Alfred Henry Allen.

Commercial organic analysis . (Volume vol. 3, part 2) online

. (page 17 of 65)
Online LibraryAlfred Henry AllenCommercial organic analysis . (Volume vol. 3, part 2) → online text (page 17 of 65)
Font size
QR-code for this ebook


cotism of morphine to the paralysis of conine and the tetanus of
strychnine.

In making experiments on animals it is often advantageous to
administer the poison by hypodermic injection of a solution of
alkaloid in water, or weak spirit acidulated with acetic acid. Such
a plan obviates the loss of the poison by vomiting, which some-
times eliminates the greater part of the poison from the system.
On the other hand, the subcutaneous injection of small animals is
open to certain obvious objections, and in many cases internal
administration may be advantageously substituted for it, especially
if the animal employed be a rabbit or guinea-pig, and hence not
liable to vomit. In many instances, such animals are hopelessly
large, and mice, small birds, or frogs must be employed. "W y n t e r
B 1 y t h has used blowflies with success in some cases, and occasion-
ally fish are of service. When the poison is to be given internally,
the extract or very strong solution should be made up into one or
more small pills with oatmeal, which the animal is either induced
to eat or forced to swallow. In the case of linnets and other small
birds, a drop of the liquid to be tested should be introduced into
the open beak by means of a pipette or feather.

In some cases, physiological tests may be advantageously made
on human subjects. Besides observing the bitter taste possessed
by most alkaloids, the tingling sensation produced on the tongue
by aconitine and cocaine can be thus detected.

A marked physiological characteristic of many of the alkaloids,
sufficiently striking in some cases to serve as actual evidence of
their presence, is their effect on the pupil of the eye. The test is



150 EFFECT OF ALKALOIDS ON THE PUPIL.

generally made by placing a drop of the alkaloidal solution to be
examined, as nearly neutral as possible, on the eye of a rabbit,
dog or cat, when, in a time varying from a few minutes to about
half an hour, a marked contraction or dilation of the pupil will be
observed.

A. The pupil is dilated by :

1. Atropine and belladonna ; hyoscyamine and h y o-

seine, and preparations of henbane and stamonium ;
solanine; and extracts from solanaceous plants
generally.

2. Cocaine, and preparations of coca.

3. Conine, and preparations of hemlock and other umbel-

liferous plants.

4. Cytisine, and preparations of laburnum.

5. Digitali n, and preparations of foxglove.

6. Gelsemine, and preparations of gelsemium (yellow

jesamine).

7. Sparteine, and preparations of broom.

8. Veratrine, jervine, and preparations of hellebore.

9. Hydrocyanic acid and cyanides.

Mydriasis, or dilation of the pupil, is so striking a characteristic
of atropine and the isomeric and associated bases that these are
often grouped together as the "mydriatic alkaloid s." The
mydriasis is only observed in the eye to which the alkaloid is
applied.

B. The pupil is contracted by :

1. M o r p h i n e, and other opium alkaloids and preparations

of opium.

2. Aconitine, and preparations of aconite and other mem-
bers of the RanunculacecB.

3. Physostigmine, and preparations of the Calabar

bean.

4. Strychnine, brucine, and preparations of nux

vomica.

A similar effect on the pupil is produced by the poisons when
taken internally or hypodermically in sufficient quantities. Some-
times, as in the case of morphine and preparations of opium,
the pupils are contracted during the early stages of the poisoning,
but dilated subsequently, especially after death. Nicotine and
preparations of tobacco in some cases cause contraction, and in
others dilation, of the pupil. In poisoning with aconitine alter-
nate contraction and dilation of the pupil is sometimes observed.



EXTRACTION OF ALKALOIDS.



151



ISOLATION AND PURIFICATION OF ALKA
LOIDS.

The vegetable alkaloids are found in all parts of plants, and in
many cases constitute their characteristic active principles. It
must not be assumed that the active principle is necessarily of an
alkaloidal character, though plants and plant-products, which act
primarily on the nervous system, producing tetanus, paralysis, or
narcosis (e.g., nux vomica, aconite, opium), owe their activity, as a
rule, to the presence of an alkaloid. On the other hand, in plants
which act primarily on the muscular system (e.g., digitalis), the
active principle is usually of a non-alkaloidal character. Where
the action of the plant is emetic, cathartic, or purely astringent,
the active principle is usually of a neutral or resinous character ;
but this statement has some marked exceptions, for ipecacuanha,
a typical emetic, owes its activity to the alkaloid emetine.

