Dimethyl sulfide occurs as a normal constituent in peppermint oils. Upon rectification of the oil obtained by steam distillation from the plant, most of this volatile compound is lost because of its low boiling point (boiling point = 37.5o-38o). Hence, the presence of dimethyl sulfide in a peppermint oil is an indication that such an oil has not been rectified. "The United States Pharmacopoeia"141 has made use of the following procedure to assure the absence of dimethyl sulfide found in nonrectified oils:
Apparatus for the determination of Di-methyl sulfide
DIAGRAM 4.12. Apparatus for the determination of Di-methyl sulfide.
Procedure: Distill 1 cc. from 25 cc. of peppermint oil and carefully superimpose the distillate on 5 cc. of a 6.5% mercuric chloride solution in a test tube. A white film does not form at the zone of contact within 1 min.
This test is based upon the reaction of dimethyl sulfide with mercuric chloride, giving a white sulfonium compound which is insoluble in saturated mercuric chloride solution. The following modification142 of this official test is more sensitive and somewhat more reliable:
Dry the oil by shaking thoroughly with a small amount of anhydrous sodium sulfate in a stoppered bottle and filter. Place 25 cc. of this dried and filtered oil in a large Pyrex test tube (diameter = 22 mm., length = 200 mm.) which is clamped to a ring stand at an angle of approximately 45. Add a small piece of clay chip. Insert a tight-fitting cork equipped with a bent glass tube which extends 2 cm. through the cork. The other leg of this bent tube is inserted into a second test tube which contains 5 cc. of a 6.5% aqueous solution of mercuric chloride (see Diagram 4.12). The tube should not dip into the solution, but should extend to within 1 cm. of the surface. Apply gentle heat until the oil begins to boil. Heating is continued until the ring of condensing vapor rises to within 1 cm. of the end of the glass tube. If the oil has been carefully dried and the heating has been carried out slowly, no oil will distill over into the second test tube. The formation of a white scum on the surface of the mercuric chloride solution or on the sides of this second test tube indicates the presence of dimethyl sulfide in the oil.
141 Thirteenth Revision, 390 (Peppermint Oil).
148 This modification has proved satisfactory during the last five years in the laboratories of Fritzsche Brothers, Inp.

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