TESTS FOR HALOGENS (The Essential Oils)


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."
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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