424 THE CHEMISTEY OF ESSENTIAL OILS
liquor again placed in the freezing mixture for fifteen to twenty minutes,
and the crystals which are thus still formed are added to those first ob-
tained. The crystals, well washed with alcohol, are, after drying, dis-
solved in as little chloroform as possible, and a little hot methyl alcohol
is then added so that on cooling crystallisation takes place. Finally, a
little more methyl alcohol is added, and the crystals are then filtered off.
FOP the examination under the microscope the crystals are best embedded
in oil. In this manner Chace claimed to be able to detect an admixture
of even 2 per cent, turpentine oil, whilst as little as 0*5 per cent, turpen-
tine oil could be detected when a three-bulb Glinsky fractionating head
was employed.
Bocker l has elaborated the following method for the determination
of the hydrocarbons present in the concentrated or so-called terpeneless
and sesquiterpeneless oils of lemon of commerce :—
Citral is first estimated in 10 c.c. of oil by the sulphite method. When
the oil which has not entered into reaction is less than 6 c.c., the estima-
tion is repeated with another 5 or 10 c.c. The oil left over from these
tests is bulked, and 5 c.c. of this citral-free oil is placed in a 600 to
700 c.c. separator, into which, immediately previously, 500 c.c. of alcohol
of precisely 51 per cent, by volume, cooled down to from 0 to - 2° C.,
has been introduced. The separator is closed with a cork and the con-
tents are repeatedly shaken, when the aromatic bodies of the oil are dis-
solved by the alcohol, while the hydrocarbons are left behind almost
quantitatively. The separator is then placed in a cooling-bath at 0° C.,
cork downwards, for six to ten hours. It is then taken out of the freez-
ing mixture, carefully turned back to its proper position and placed in a
stand. When the alcoholic solution has cleared to a point at which only
a slight film remains (which in certain conditions may take up to two
days) it is drawn off to about 10 c.c., any oil-drops which may still adhere
to the sides of the funnel being rinsed down with ice-cold 51 per cent,
alcohol, so that all the oil which has remained undissolved is brought
together. When the mixture has become perfectly clear, the oil, being
freed as far as possible from the last traces of alcoholic solution, is trans-
ferred to a measuring tube calibrated to ^ c.c, and the vessel rinsed out
with a little more ice-cold 51 per cent, alcohol. It is advisable to use a
separating funnel of which the lower part ends in a narrow graduated
tube. As soon as the oil is completely cleared (when the froth is very
persistent a few drops of dilute acetic acid should be added), the volume
is read off and calculated to the original oil.
The method may be completed by estimating not only the quantity
but also the character of the hydrocarbons. Thus, from 100 to 200 c.c.
of oil is fractionally distilled in vacuo. The distillation is only continued
up to the point where the separate fractions of about 10 c.c. each still
show dextro-rotation. These fractions are put together as the " terpene-
containing portion " of the oil, the rest constituting the " sesquiterpene-
containing portion ". Each of these portions is then freed from citral
by means of Na 2 SO 3 , and in both cases the residue is treated with 100
times its quantity of ice-cold 51 per cent, alcohol, as described above, a
large glass flask being, if necessary, substituted for the separator. The
alcohol solution is separated by means of a siphon from the portion which
has remained undissolved. The oils which have been separated are
estimated quantitatively, and from the values thus obtained the per-
] Jour. Prakt. Chem. (1914), 89, 199.