An alkaloid never exists in a plant in a free state. It is most
frequently present as a salt, often an acid salt, of some organic
acid, especially malic acid or one of the varieties of t a n n i c
acid. In some instances the acid with which the alkaloid is
united is peculiar to the plant in question, as, for instance,
meconic acid in opium, quinic acid in cinchona bark, and
igasuric acid in nux vomica. In other cases the alkaloid is
combined with an inorganic acid, as is the case, in part at least,
with the morphine in opium. The natural forms of combina-
tion of the alkaloids are almost invariably readily soluble both in
water and in alcohol, but insoluble in ether.

The general action of solvents on the leading constituents of
plants will be seen from the following table, which will also serve
to indicate the nature of the bodies likely to be co-extracted with
the alkaloid when the respective solvents are employed :





Water.


Alcohol.


Ether.


Alkaloidal salts, .


Soluble.


Soluble.


Insoluble.


Other salts of inorganic


Mostly soluble.


Mostly insoluble.


Insoluble.


acids,








Other salts and organic


Soluble.


Soluble.


Mostly insoluble.


acids,








Free organic acids,


Soluble.


Soluble.


Mostly insoluble.


Tannins and colouring


Soluble.


Soluble.


Variable.


matters,








Sugars


Soluble.


Soluble.


Insoluble.


Gums and pectous bodies,
Albuminoids, &c..


Soluble.
Soluble.


Mostly insoluble.
Insoluble.


Insoluble.
Insoluble.


Starch, .


Soluble in hot


Insoluble.


Insoluble.




water.






Cellulose,


Insoluble.


Insoluble.


Insoluble.


Resins, .


Insoluble.


Soluble.


Variable.


Fixed oils, .
Essential oils,


Insoluble.
Insoluble.


Sparingly soluble.
Soluble.


Soluble.
Soluble.


Chlorophyll, .


Insoluble.


Soluble.


Soluble.



152 EXTRACTION OF ALKALOIDS.

Alcohol is the solvent best adapted for the extraction of
alkaloids from plants, which should, of course, be reduced to a
suitable condition. The treatment may with advantage be re-
peated several times, the residue being well pressed between each
exhaustion, which is preferably effected by a percolator, or some
equivalent arrangement. In the final extraction, the addition
of a little sulphuric or tartaric acid is often an advantage,
but the amount of acid used should be very limited, and its
employment is vetoed in the case of readily changeable alkaloids.
Hot water may be substituted for alcohol in some cases. When
alcohol has been used for the extraction, it should be removed
partially or wholly by gentle evaporation before proceeding to
the next stage of the treatment.

The method to be adopted for the isolation of the alkaloid
from the infusion or tincture obtained depends much on its nature,
and the object of the experiment. Extraction by immiscible
solvents permits the detection of small quantities of alkaloids, which
defy methods based on precipitation, and hence this principle is
very valuable in toxicological investigations ; but, on the other
hand, the alkaloids so extracted are usually less pure than when
isolated by other means.

Where it is intended to attempt the separation of the alkaloid
by conversion into an insoluble or nearly insoluble compound, a
variety of precipitants are available, each one of which has special
advantages in particular cases. But before resorting to these
general precipitants, it is desirable, and in many cases absolutely
necessary, to remove from the liquid as much as possible of the
inert organic matters. The best reagent for this purpose is lead
acetate, which should be added gradually to the previously
neutralised liquid, as long as a precipitate continues to be produced,
avoiding the use of any considerable excess of the reagent. The
precipitate having been filtered off, the filtrate should be treated
with basic acetate of lead, which in many cases will produce a
further precipitate, to be removed by the filter as before. On
adding ammonia to the filtrate, a third precipitate will frequently
be produced, but it must be remembered that cinchonine and other
sparingly soluble alkalies are liable to be thrown down at this
stage. 1 (On this account it is undesirable to add basic acetate of
lead and ammonia at once, and filter off the joint precipitate.)

1 The threefold treatment with neutral lead acetate, basic lead acetate,
and ammonia in presence of lead acetate causes the precipitation of tannins ;
most vegetable acids (e.g., malic, tartaric, oxalic) ; albuminoids, starches, and
gums ; many glucosides, sugars, and dextrin ; and the majority of colouring
matters.



ISOLATION OF ALKALOIDS. 153

The liquid, whida should smell distinctly of ammonia, is next
evaporated at a gentle heat till the odour of ammonia has dis-
appeared, when the excess of lead is precipitated by a stream
of sulphuretted hydrogen or the addition of a moderate excess of
dilute sulphuric acid. Of these plans, the first is much to be
preferred. The lead sulphide often carries down with it a notable
quantity of colouring matter, otherwise difficult to remove, and the
excess of sulphuretted hydrogen is easily got rid of by concen-
trating the filtrate at a gentle heat. When sulphuric acid has been
employed to precipitate the lead, the filtrate should be carefully
neutralised before attempting to further concentrate the liquid,
otherwise the alkaloid may suffer partial or complete decom-
position.

