A number of volatile oils contain as important constituents methyl and ethyl ethers of phenols and acids, the alkyl groups of which can be determined according to Zeisel's method.1) Bene-dikt and Grussner2) have recommended the quantitative methoxyl determination for the practical and scientific investigation of volatile oils, and have shown its usefulness by a series of illustrations. They designate as methyl number that number which tells how many mg. of methyl 1 g. of substance splits off when boiled with hydriodic acid. Ethyl or propyl and isopropyl are here considered as having been replaced by the equivalent amount of methyl. The weighed amount of silver iodide is therefore in all cases calculated as methyl.
The vapors of methyl iodide formed by boiling from 0,2 to 0,3 g. of the oil to be investigated with hydriodic acid (sp. gr. 1,70, to which, according to Herzig3) 8°/o of acetic acid are added) are first passed through some warm water in which some phosphorus is suspended, so as to retain any iodine vapors which may have been carried over. After the methyl iodide has passed through this apparatus it is passed into an alcoholic solution of silver nitrate and the separated silver iodide weighed.
1) Monatsh. f. Chem. 6 (1885), 989.
2) Chem. Ztg. 13 (1889), 872, 1087. 3) Monatsh. f. Chem. 9 (1888), 544.
A very convenient apparatus has been designed by L. Ehmann *) for carrying out this determination.
Gregor2) has recently suggested to replace the phosphorus suspended in water by a solution of one part each of potassium bicarbonate and arsenous acid in 10 parts of water, by which not only the iodine vapors but also any hydriodic acid carried over is retained. For collecting the methyl iodide Gregor uses a Vio normal silver nitrate solution which has been acidified with nitric acid and titrates back the silver not used for the precipitation of the silver iodide with 1/10 normal potassium sulphocyanate solution according to Volhard.
Of the oils investigated by Benedikt and Griissner the following gave no methyl numbers: the oils of wormwood, bitter almond, angelica, bergamot, caraway, lemon, copaiba-balsam, coriander, cubeb, elemi, eucalyptus, geranium, juniper, cherry laurel, lavender, spearmint, peppermint, olibanum, oil of Pinus montana, savin, East Indian and West Indian sandalwood, turpentine and valerian.
High methyl numbers were given by anise oil, star anise oil and fennel oil on account of their content of anethol and methyl chavicol, by clove oil, clove stem oil and oil of cinnamon leaves on account of their content of eugenol. In wintergreen oil the high methyl number is due to the methyl salicylate, in parsley oil to the apiol, and in calamus oil to its asarone3) content.
The determination is applicable only to oils absolutely free from alcohol, as ethyl alcohol itself gives a methyl number, from which it follows, that this method may also be used for the quantitative estimation of alcohol in those oils which in their pure condition do not contain any methoxyl groups.4)
1) Description and illustration of the apparatus are to be found in Chem. Ztg. 14 (1890), 1767.
2) Monatsh. f. Chem. 19 (1898), 116. 3) Thorns and Beckstroem, Berl. Berichte 85 (1902), 3191.
4) The methyl numbers of some of the oils examined by Benedikt and Griissner can be explained only by assuming the presence of alcohol. It is to be regretted that the physical constants of the oils were not recorded so that some judgement might be passed on their purity. For the Ceylon cinnamon oil marked No. 22 in the paper of Benedikt and Griissner they find 25,7 as methyl number and compute therefore a 28,1 p. c. eugenol content. In as much as pure Ceylon cinnamon oil contains only 4 to 8 p. c. eugenol, it would seem to follow (provided the methyl number found was due only to eugenol) that the oil had been adulterated with cinnamon leaf oil rich in eugenol. This example reveals that the methyl numbers reported should be adopted cautiously. It would be desirable indeed to repeat the determinations in connection with oils the purity of which has previously been established by other means. Only thus can desirable data be obtained which will give this method practical value. For the scientific investigation of volatile oils the methoxyl determination should prove exceedingly valuable since it reveals the presence or absence of phenol ethers or acid esters containing methyl, ethyl or propyl groups.