2. TESTS FOR HALOGENS
The presence of chlorine in a synthetic is usually
indicative of insufficien purification. The detection of halogen in a reputedly
natural essential oil or fraction is indicative of adulteration with a chlorine-containing
synthetic. For example, cassia oils showing the presence of chlorine have probably
been adulterated with impure synthetic cinnamic aldehyde.
Of the numerous procedures which hve been suggested,
the classical test with copper oxide (the so-called Beilstein test) groves by far
the most convenient and rapid. Should this test prove inconclusive, the
presence of absence of halogen should be confirmed by the combustion method, a more
sensitive test.
Procedure
I. Beilstein Method: Wind the
end of a No. 16 gage130 copper wire into a tight spiral about 6 mm. long
and 6 mm. in diameter. Fasten the other end of this wire to a wooden handle. Heat
the wire in the nonluminous flame of a Bunsen burner until it glows without coloring
the flame green.131 Permit the wire to cool and reheat several times
until a good coat of oxide has formed on the coil.132 Add to the
cooled spiral 2 drops of the material to be tested. Ignite and permit it to burn
freely in air. The wire is again cooled and 2 more drops of the material are added
and burned. This process is continued until a total of 6 drops has been added
and ignited. Then hold the spiral in the oxidizing portion of a Bunsen flame, adjusted
to about 1/2 in. high. If the material is free from halogen, the flame will show
no green color. The degree of persistence of green color is a rough indication
of the amount of halogen present. A highly purified synthetic, free from halogen,
will not show even a transient green color or flash of green.
Instead of the wire spiral described above, a
piece of 30 mesh copper screening (1.5 cm. X 5.0 cm.) may be used. The screening
should be rolled tightly around a copper wire and held in position by bending
back the wire and twisting securely. Such a roll of copper screening will hold about
1 cc. of oil because of surface tension.
Certain nitrogen-containing compounds may give
a positive test although no halogen is present. Also, the presence of free organic
acids may cause a green colored flame since the copper salt may be sufficiently
volatile; e.g., phenylacetic acid. Therefore, if a positive test is obtained, it
is best to confirm such findings by the combustion method.
Procedure
II. Combustion Method:1 A piece
of filter paper about 5X6 cm. is folded and saturated with the oil to be tested.
The paper is placed in a small porcelain evaporating dish which rests in a larger
watch glass. The paper is ignited and covered immediately with a 2 liter beaker,
the inner surface of which has been previously moistened with water. (The watch
glass should be sufficiently large to extend beyond the rim of the beaker.) After
the flame has died out, the beaker is permitted to remain in position for 5 min.
The porcelain evaporating dish is removed, and the products of combustion, which
have condensed on the inner surface of the beaker, are washed into the watch glass
with about 10 cc. of distilled water and then poured into a filter.134
Add to the filtrate 1 drop of nitric acid and 1 cc. of 0.1 N silver nitrate
solution. If the oil is free from halogen, no turbidity should result. Since this
method will detect even minute traces of chlorine, it is absolutely necessary to
run a blank.135
Several other tests have been described, such
as the lime test,136 the test employing sodium peroxide,137
and the classical test employing molten metallic sodium.138 The last
named test is perhaps the most sensitive, but it suffers from the inherent disadvantages
of working with metallic sodium.
A special test for the detection of side chain
chlorine in cinnamic aldehyde has been accepted as official in the "National
Formulary” Eighth Edition, Monograph on cinnamic aldehyde. It indicates the
presence of chlorine only when it appears in the side chain. This is not
intended as a general test for the detection of side chain halogen in all compounds.
However, it has proven satisfactory for such synthetics as cinnamic aldehyde.
Procedure:1 To a 1 cc. sample of cinnamic aldehyde add
10 cc. of commercial isopropanol, 1 cc. of nitric acid (1:1) and 1 cc. of 1 0% silver
nitrate solution. Shake the mixture after the addition of each reagent. Heat to
incipient boiling and permit the test tube to stand for 5 min. If chlorine is present
in the side chain, opalesconce or turbidity will result. Carry out simultaneously
a blank in order to assure absence of chlorine in the reagents.
When recording the presence of halogen always
designate the method employed. Also, an estimate of the relative amount of halogen
present should be given; use may be made of such relative phrases as
"strongly positive," "moderately positive," "slightly
positive," "positive :traces” and "negative."
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130 A No. 16 gage wire has a
diameter of 0.065 in.
131 Too intense a heat is to
be avoided since the copper spiral will then fuse and will offer less surface in
the subsequent test.
132 These wires may be used repeatedly.
After many determinations, the wire becomes somewhat porous and well coaied with
oxide. Such wires prove very satisfactory.
183 Mr. Schimmel & Co. April
(1890), 29; October (1904), 57.
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