The alkaloidal solution, having been purified by the foregoing
treatment, may be treated with one of the general reagents for
alkaloids, the choice of which will necessarily depend on the
nature of the base supposed to be present. Where this is
unknown, preliminary tests with various precipitants should be
made on small aliquot fractions of the solution. Although other
reagents may be preferable in particular cases, the choice will
generally lie between one of the following precipitants :

1. A fixed alkali, carbonate of alkali-metal, lime, or ammonia;

suitable for precipitating morphine, the cinchona alka-
loids, the aconite bases, &c.

2. Picric acid (page 134); very suitable for precipitating

the cinchona bases, emetine, berberine, and veratrine.

3. Tannic acid (page 135).

4. Phosphotungstic or phosphomolybdic acid (page 136);

available for the great majority of alkaloids, and
especially for strychnine.

5. Iodised iodide of potassium (page 137), which produces

very insoluble precipitates with the great majority of
alkaloids.

6. Mayer's solution (potassio-iodide of mercury) (page 139);

valuable for precipitating emetine and the opium bases.

With the exception of tannic acid, which should be applied
to the neutral or even faintly alkaline solution of the alkaloid,
the reagent should be added to the acidulated solution, sulphuric
acid being the most suitable acid to bring the liquid to the
proper condition. In most cases precipitation is tolerably rapid,
but it is desirable, as a precaution, to wait 24 hours before
proceeding with the filtration. This is especially necessary perhaps
in the case of precipitants 1 and 2. The alkaloid may be



154 USE OF IMMISCIBLE SOLVENTS.

recovered from the precipitate in the manner described on page
135 et seq.

As a rule, the salts of the alkaloids are not soluble in immiscible
solvents, and hence when the acidulated solution of an alkaloid is
agitated with chloroform, ether, petroleum spirit, benzene, or amylic
alcohol, the solvent does not remove the base from the aqueous
liquid. This behaviour broadly distinguishes alkaloids from
glucosides; but, owing chiefly to their weak basic character
and the instability of their salts, caffeine, colchicine, delphinine,
narcotine, papaverine, thebaine, and theobromine are partially or
wholly removed from their acidulated solutions on agitation with
chloroform, while amylic alcohol is stated to extract berberine and
veratrine in addition to the above bases.

EXTRACTION BY IMMISCIBLE SOLVENTS.

The behaviour of the alkaloids, when their acid and alkaline
solutions are agitated with immiscible solvents, is of the highest
practical value for their isolation and identification. 1

The immiscible solvents used for the extraction of alkaloids, &c.,
should be free from any trace of fixed or difficultly volatile organic
matter. This is best ensured by shaking the solvent with water
slightly acidulated with sulphuric acid, separating the aqueous
liquid, and redistilling the immiscible solvent at a moderate tem-
perature rejecting the last portion. The distillate should then
be agitated with water rendered faintly alkaline by caustic soda,
and indeed may be advantageously kept in contact with faintly
alkaline water. The agitation with water is essential in the case
of solvents liable to certain alcohol (e.g., ether, chloroform, amylic
alcohol), the presence of which might seriously modify their
action.

In using immiscible solvents, it must be borne in mind that
extraction is never theoretically perfect with a single treatment.
The dissolved body is distributed between the two solvents in
proportions which are probably dependent on the relative solu-
bility of the substance in the two media, and the relative
quantities of the two media employed. Thus, it may be sup-
posed that if a substance be 99 times more soluble in chloroform
than in water, and its aqueous solution be shaken with an equal

1 The principle appears to have been first adopted by Otto in 1856, who
employed ether in his modification of S t a s ' process for the detection of
poisonous alkaloids. In 1856, Rodgers and Gird wood employed the
method with chloroform, and in 1861 Uslar and Erdmann recommended
the use of amylic alcohol. In 1867, Dragendorff published his well-
known elaborate scheme for the separation of plant-principles by immiscible
solvents.



JEJSE OF IMMISCIBLE SOLVENTS.



