DETERMINATION OF CINEOLE

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
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
 TABLE 4.14. PERCENTAGE OF CINEOLE
PERCENTAGE OF CINEOLE
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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