120 THE CHEMISTEY OF ESSENTIAL OILS
determination of the citral-content. A great many methods have been
suggested for this, most of which yield only approximate results.
From a lengthy experience in these processes, the author has no
hesitation in saying that the most accurate method fo*r the estimation of
citral in lemon oil is that devised by J. Walther,
1
with a slight modifica-
tion suggested by A. H. Bennett.^2
This process depends on the formation of an oxime with hydroxy-
lamine in alcoholic solution, and determination of the amount of hydroxy-
lamine used in the process, according to the equation :—
C 9 H 15 CHO + H 2 NOH = C 9 H 15 : NOH + H,O.
The process is carried out as follows : A mixture of 20 c.c. of lemon
oil, 20 c.c. of 90 per cent, alcohol, 20 c.c. of a 5 per cent, solution of
hydroxylamine hydrochloride in 80 per cent, alcohol, and 8 c.c. of normal
alcoholic potash solution is gently boiled for thirty minutes under a
reflux condenser. A similar mixture, without the lemon oil, is treated
in exactly the same manner. After cooling, the condensers are washed
down with water to restore any traces of hydroxylamine to their respec-
tive flasks, and the contents of the flasks diluted with about 200 c.c. of
water. A few drops of phenolphthalein are added to the flasks, and the
contents first neutralised with semi-normal alkali. The hydroxylamine
is now titrated with semi-normal sulphuric acid solution, using methyl
orange as indicator. A few drops of the indicator should be added to the
flask, so that a general indication of the approach of the end reaction is
given, but the end reaction itself should be observed by the familiar pro-
cess of " spotting " on a white tile, on drops of the indicator. Each cubic
centimetre of half-normal acid used in the blank experiment, over that
used in the assay experiment, is equivalent to 0
- 076 grm. of citral.
A process depending on the use of cyanacetic acid was proposed by
the author
3
some years ago, which depends on the condensation of
cyanacetic acid with citral in alkaline solution, according to the following
equation:—
ON
C 9 H1SCHO + CH 2 CN. COOH = C 9 H 15 CH. C<f , TTnN
COOH
The procedure is as follows : 200 c.c. of the oil are concentrated to
25 c.c. in a Wurtz flask under a pressure which should not exceed 15
mm. The 175 c.c. of terpenes contain a little citral, but the quantity is
so small that it may be neglected. Ten c.c. of the residue, of which the
specific gravity is known, are shaken with a solution of 5 grms. cyan-
acetic acid and 5 grms. ordinary KOH in 30 c.c. water in a small
absorption flask with a long graduated neck, and the non-absorbed
residue read off as usual. (The specific gravity of citral is so near that
of the concentrated oil used that it may be taken as identical.)
This process does not give satisfactory results, and is only quoted
here so that the author may definitely withdraw it as a process as-
sociated with his name.
E. M. Chace^4 has elaborated a method for the determination of citral
depending on the fact that fuchsine-sulphurous acid develops a red
coloration with aldehydes, the depth of which depends on the amount
*Pharm. Zentral., 40 (1899), 621 ; 41 (1900), 585. 2Aiwlyst, 34 (1909), 14.
3
Chemist and Druggist, 56 (1900), 376. 4 Jour. Amer. Chem. Soc., 28, 1472