155



measure of chloroform, 99 per cent, of the whole substance will
pass into the chloroform. On separating this layer, and again
agitating the aqueous liquid with an equal quantity of chloro-
form, 99 per cent, of the remaining substance will be dissolved,
thus making the exhaustion practically complete. In the case
of ether and amylic alcohol, the solubility of the solvent itself
in the aqueous liquid is also an important consideration ; for, as
ether is soluble in about ten times its measure of water, on
agitating together equal measures of ether and an aqueous liquid,
it may be assumed that one-tenth of the ether will be dissolved,
and will remain in the aqueous liquid together with its one-
tenth share of the alkaloid or other substance to be extracted.
On separating the ethereal layer, and again shaking the aqueous
liquid with an equal measure of ether, it may be considered
that nine-tenths of the previously dissolved ether and its alkaloid
will be recovered in the immiscible solvent. On the other hand,
the ethereal layer is not wholly free from water, which may be
expected to take up certain substances not soluble in anhydrous
ether; but practically such traces of impurity are removed on
agitating the ether with a limited quantity of water. Similar
considerations of solubility apply to treatments with chloroform,
but with considerably less force owing to its slight solubility
in water and vice-versa ; and in the case of petroleum-ether and
benzene they have no practical bear-
ing, as these solvents are almost abso-
lutely insoluble in aqueous liquids.

In making a proximate analysis by
means of immiscible solvents, much of
the success in practice depends on the
care and skill with which the manipu-
lation is conducted. The most con-
venient apparatus for effecting the
treatment consists of a pear-shaped
(fig. 1) or cylindrical (fig. 2) glass
separator, furnished with a tap below
and a stopper at the top. The tube be-
low the tap should be ground obliquely
so as to prevent loss of liquid by
imperfect delivery. Supposing that it
be desired to effect the separation of a substance from an aqueous
liquid by agitation with ether, the former is introduced into the
separator, of which it should not occupy more than one-third, acid
or alkali added as may be desired, and next a volume of ether
about equal to that of the aqueous liquid. The stopper is then




A



Fig. 1.



Fig. 2.



j
156 REPARATION BY IMMISCIBLE SOLVENTS,

inserted and the whole thoroughly shaken together for a minute
or two, and then set aside. As a rule, the contents will readily
separate into two well-defined layers, the lower of which is
aqueous, and the upper ethereal. Sometimes separation into layers
does not occur readily, the liquid remaining apparently homo-
geneous, forming an emulsion, or assuming a gelatinous consistency.
In such cases, separation may sometimes be induced by thoroughly
cooling the contents of the separator. In the case of ether,
the separation may usually be effected by adding an additional
quantity of ether and re-agitating, or, when the employment of
a sufficient excess of ether is inconvenient or impracticable, the
addition of a few drops of alcohol, followed by a gentle rotatory
motion of the liquid, will almost invariably cause separation to
occur promptly.

The tendency to form an obstinate emulsion is greatest when
the aqueous liquid is alkaline, and is often very troublesome
when chloroform, benzene, or petroleum-ether is substituted for
ether. In such cases, the employment of a larger quantity of
the solvent sometimes causes separation, but, when admissible,
a better plan is the addition of ether. This answers very
successfully for the isolation of strychnine, which is nearly
insoluble in unmixed ether, but readily soluble in a mix-
ture of equal measures of ether and chloroform. This solvent
is heavier than water, and is capable of very extensive appli-
cation.

Separation having taken place, the aqueous layer should be
run off by the tap into another separator, where it can again
be agitated with ether to insure the complete removal of the
body to be dissolved therein. The ethereal liquid remaining in
the first separator should be shaken with a fresh quantity of
alkalised or acidulated water, which is then tapped off as before,
and the remaining traces removed by treating the ether with a
little pure water. This having in turn been run off to the
last drop, the ethereal solution can next be removed by the tap,
but a preferable plan is to pour it off from the mouth of the
separator, taking care to avoid the draining of any drops of
aqueous liquid from the sides of the glass.

When amylic alcohol, benzene, or petroleum ether is employed,
the manipulation is the same as that just described ; but when
chloroform is used, or a mixture containing a considerable pro-
portion of that solvent, the aqueous liquid forms the upper
stratum, and the chloroformic solution can at once be removed
by the tap.

When the volume of fluid treated with the immiscible solvent



SEPARATION BY IMMISCIBLE SOLVENTS.



157




a



>c




is very small, the syringe pipette shown in fig. 3 may be con-
veniently substituted for a tapped separator. It is readily con-
structed by drawing out a test-tube, so as to form a narrow
prolongation, the orifice of which should be turned up so as
not to disturb the liquid in
which it is immersed. A
narrow test-tube fashioned
into a handle at the upper
part serves as a piston, a
short length of india-rubber
tubing uniting it to the outer
tube, while allowing of easy
movement both in a vertical
and a horizontal direction.

