MYBTACE^E 333
The value of oil of cloves depends almost entirely on the percentage of
the phenol eugenol (especially when it is required for vanillin manufacture),
and as will be seen from the following paragraphs, the method of deter-
mination used should always be stated if results are to be comparative :—
Several methods have been suggested for the determination of the
eugenol, none of which give absolutely accurate results, but from them
useful comparative results may be obtained. Umney suggested the ab-
sorption of the eugenol in a Hirschsohn flask, as used for the examina-
tion of cassia oil, with a 10 per cent, solution of caustic potash. This
strength was found to be too high, and a 5 per cent, or even 3 per cent,
solution is now preferred. The unabsorbed portion has a specific gravity
of about 0*910, so that this may be allowed for (although rarely done in
practice), since the eugenol has a much higher specific gravity. Spurge
prefers to saponify the esters first, and to make an allowance for them,
but if a 5 per "cent, solution be used, this method without any such
allowance gives a close approximation to the total amount of eugenol,
both free and combined being reckoned as eugenol. The globules of
uncombined hydrocarbons have a great tendency to stick round the
top of the flask and require some " coaxing " to rise and agglomerate
in the neck of the flask. Heat will accelerate and assist this, however.
But the source of error lies in the fact that the aqueous solution of
potash and potassium eugenate dissolves some of the sesquiterpene,
which is thus reckoned as eugenol, and a too high result is obtained.
The process proposed by Thorn, although more tedious, gives more
exact results. This depends on the conversion of the eugenol into
benzoyl eugenol. The following are the details, which should be care-
fully observed in order to secure accurate results :—
About 5 grms. of clove oil are weighed into a beaker of about 150 c.c.
capacity, 20 grms. of 15 per cent, sodium hydroxide solution added, and
then 6 grms. benzoyl chloride. On stirring, the solid mass of eugenol
sodium salt at first formed goes into solution again as it is converted
into benzoic ester, with evolution of much heat. In the course of a few
minutes the reaction ends, and on cooling a solid crystalline mass of
benzoyl eugenol is obtained. To this, 50 c.c. water is added, and the whole
warmed on a water-bath until the ester is completely melted to an oil,
well stirred, cooled, and the clear supernatant aqueous solution filtered
off. The crystalline mass is again washed with two successive quan-
tities of 50 c.c. water, and the resulting impure benzoyl eugenol is re-
crystallised from alcohol, due allowance being made for its solubility in
that medium. 25 c.c. of hot alcohol (90 per cent, by weight strength)
are poured through the filter employed in the previous washing opera-
tions, in order to dissolve any adherent crystals, into the beaker, and
the whole warmed upon the water-bath until complete solution is
effected. The solution is then cooled to 17° C., and the crystalline
precipitate thrown upon a small weighed filter paper, filtered into a
25 c.c. cylinder, and washed with 90 per cent, alcohol until the filtrate
exactly measures 25 c.c. The filter and crystals are then removed to
a weighing bottle, dried at 100° C. until constant, and then weighed.
From the total weight the weights of the filter paper and of the weigh-
ing bottle are deducted, from which the benzoyl eugenol is calculated.
To the latter weight 0 * 55 grm. are added, being the weight of pure
benzoyl eugenol dissolved by 25 c.c. 90 per cent, alcohol at 17° C. as
determined by experiment.