5. TESTS FOR IMPURITIES IN NITROBENZENE
a. Test for Thiophene.
Nitrobenzene
which has been manufactured from an impure grade of benzene will give a positive
thiophene test, if insufficiently purified. This is due to the fact that inferior
grades of benzene contain thiophene, SCH : CHCH : CH, and that all thiophene compounds
give an intense blue coloration when mixed with isatin, C6H4NHCOCO, and
concentrated sulfuric acid, because of the formation of indophenin, (C12H7NOS)x.
Procedure:143 Shake thoroughly 5 cc. of nitrobenzene and 0.5 cc. of concentrated sulfuric
acid, in a test tube, and add a pinch of isatin, and again shake the mixture thoroughly.
Permit the test tube to stand for 2 hr. No blue coloration should appear during
this interval.
b. Soap Test.
The soap test144 is
an empirical method of testing the purity of nitrobenzene. Since a large quantity
of this synthetic is used to perfume soaps, it is necessary to carry out a soap
test to determine whether or not the nitrobenzene in question will cause a discoloration
of the soap.
Procedure: Into a large, wide Pyrex test tube of approximately
75 cc. capacity introduce 5 cc. of the nitrobenzene and 10cc. of a 15% aqueous solution
of potassium hydroxide. Heat the mixture to boiling over an open flame. It is important
to shake thoroughly the test tube while the mixture is heated and boiled, in order
to prevent the formation of two layers. (The nitrobenzene and potassium
hydroxide solution will then be thrown out of the test tube with explosive violence.)
After boiling for 2 min., permit the test tube to stand, at room temperature, for
one-half hour, and then filter the mixture through filter paper previously wetted
with water. The potassium hydroxide solution passes through the wetted paper;
the nitrobenzene is retained. The alkaline filtrate should be colorless, or at most
show only a light yellow color. A full deep yellow indicates that the
nitrobenzene has been insufficiently purified or is old. Such a product will require
rectification before it is satisfactory for use in soaps.
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143 This is a modification
of the test described in "A. C. S. Analytical Reagents," American Chemical Society, Washington,
D. C., March (1941), 38.
144 Gildemeister and
Hoffmann, "Die atherischen Ole," 3d Ed., Vol. I, 673.
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