A laboratory distillation of essential oil from plant material is often necessary in order to evaluate raw material to be used for large-scale com- mercial distillations. The determination of the essential oil content is also important in appraising the quality of spices and oleoresins.
Such determinations may be conveniently carried out in a special apparatus devised by Clevenger.163 This apparatus offers the following advantages: compactness, cohobation of distillation waters, the actual distillation and separation of the essential oil (so that certain chemical and physical properties may be determined, and so that the odor and flavor of the oil may be studied) and an accurate determination of the essential oil content using only small quantities of plant material. Furthermore, this apparatus may be used to advantage for steam rectification of small amounts of essential oils.
Apparatus fur the determination of the volatile oil content of plant materials

 DIAGRAM 4.14. Apparatus fur the determination of the volatile oil content of plant materials.
The apparatus consists of two specially designed oil traps and a small condenser of the "cold-finger" type154 (see Diagram 4.14). Two traps are supplied; one, for oils lighter than water; the other, for oils heavier than water. The diagrams are self-explanatory.
Procedure: Place a sufficient quantity of the ground or chipped material156 to yield 2 to 6 cc. of oil (preferably 4 cc.) in a round bottom, short-necked flask of 2 liter capacity. Add sufficient water to the flask to correspond to 3-6 times the weight of the plant material; in general, 4 times the weight is sufficient. Attach the proper essential oil trap and the condenser to the flask, and add enough water to fill the trap. Place the flask in an oil bath, heated electrically or by a Bunsen burner to approximately 130o. Adjust the temperature of the bath so that a condensate of about 1 drop per sec. is obtained.
Continue the distillation until no further increase of oil is observed. Usually 5 to 6 hr. are sufficient, although in the case of the distillation of certain woods and roots a much longer period may be necessary. When the distillation has been completed, permit the oil to stand undisturbed so that a good separation is obtained, and so that the oil may cool to room temperature. Determine the number of cc. of oil obtained, and express the yield as a volume/weight percentage; i.e., number of cc. of oil per 100 g. of plant material.
In the event that a good separation is not obtained, the oil and water may be withdrawn from the trap into a graduated cylinder; the addition of sufficient salt to saturate the aqueous layer often aids in obtaining a sharp separation. Periodic withdrawal of the oil and water into such a cylinder is sometimes necessary when the oil being distilled has a specific gravity close to that of water, or when the oil consists of two main fractions one lighter than water, and the other heavier than water.
The volume/weight may be converted into a weight/weight relationship by means of the following formula :
P = pD
where: P = wt./wt. percentage;
           p = vol./wt. percentage at temperature to;
           D = density of oil at temperature to.
This necessitates the determination of the specific gravity of the separated oil. It is advisable to permit the separated oil to remain overnight in an uncorked bottle before evaluating the odor and flavor. Freshly distilled oils often have a peculiar weedy note, which soon disappears. The yield and the physicochemical properties of the resulting oil will agree closely with the results of a commercial distillation. However, the oil from a commercial or pilot still is generally superior to that obtained in the Clevenger apparatus in respect to odor and flavor. improved. The distillation should be carried out immediately after the material has been ground to prevent loss of oil by evaporation. The sample to be examined should be representative of the lot or shipment in question.
When determining the essential oil content of oleoresins, it is best to bring the water to incipient boiling before adding the oleoresin156 and to distill at a faster rate. The addition of clay chips and boiling tubes will prevent undue bumping.
152 The use of an efficient trap is mandatory to prevent any sodium salicylate from being carried over mechanically into the distillate.
153 Am. Perfumer 23 (1928), 467.
154 An Allihn type condenser having at least four bulbs is often substituted to prevent loss of oil and water vapor. However, if a moderate rate of distillation is maintained, the "cold-finger" condenser proves satisfactory, since the thin layer of condensed water forms an effective seal.
155 It is very important that the material be ground or chipped into small pieces, especially in the case of woods, roots, and berries. The yield of oil is greatly increased, the time required for distillation is materially reduced, and the quality of the resulting oil is

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