3. DETECTION OF ROSIN
In testing for rosin as an adulterant in oils,
the following pertinent properties of this substance should be borne in mind. It
is a nonvolatile material, and consequently may be concentrated in the residue by
distillation of the oil under vacuum or at atmospheric pressure; it is found also
in the evaporation residue. Rosin consists primarily of complex acids and, therefore,
will increase the acid number of an oil or of the evaporation residue if such residue
normally consists of solid esters or paraffins; this is specifically of importance
in the case of citrus oils. Rosin is soluble in most organic solvents, including
petroleum ether, benzene, and xylene; since cinnamic aldehyde (the main constituent
of cassia oil) is practically insoluble in petroleum ether, this permits a
convenient separation of added rosin for this oil, and is the basis of "The
United States Pharmacopoeia" test described below. Rosin gives a dark green
copper salt when treated with cupric acetate ; this salt is sufficiently soluble
in petroleum ether to impart to this solvent a green color. Rosin is a relatively
high melting solid, normally a hard, noncrystalline material which fractures
readily; hence the consistency of the evaporation residue is frequently altered
if rosin is present.
a. Detection of Rosin in Balsams and Gums.
Procedure:193 Place in a small mortar 1 g. of the substance, powdered or crushed if necessary,
and add 10 cc. of purified petroleum ether. Triturate well for 1 or 2 min. Filter
into a test tube and add to the nitrate 10 cc. of a freshly prepared aqueous solution
of cupric acetate (1 g. in 200 cc.). Shake well and allow the liquids to separate.
The petroleum ether layer should not show a green color.
b. Detection of Rosin in Cassia Oils.
Procedure
I:184 Shake about
2 cc. of the oil in a test tube with 10 cc. of petroleum ether. Permit the liquids
to separate and decant the benzene layer into a second test tube. Add an equal volume
of cupric acetate solution (1 in 1000); a green color indicates the presence of
rosin in the oil.
It is well to carry out simultaneously a test
with an oil known to be free of rosin, to act as a blank. Unfortunately, tests based
upon color reactions have not proved too reliable in mixtures as complex as
essential oils; nevertheless, this test will give an indication of the presence
or absence of rosin.
Procedure
II:185 About 50 g.
of the oil, accurately weighed, are distilled from a tared distilling flask over
an open flame. Continue the distillation until decomposition is evidenced by the
formation of white fumes within the flask; this usuallyoccurs at a temperature of
about 280o. Cool the flask and weigh; calculate the percentage of
residue.
This test will reveal adulteration with nonvolatile
material such as rosin, if large amounts have been added. Normal oils show a distillation
residue of 6 to 8 per cent, or at most 10 per cent, according to Gildemeister and
Hoffmann.186 Furthermore, the residue should be tacky, but not hard and
brittle. According to Allen,187 formerly of Hongkong, the residue
should not be higher than 5 per cent for a pure oil. Treff188 has pointed out that
distillation should be carried out rapidly, since the amount of residue
obtained is greatly dependent upon the rate of distillation.
Procedure
III: Determine the acid number
of the oil in the usual manner. If the oil is pure and has been properly stored,
the acid number should not be greater than 15.
c. Detection of Rosin in Orange Oils.
Procedure: Determine the evaporation residue in the usual
manner. In the case of pure oils this residue upon cooling should be soft and waxy,
not hard, brittle or tacky. The acid number of the residue should lie between 11
and 28, the ester number between 118 and 157.189
-----------------------------
182 Schimmel & Co., April (1904), 29; April
(1910), 32; April (1911), 47.
183 United States Pharmacopoeia," Thirteenth Revision,
688.
184 "The United States Pharmacopoeia," Thirteenth
Revision, 132.
185 Ber. Schimmel & Co., October (1889), 15. Gildemeister
and Hoffmann, "Die atherischen Ole," 3d Ed., Vol. II, 631.
186 "Die atherischen Ole," 3d Ed., Vol. II, 631.
187 D. Allen, private communication.
188 Z. angew. Chem. 39 (1926), 1308.
189 Gildemeister and Hoffmann, "Die atherischen Ole”
3d Ed., Vol. Ill, 79. These data apply to Italian orange oils. However, the
values for oils from other origins do not appear to differ materially from these
limits.
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