On a New Test for Distinguishing the Russian, Indian, and English
Rhubarbs. By Mr. John S. Cobb.—The absence of a good test for
distinguishing the Russian from East Indian rhubarb has been too much
felt and too often expatiated on to require any comment. The want of
it has led to a great many analyses, undertaken with a view to the dis-
covery of some constituent peculiar to one or the other species; I am
not, however, aware of any successful results having, up to the present
time, ensued from these researches, at least so far as concerns the dis-
covery of any reagent capable of distinguishing the varieties of this drug.
Several tests have indeed been proposed, but none of these are to be relied
upon. The grittiness, when chewed, caused by the presence of oxalate
of lime in the Russian variety, is far from being a satisfactory test. I
have met with specimens of Indian rhubarb possessing this quality to a
great degree.
Geiger states that “ iodized hydriodic acid ” gives with a decoction of
Russian rhubarb a green tint, with Indian a brownish color, and a deep
red with the English rhubarb; but in my hands this test has proved
singularly fallacious; with it I obtained a green color with the Russian,
a dirty green with Indian, and a sea-green with English rhubarb. More-
over, the color varies according to the quantity of the test added; and if
the latter be in excess, any of the varieties when viewed by transmitted
light, will seem of a red color. If, however, the solution be subsequently
plentifully diluted with water, the green tint is rendered evident.
Thomson remarks, first, that a solution of isinglass gives a more abun-
dant precipitate in the Indian than in the Russian rhubarb; and second,
that the decoction of yellow bark gives a more abundant greenish pre-
cipitate with the infusion of Russian than with the East Indian rhubarb,
in the latter of which the precipitate is bright yellow.
These tests do not appear to me to be conclusive. The precipitate
caused by the solution of isinglass, gives a bulkier but not really more
abundant precipitate in the one case than in the other, as is evident when
the precipitate has subsided, and the difference of color in the other test
is so small that it requires a somewhat artistic eye to appreciate the dis-
tinction, and it certainly would not serve otherwise than as a compara-
tive test. I may incidentally mention here, that in the course of my ex-
periment, I found that the acetate of lead, when ammonia had previously
been added, gave with infusion of Indian rhubarb a violet-red precipitate,
and one approaching to a brick-red with Russian rhubarb, the precipitate
in the latter being of a coarser description than in the former. The
same objection, however, applies to this as to the previously cited tests,
for any reagent depending for its efficiency on a slight variation in color
or bulk, must practically be of little value. Bearing this in mind, I have
endeavored to find one not liable to these objections.
The test to which I am about to direct your attention is based on the
supposition which I had the honor of suggesting to you some two years
since, that the precipitate formed in the tincture of rhubarb is the result
of the oxidation of the active principle of the rhubarb. I am not at
present prepared to maintain the correctness or inaccuracy of this view,
but I am now engaged in some experiments on the subject, which, if
satisfactory, I shall have the pleasure of submitting to you on a future
day. Starting then with this hypothesis, and remarking that the tinc-
ture, when made with Indian, deposited more than with the Russian
rhubarb, I thought it probable that an oxidizing agent might exhibit
sufficient difference in its modus agendi on the species of rhubarb to
enable us to identify them thereby. In this, though at first unsuccessful,
my anticipations were ultimately realized. If two drachms of simple
(proo/) tincture, of the P. L. strength, be placed in a test tube, and
treated with one drachm of a mixture of equal parts of nitric acid and
distilled water, the following results will take place :—
East Indian soon becomes cloudy, and in from five to twenty minutes
is turbid.
Russian remains unchanged for three or four hours.
English loses its brightness in half an hour; on holding it before a
light a precipitate may be seen diffused through it.
If, instead of 3ij. of tincture, one of tincture and one of water be used,
the test as regards the Indian is still more prompt, but the difference
between the English and Turkey not so clearly defined.
There are one or two precautions to be observed in the application of
this test: the dilute acid should be gradually added, the test tube being
shaken in the meantime. Again, if rectified tincture or strong acid be
used, very equivocal results are obtained. My first experiments were
made with strong acid and rectified tincture. The results were excel-
lent, as far as regarded three or four samples; but when I came to essay
another, the very reverse of the former experiments took place. I thought
at first that the samples of rhubarb could not be rightly marked, but on
again returning to the former samples, they also gave contradictory re-
sults. I procured other samples, but ultimately discovered that this
anomalous reaction was due to the time the nitric acid was left in con-
tact with the supertangent film of tincture, in which it seemed to set up
a sort of fermentation, which, after the two were mixed by agitation,
propagated itself throughout the liquid. Thus, if two drachms of tinc-
ture of East Indian rhubarb be placed in two separate test tubes, held
in the hand, and the acid added; if the tube to which the acid is last
added be immediately agitated, and then the other treated in a similar
manner, the one will shortly let fall a ropy deposit, while the other re-
mains clear. By using the dilute acid and proof tincture these fallacies
are avoided.—Lond. Pkarm. Journ., Feb. 1853.