TEST FOR PHENOL IN METHYL SALICYLATE (The Essential Oils)

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.
Apparatus for the detection of Phenol in Methyl Salicylate

 DIAGRAM 4.13. Apparatus for the detection of Phenol in Methyl Salicylate.
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.
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150 Ibid. m
151 Drug Markets 24 (1928), 509.

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