498 THE CHEMISTRY OF ESSENTIAL OILS
cooled down to the temperature of the room, filled up with water to the
mark, shaken up, and filtered. Fifty c.c. of the filtrate are titrated with
one-tenth normal solution of ammonium sulphocyanide, after adding 6 c.c.
nitric acid (d1&o T153) and a small quantity of solution of iron alum, until
a change of colour from white to red takes place. In order to ascertain
the whole quantity of silver solution which has entered into reaction, the
number of c.c. of ammonium sulpho-cyanide solution used up is doubled,
and the product subtracted from 50. The percentage of allyl iso-thio-
cyanate in the mustard oil is obtained by means of the following formula:—Per cent. CSNC 3 H 5 =a — number of c.c. of deci-normal solution of silver nitrate used up, b =
spirit of mustard used, in grams. Mustard oil determinations carried out
by them in the manner described showed in the case of natural oil a con-
tent of about 94 per cent, allyl iso-thiocyanate, whilst in artificial oil about
98 per cent, was determined.
Kuntze l has examined the differences to be observed in the results
obtained by the gravimetric and the volumetric processes above described,
those of the gravimetric process being higher than those of th3 volumetric
processes.
Kuntze finds that the precipitate of silver sulphide, which is formed
by both methods under the same conditions, is always contaminated with
more or less silver salt of allyl-thiocarbaminic acid ester.cso that the ester which is always present even in freshly-prepared alcoholic
solutions of mustard oil does not decompose quantitatively with ammonia-
<cal silver solution, with formation of sulphide of silver, but is partly pre-
cipitated as a silver salt of the above-mentioned ester. Whilst this
admixture, owing to its molecular weight 252, which differs but little from
that of silver sulphide 248, cannot cause an appreciable error in the
gravimetric determination, it caus s a considerable difference in the volu-
metric estimation, because in the formation of this ester salt only 1 atom
of silver corresponds to 1 molecule of mustard oil, whereas for the con-
version into silver sulphide, 2 atoms of silver are required, so that the
titration results are obviously too low.
Kuntze has found that the separation of the ester salt can be avoided
by immediately heating the alcoholic mixture for one hour, because the
salt is unstable at a high temperature, and proposes the following volu-
metric method to deal with this difficulty:—
Five c.c. of the alcoholic solution of mustard oil are mixed in a graduated
tube of 100 c.c. capacity with 10 c.c. of solution of ammonia and 50 c.c.
deci-normal silver nitrate solution, and the whole is immediately heated
for one hour on a briskly boiling water-bath under a reflux condenser.
After cooling to 15°, and filling up to the mark with distilled water, add-
ing nitric acid until a feeble acid reaction takes place, and 1 c.c. ferri-
ammonium sulphate solution, deci-normal ammonium thiocyanate solu-
ti'on should be added to the clear filtrate, until a red coloration is produced.
1
Arch, der Pharm., 246 (1908), 58.