Another convenient form
of separator, devised by W.
C h a 1 1 a w a y, is shown in
fig. 4. It is practically a
small wash - bottle fitting,
which is adjusted to the
tube or cylinder containing
the layers of liquid it is
desired to separate. It is
so arranged that the exit-
tube (B) can be adjusted in
height by sliding it through
the india-rubber collar C, so as to bring the turned-up end just
above the junction of the two layers. On then blowing through
the side-tube (A), the upper stratum is forced up the inner tube,
and can be removed, almost to the last drop, without disturbing
the lower layer.

The following table shows the behaviour of various classes of
organic substances when shaken in acidulated or alkalised solution
with immiscible solvents, such as ether, chloroform, amylic alcohol,
benzene, and petroleum ether. It must not be supposed, how-
ever, that the immiscible solvents can be employed indifferently,
as some of the bodies are readily removed by certain solvents,
but are unaffected by others owing to their limited solubility
therein. This is especially the case with the alkaloids and
glucosides, and hence the table must merely be regarded as
showing their general tendency, their special behaviour with
the different solvents being deferred for fuller description later
on.



Fig. 3.



Fig. 4.



158



SEPARATION BY IMMISCIBLE SOLVENTS.



Table showing the behaviour of Organic Substances with
Immiscible Solvents.



Oil agitating the substance with water, acidulated with sulphuric acid, and a suitable
solvent immiscible therewith (such as ether, chloroform, amylic alcohol, benzene,


or petroleum ether), the following distribution will occur :


THE IMMISCIBLE LATER will contain


THE ACIDULATED AQUEOUS LIQUID will


hydrocarbons, oils, various acids, resins,


contain carbohydrates, soluble alkaloids


colouring matters, phenols, glucosides,
<fcc., which may be further separated by
agitating the liquid with water con-


and acids, organic bases, proteids, &c.,
which may be further separated by add-
ing a moderate excess of soda, and again


taining caustic soda, when there will be


shaking with a suitable immiscible sol-


obtained :


vent, when there will be obtained :


IN THE IMMISCIBLE


IN THE ALKALINE


IN THE IMMISCIBLE


IN THE ALKALINE


LAYER


AQUEOUS LIQUID


LAYER


AQUEOUS LIQUID


Solid Hydrocarbons ;


Fatty Acids; as


Most Vegetable Alka-


Carbohydrates ; as


as paraffin, naph-
thalene, anthra-


stearic, oleic,
valeric.


loids; as quinine,
strychnine, acoui-


sugars, gums,
dextrin.


cene.


Various other Adda ;


tine, atropine,


Soluble Alcohols; as


Liquid Hydrocar-
bons ; as petroleum


as benzoic, sali-
cylic, phthalic,


nicotine (cincho-
nine, morphine ;


methyl alcohol,
ethyl alcohol, gly-


products,rosin-oil,


meconic.


the last two with


cerin.


benzene.


Acid Dyes and Col-


difficulty).


Soluble Acids; as


Essential Oils; as


ouring Matters ;


Coal-Tar Bases; as


acetic, oxalic, lac-


turpentine.
Nitro - compounds;


as picric and chry-
sophanic acids,


aniline and its
homologues (ros-


tic, malic, tartaric,
sulphophenic.


as nitrobenzene.


alizarin, aurin,


aniline), chryso-


Certain Alkaloids and


Ethers and their


bilirubiu.


toluidine (pyri-


Organic Bases ; as


Allies; as ether,


Acid Resins ; as


dine), homologues


curarine, cytisine,


chloroform, ethe-
real salts, nitro-


colophony.
Phenoloids ; as car-


of pyridine.


narceine, urea,
glycocine, sola-


glycerin.
Fixed Oils, Fats, and


bolic and cresylic
acids, thymol,




nine, and possibly
cinchonine. mor-


Waxes.


creosote.




phine and pyri-


Neutral Resins and


Certain Glucosides,




dirie.


Colouring Matters.


<kc. ; as santonin,




Certain Colourina


Chlorophyll.


cantharidin, picro-


Matters ; as indigo-


Camphors; as laurel-


toxin.


products.


camphor, borneol,






Proteids and their


menthol.






Allies; as albumin,


Alcohols insoluble or






casein, gelatin.


nearly insoluble in








water ; as arnyl








and cetyl alcohols,








cholesterin.








Certain Glucosides,








<&c. ; as saponin,








digi talin, santonin.








Certain Weak Alka-






loids; as caffeine,






colchicine, narco-



Online LibraryAlfred Henry AllenCommercial organic analysis . (Volume vol. 3, part 2) → online text (page 17 of 65)