SPECIFIC GRAVITY-DETERMINATION OF PHYSICAL PROPERTIES

1. SPECIFIC GRAVITY

Specific gravity is an important criterion of the quality and purity of an essential oil. Of all the physicochemical properties, the specific gravity has been reported most frequently in the literature. Values for essential oils vary between the limits of 0.696 and 1.188 at 15o ;9 in general, the gravity is less than 1.000. For each individual oil, however, the limits are much narrower and in most cases have been established during the course of years.
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Gildemeister and Hoffman, "Die atherischen Ole," 3d Ed., Vol. I, 699.
Pycnometers
Procedure: Clean the pycnometer by filling it with a saturated solution of chromium trioxide in sulfuric acid and allow it to stand for at least 3 hr. Empty the pycnometer and rinse thoroughly with distilled water. Fill the pycnometer with
recently boiled distilled water which has been cooled to a temperature of about 12 and place it in a water bath, previously cooled to 12. Permit the temperature to rise slowly to 15. Adjust the level of the water to the top of the capillary side arm,
removing any excess with a blotter or cloth, and put the ground glass cap in place. Remove the pycnometer from the water bath, dry carefully with a clean cloth, permit it to stand for 30 min. and weigh accurately. Empty the pycnometer, rinse several times with alcohol and finally with ether. Remove the ether fumes with the aid of an air blast and permit the pycnometer to dry thoroughly. Weigh accurately after standing 30 min. The "water equivalent" of the pycnometer may be found by subtracting the weight of the empty pycnometer from its weight when full.

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10 The density of a liquid is the weight of a unit volume. Thus, density may be expressed in pounds per cubic foot, or more frequently in grams per cubic centimeter. A 3.98o (the temperature of maximum density for pure water, free from air) 1 cc. of water weighs 0.099073 g. ; furthermore, at this temperature 1 ml. of water weighs exactly 1 g Since the coefficient of expansion of water  Is small, the density of a liquid expressed in grains per cubic centimeter corresponds doyly to the specific gravity. However, the fundamental difference in the two concepts shout* be thoroughly understood.
11 This pycnometer is similar to that described in A.S.T.M. Designation D 153 with the exception that the capacity is approximately 10 cc. instead of 50 cc.

Fill the clean, dried pycnometer with the oil previously cooled to a temperature of 12o. Following the same procedure as above, place the pycnometer in a water bath and permit it to warm slowly to 15. Adjust the oil to the proper level, put the cap in place, and wipe the pycnometer dry. Accurately weigh after 30 min.
The weight of the oil contained in the pycnometer divided by the water equivalent gives the specific gravity of the oil at 15o/15o (in air).
For a given pycnometer the water equivalent need be determined only once; therefore, it is important that this determination be performed with great care and accuracy.
For scientific work or for cases where the gravity is in question, the determination should be carried out exactly as described above. However, for routine analyses it is permissible to determine the specific gravity of an oil at room temperature compared with water at 15 and then to reduce this value to a temperature of 15o/15o by use of a proper correction factor. Numerous workers have determined correction factors for various oils and have recommended a general value from 0.0004212 to 0.0008413 per degree centigrade. However, as Bosart11 pointed out, it would be unsatisfactory to take the average figure obtained from a variety of oils and apply it to a particular oil, all the more so when there is a difference of opinion as to what that figure should be.
In the investigation carried out by Bosart, values were obtained which ranged from 0.00070 to 0.00099 per degree15 for the forty-two essential oils he examined. For synthetics and isolates normally encountered by the essential oil chemist or perfumer, values ranged from 0.00007 to 0.00114 per degree.16 Hence, it is unjustifiable to use an average correction factor if
TABLE 4.2. VARIATIONS IN SPECIFIC GRAVITY PER DEGREE CENTIGRADE
FOR ISOLATES AND SYNTHETICS
VARIATIONS IN SPECIFIC GRAVITY PER DEGREE CENTIGRADE FOR ISOLATES AND SYNTHETICS
TABLE 4.3. FACTORS FOR CONVERSION OF SPECIFIC GRAVITY FROM TO 15o/15o to 25o/25o
FACTORS FOR CONVERSION OF SPECIFIC GRAVITY FROM TO 15o/15o to 25o/25o

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