EUTACE.E^423
duced after a further addition of SO 2. The oil is then measured by add-
ing sufficient sulphite solution to drive the oil into the neck of the flask.
The obvious advantage of this method is that the end of the reaction
may be ascertained to a certainty, while the above bisulphite method
depends on the continual shaking for a period of not less than one hour.
The only objection to this process is that the reading of the meniscus
in the neck of the flask is a little difficult owing to the presence of
flocculent matter.
Numerous other methods for the determination of citral in lemon oil
have been proposed, but the foregoing may be taken to cover the most
accurate, and to embrace all that are likely to be of any practical value.
E. M. Chace l has published an elaborate study on the detection of
traces of pinene in lemon oil, deducing from the presence of even a very
FIG. 39.—Photomicrograph of crystals from lemon oil ( x 100).
a, 6, Limonene nitroso-chloride crystals from lemon oil; c, Limonene and
pinene nitroso-chloride crystals from a lemon oil mixed with 5 per cent, of turpen-
tine ; d Pinene nitroso-chloride crystals from turpentine.
minute quantity in the oil that turpentine is present. As pinene ap-
pears to be a natural constituent of the oil and adulteration with tur-
pentine would not be practised except on a scale that would render it
remunerative, there appears to be no doubt that Chace's deductions are
inaccurate, and the presence of traces of pinene cannot be held to in-
dicate adulteration of lemon oil.
Chace points out that the nitrosochlorides of pinene and of limonene
crystallise in different forms, and makes use of this property to detect
pinene. For this purpose he distils, in a Ladenburg flask, 5 c.c. from
50 c.c. oil, and from this he prepares the nitrosochloride with the use of
ethyl nitrite according to Wallach's method. The mixture is left for
fifteen minutes in a freezing mixture; the crystals which separate off
are then collected, washed with 50 c.c. 95 per cent, alcohol, the mother-
lJour. Amer. Chem. 8oc.t 30 (1908), 1475.