8. TEST FOR PHENOL IN METHYL SALICYLATE
Methyl salicylate which has been insufficiently
purified will frequently contain phenol. The presence of this impurity affects markedly
the odor and flavor of the synthetic. Hence, it is well to test all samples of methyl
salicylate for phenol. For routine analyses the following simple procedure has
proved quite satisfactory :
Procedure
I: Dissolve 5 cc. of the oil
in 50 cc. of a 1 N aqueous potassium hydroxide solution. Heat on a steam bath for
2 hr., cool to room temperature, and acidify with suifuric acid (1:3). Cautiously
smell the flask for the distinct characteristic odor of phenol. If no such odor
is apparent the sample may be considered free of objectionable amounts of phenol.
If a phenolic "by-note" is observed, the presence of phenol should be
confirmed by the method of Dodge. (See "Procedure II.")
The Dodge method151 for the
detection of phenol in methyl salicylate has proved of value as a qualitative method.
Attempts have been made to convert this method to a quantitative procedure. However,
these have proved unsatisfactory, particularly when applied to the natural oils
of sweet birch and wintergreen.
Procedure
II: Into a 100 cc. Pyrex saponification
flask introduce 10 cc. of the oil in question, and add 35 cc. of a 10% aqueous solution
of sodium hydroxide, measured from a graduated cylinder. Connect an air-cooled
reflux condenser and heat the flask on a steam bath for 2 1/2hr. Remove the flask
and allow it to cool for 15 min. Neutralize the saponified mixture with dilute hydrochloric
acid (1 : 3) until the solution is distinctly acid to blue litmus paper ; this requires
from 3.5 to 5 cc. of acid. The hydrochloric acid should be added slowly from a
burette so that no precipitation occurs. Then slowly add from a burette enough of
a saturated, freshly prepared sodium bicarbonate solution to just neutralize the
mixture, and then an additional 0.5 cc. of the sodium bicarbonate solution.
Filter into a 500 cc. distillation flask and distill with steam, using an efficient
trap to prevent the mechanical carrying over of any of the solution. A 500 cc. Erlenmeyer
flask may conveniently be used for this trap; the delivery tube from the side arm
of the distillation flask should extend to within in. of the bottom of the
Erlenmeyer flask, and the delivery tube from the trap to the condenser should not
extend more than 1 in. below the rubber stopper into the flask152 (see Diagram 4.13).
Collect three 5 cc. portions of distillate and filter each distillate. Test for
the presence of phenol by the addition of enough bromine water to give a permanent
light brown color. If phenol is present to the extent of 0.01% or more, a crystalline
precipitate of tribromophenol (melting point, 95o) will form within an
hour.
Procedure II is based upon the well-known fact
that acids will react to form the corresponding sodium salts, but phenols will not
form the corresponding phenolates when treated with sodium bicarbonate solutions.
Thus the free phenol is steam distilled out of
the solution in which is dissolved the nonvolatile sodium salicylate. Normal, pure
wintergreen and sweet birch oils do not give a positive reaction with this procedure;
certain constituents of the oils will distill which are capable of decolorizing
the bromine water, but which do not form crystalline derivatives under the conditions
outlined. Hence, a positive test for such oils indicates adulteration with
phenol-containing synthetic methyl salicylate. The test is to be considered positive
only when definite crystal formation is observed within 1 hr. at room temperature
in the bromine treated distillates.
----------------------
150 Ibid. m
151 Drug Markets 24 (1928),
509.
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