6. DETERMINATION OF CINEOLE
Of the numerous methods that have been
proposed for the determination of the cineole content of essential oils, the method
of Kleber and von Rechenberg,100 the method of Cocking,101 and that of
Scammell102 (modifiedby Baker and Smith103) have proved the
most valuable.
According to the Kleber and von Rechenberg
method, the congealing point of the oil itself is determined, from which the cineole
content may be determined by reference to a table or graph. The presence of oxygenated
constituents other than cineole has little effect upon the values obtained; an accuracy
of about dbl per cent may be obtained. "The United States
Pharmacopoeia"104 makes use of
this method to establish a minimum of 70 per cent cineole in official eucalyptus
oils. The main criticism of the Kleber and von Rechenberg method is the
inconvenience of working at greatly reduced temperatures. The exact determination
of a congealing point at a temperature much below often presents difficulty.
Procedure:
Place about 10 cc. of the oil in a heavy walled
tube which is preferably equipped with an air or vacuum jacket. Immerse the tube
in a mixture of ice and salt, or in a cooling bath of solid carbon dioxide in acetone.
The true solidification point is determined ; i.e., the temperature at which the
crystals of cineole first appear as the oil is cooled, and at which the crystals
disappear as the temperature is permitted to rise. Several determinations of the
solidifying point should be made in order to obtain an exact reading. The percentage
of cineole, corresponding to this temperature can be determined directly by reference
to Table 4.13.
TABLE 4.13. DETERMINATION OF EUCALYPTOL
CONTENT BY CONGEALIN POINT
The o-cresol method of Cocking offers certain
advantages. Since the congealing point is well above room temperature, the determination
is greatly simplified. Results are easily reproducible. According to Cocking, the
accuracy is approximately 3 per cent in the case of eucalyptus and cajuput oils.
The o-cresol method has been accepted as the official method of "The British
Pharmacopoeia."106 The procedure as given below is essentially the
official method of "The British Pharmacopoeia."
Procedure: Into a stout walled test tube, about 15 mm.
in diameter and 80 mm. in length, place 3 g. (accurately weighed) of the oil, previously
dried with anhydrous sodium sulfate, together with 2.1 g. of melted o-cresol. The
o-cresol used must be pure and dry, with a freezing point not below 30. It is hygroscopic
and should be stored in a small well-stoppered bottle, because the presence of moisture
may lower the results.
Insert a thermometer, graduated in fifths of a
degree, and stir the mixture well in order to induce crystallization; note the highest
reading of the thermometer. Warm the tube gently until the contents are thoroughly
melted, and insert the tube through a bored cork into a widemouthed bottle which
is to act as an air jacket. The thermometer should be suspended from a ring stand
in such a way that it does not touch the walls of the inner tube. Allow the mixture
to cool slowly until crystallization commences, or until the temperature has fallen
to the point previously noted. Stir the contents of the tube vigorously with the
thermometer, rubbing the latter on the sides of the tube with an up and down motion
in order to induce rapid crystallization. Continue the stirring and rubbing as long
as the temperature rises. Take the highest point as the freezing point. Repeat this
procedure until two readings agreeing within 0.1 are obtained. The percentage of
cineole in the oil can be computed from Table 4.14.
It should be noted that in both methods relatively
high cineole contents give more accurate analytical results. Therefore, if the cineole
content of the oil is low, the determination is best carried out on a mixture of
equal parts (by weight) of oil and pure cineole, (melting point, 1.2 or higher).
In the Kleber and von Rechenberg method, if the cineole content is less than 65
per cent, this modified procedure should be followed; in the o-cresol method, of
Cocking, if less than 50 per cent. The cineole content of the original oil may then
be calculated by means of the following formula :
Percentage of cineole in original oil = 2 X (%
of cineole in mixture - 50)
The phosphoric acid method of Scammell as modified
by Baker and Smith is based on the formation of a solid, loose molecular compound
of cineole and phosphoric acid from which the cineole may be regenerated by the
addition of water. The procedure recommended by these authors is given below :
Procedure:
Place 10 cc. of the oil in a 50 cc.
beaker or other suitable vessel and cool thoroughly in a bath of ice and salt. Slowly
add 4 cc. of phosphoric acid,107 a few drops at a time, mixing the acid
and oil thoroughly between each addition by careful stirring. After all of the acid
has been added, permit the mixture to remain in the bath for 5 min. to insure complete
formation of the cineole-phosphoric acid addition compound. Then add 10 cc. of petroleum
ether (boiling below 50) which
has previously been well cooled in the ice bath
and incorporate well with the aid of a flat ended rod. Immediately transfer the
mixture to a cooled Buchner funnel, 5 cm. in diameter. Filter off the noncombined
portion rapidly with the aid of a water pump. Transfer the cake from the Buchner
funnel to a piece of fine calico and spread the cake with a spatula so that it covers
an area of about 6 X 8 cm. Fold over the calico into a pad and place between several
layers of absorbent paper. Press well for 3 min. Break up the cake on a glazed tile
with a spatula and transfer to a cassia flask. Decompose the cineole-phosphoric
acid addition compound with warm water and force the liberated cineole into the
neck of the flask by the further addition of water. After the separation is complete
and the contents of the flask have cooled to room temperature measure the amount
of cineole.
If the cineole content is found to be above 60
per cent, repeat the determination using a sample of the oil diluted with freshly
distilled pinene or turpentine oil : three volumes of oil plus one volume of pinene.
Make the necessary correction in calculating the percentage of cineole.
This test gives satisfactory results with oils
containing as little as 20 per cent cineole and as high as 100 per cent; in the
latter case, the oil must be previously diluted as described. It is important to
add the acid slowly, to have the mixture very cold and to cool the petroleum ether
thoroughly before adding.
Other methods for the quantitative determination
of cineole have been suggested. In their monumental work on the eucalypts Baker
and Smith108 give a brief criticism of many of these methods.
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100 J. prakt. Chem. [2] 101 (1920), 171.
101 Analyst 52 (1927), 276.
102 British Patent 14138 (1894).
103 "Eucalypts and Their Essential Oils," 2nd
Ed., Sydney, 1921, 364.
104 Thirteenth Revision, 217.
105 (1932), 584.
106 The use of calcium chloride for drying the oil as suggested
by "The British Pharmacopoeia" is not to be recommended ; anhydrous
sodium sulfate is much to be preferred, eliminating the possibility of formation
of addition products with primary alcohols that may be present in the oil.
107 If the cineole content is below 30 per cent, add only
3 cc. of phosphoric acid.
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