338 THE CHEMISTEY OF ESSENTIAL OILS
The method has been criticised by H. G. Harding,
1
who states that
•a pure white oil from Eucalyptus dives, which contained no cineol,
showed an absorption of 32 per cent, by the resorcinol test. Kectifying
the oil and applying the test to the portion distilling between 170° and
190° does not entirely remove the difficulty, as the results are always
slightly high, owing to retention of other constituents by the resorcinol
solution. His experiments show that the method is more reliable when
the percentage of cineol does not exceed 40 to 50 per cent., oils contain-
ing a higher percentage requiring to be diluted. Ordinary turpentine
oil must not be used for dilution, as it is likely to produce serious errors.
By employing the fraction of turpentine distilling between 156° and 160°
for dilution the error is minimised, and the following method is recom-
mended : 100 c.c. of the oil to be tested is distilled, the portion distilling
between 170° and 190° collected, and this is diluted to 100 c.c. with the
turpentine fraction. If a trial shows the percentage of cineol to be
above 70 per cent., the cineol fraction is further diluted with the tur-
pentine so that the percentage is not over 50. The temperature is
noted, and 6 to 10 c.c. is shaken with warm 55 per cent, resorcinol
solution. After five minutes' shaking more solution is added so as to
bring the oil into the graduated neck. It is then cooled and the volume
read.
A method for the determination of cineol has recently been proposed
by Dodge.^2
The process is based on the fact that the terpenes of the essential
oils to be examined are readily oxidised at 0° C. by a 5 to 6 per cent,
solution of potassium permanganate, whilst the eucalyptol remains un-
attacked. Ten c.c. of essential oils are gradually added to 400 or to 100 c.c.
of permanganate solution, according to whether the essential oil is more
or less rich in terpenes. When the reaction is finished, the solution
is allowed to stand in the cold for twelve to eighteen hours, with occa-
sional agitation ; sulphurous acid or a mixture of sodium sulphite and
hydrochloric acid is added and the oily portion of the residue is brought
iup into the neck of the flask, from which it is removed by means of a
slender pipette. This oil is washed with a little alkali, then transferred
to a graduated tube where its volume is determined which indicates the
percentage of eucalyptol.
This method cannot be relied on, as the action of potassium per-
manganate is such as to react with some and not with other bodies
present in various types of eucalyptus oil.
The whole question of cineoi determination in eucalyptus oil has
recently been carefully studied by Turner and Holmes,
3
who .consider
that all methods so far published are either inaccurate or at best only
approximate. They suggest the following process:—
The determination of cineol in cineol-bearing oils by means of
arsenic acid is carried out as follows:—
Deliver from a pipette 10 c.c. of the oil into a glass dish (preferably
a round-bottom one) of 50 c.c. capacity, which is imbedded in finely
cracked ice. Add 10 c.c. of concentrated arsenic acid (containing about
j85 per cent, arsenic acid), and stir until precipitation is complete.
When the mixture ceases to congeal further, allow to stand ten minutes
in the ice. At this point if the mixture forms a hard mass, indicating
1
Analyst (1914), 475.
2
Jour. Ind. and Eng. Chem., 4, 592.
3
P. and E.O.R. (1915), 21.