422 THE CHEMISTKY OF ESSENTIAL OILS
Solution of Oil of Lemon.—Weigh from 1 to 2 grms. of the oil and
dilute to 50 c.c. with 50 per cent, alcohol.
Place 2 c.c. of the standard citral solution in a 250 mm. colorimeter
tube (using preferably a Schreiner colorimeter), add 20 c.c. of 65 per
cent, alcohol and 15 c.c. of diaminophenol reagent, and make up to 50
c.c. with 65 per cent, alcohol. Place 2 c.c. of the oil solution in the
other tube with 15 c.c. of the reagent and make up to 50 c.c. with 65
per cent, alcohol, mixing the contents of both tubes thoroughly and
allowing them to remain for five to ten minutes at rcom temperature,
when the maximum colour is reached. The reading and calculation
are made at once, or a reading can be made at the expiration of fifteen
or twenty minutes in duplicate. The calculation of percentage of citral-
content in the extract is made by placing the standard citral tube at the
30 mm. mark and adjusting the tube containing the extract under ex-
amination so that the two small disks of colour as observed through the
two immersion tubes are similar in tint.
C. Kleber's
1
- method, which gives very fair results, depends on the
use of phenyl-hydrazine.
Two c.c. of oil, accurately weighed, are mixed with 10 c.c. of 2 per
cent, alcoholic phenyl-hydrazine solution, and allowed to stand one
hour in a glass-stoppered bottle of 50 c.c.; then 20 c.c. of deci-normal
hydrochloric acid and 10 c.c. of benzene are added, and the whole
shaken thoroughly. The mixture is transferred to a separator, and the
lower layer of 30 c.c. run through a small filter. Twenty c.c. of filtrate
are titrated, after the addition of ten drops of ethyl orange 1: 2000, with
deci-normal caustic potash to distinct yellow coloration. By a similar
blank experiment without oil the acid equivalent of the hydrazine solu-
tion is determined. From the difference between the two experiments
the citral-content is calculated, each c.c. of deci-normal caustic potash
corresponding to 0'0152 grm. citral. A number of analyses made in
this way with mixtures of definite quantities of citral with tsrpenes
were found to be well in accordance with the calculated results.
An absorption method depending upon the formation of a soluble
sulphonic acid with sodium sulphite using phenolpbthalein as an indi-
cator, gives fairly good results :—
C 9 H 15 CHO + 2Na 2 SO 3 + 2H 2 O
= C 9 H 17 (NaSO 3 ) 2 COH + 2NaOH.
Prof. Tiemann, in his elaborate work on the aldehydes of lemon-
grass oils has, among other important reactions, shown that if citral be
agitated with a neutral solution of sodium sulphite, a sodium salt of the
sulphonic acid results, as shown by the above formula. By taking ad-
vantage of the sodium hydrate formed by this reaction, phenolphthalein
is used as an indicator to show when all the citral has been combined;
and the fact that the above sodium salt is quite soluble, has made the
estimation of citral fairly accurate and simple. The procedure is as
follows: A saturated solution of sodium sulphite is prepared. To
50 c.c. of such solution in a Hirschsohn flask, 25 c.c. of the oil are
added, and two drops of an alcoholic solution of phenolphthalein. The
whole is then heated on a water-bath to nearly boiling-point, constantly
shaking. A deep red colour appears almost at once, which shows that
the action has commenced. A few drops of sulphurous acid are then
cautiously added, and this is continued until no further colour is pro-
1
Schimmel's Bericht, April, 1912, 64.