for this purpose.[22] It consists of a steam-bath, W,
and a bottle, B, with the bottom cut off, resting on an iron dish, P,
containing a small quantity of mercury, enough to seal the bottom of
the bottle. The dish containing the solvent is placed on the mercury,
and the bottle placed down over it, forming a tight joint. On the
application of steam the solvent escapes into the condenser, C, and is
collected as a liquid in the flask A. In very volatile solvents the
flask A may be surrounded with ice, or ice-cold water passed through
the condenser. When an additional quantity of the solvent is to be
added to the dish for the purpose of evaporating it is poured into the
funnel F, and the stopcock H opened, which allows the material to run
into the dish in B without removing the bottle. In this way many liters
of the solvent may be evaporated in any one dish, and the total amount
of extract obtained together. At the last the bottle B is removed, and
the extract which is found in the dish is ready for further operations.