Note on U. S. P. Assay For Oil of Peppermint

A.B. Lyons
1919 The Journal of the American Pharmaceutical Association (1912)  
The requirement of the U. S. P. for Oil of Peppermint is that it shall contain "not less than 5 percent of esters calculated as menthyl acetate and not less than 50 percent of total menthol, free and as esters." The assay for esters is sound, being made by the saponification process. The effect of saponification upon the oil is to increase its weight by 2 6 percent of the weight of menthyl acetate present. It is not a matter of serious consequence that the ester actually present does not
more » ... ent does not consist wholly of menthyl acetate. For the purposes of such an assay the difference is quite negligible. For the second determination an indefinite quantity ( 1 0 mils) of the oil is acetylized in the usual manner, washed and dried. Of this oil, containing the whole of the menthol in the form of menthyl acetate, a weighed portion is taken for saponification. The result of titration shows how much half-normal alkali is required to neutralize the acetic acid which exists in combination with menthyl, each mil of the standard alkali corresponding with o 07808 Gm. of menthol or o . 09909 Grn. of menthyl acetate. The difference hetwcen these represents the difference in weight due to acetylization, viz., o 0 2 1 0 1 Gm. The formula given in the U. S. P. for calculating the result of this assay introduces this factor (0.021), but applies it to the whole of the menthol, making the weight of the oil taken = B --(A X 0.02 I), B being the weight of the acetylized oil taken and A the number of mils of volumetric (half-normal) alkali consumed in neutralizing the acid of acetylization, including that of the esters originally present. This formula will give the percentage of the oil after saponification of the esters naturally present--not that of the original sample. '1'0 find the true percent we must refer to the first step in the assay, which determines the percent of esters originally present in the oil -or by substituting for 9.909 the factor 7.808, the percent of menthol corresponding to those esters. We must therefore deduct from the (A x 0.021) of the formula a compensating correction, which in case of an oil containing about 50 percent of total menthol will make the expression (A --C + 2.5) x 0.021. The compensating quantity will, however, vary with the percent of total menthol, so that this would serve only as an empirical formula, by which, however, we could determine with certainty whether or not a given sample contained "not less than 50 percent of total menthol." The amended formula would read: A X 7 . 8 0 8 13 -(A-C + 2.5 ) X 0.021 Percentage of total menthol = If the percent of menthol differs materially from that of the pharmacopoeia1 standard the formula may be amended by substituting for the divisor 2 . 5 the expression 2 + M, in which M stands for the percent of total menthol in the sample by U. S. P. formula. The result reached is a trifle less than the correct figure, but the difference is negligible. The following is a general formula which will seem simpler, and will give a
doi:10.1002/jps.3080080103 fatcat:tvcmu3rqcfdnnh5nelctwmpphm