On account of its ready determination the specific gravity is the most frequently taken and, therefore, the best known property of the volatile oils. Even concerning the rare and little investigated oils statements of their density are to be found.
Further, as the maximum and minimum values of the more commonly used oils are fixed, the determination of the specific gravity belongs to the most important and also most convenient means of investigation. The specific gravity of an oil is changeable within certain limits, and dependent, outside of age, on the manner of the distillation and also upon the source and the state of ripeness of the plant material used. The extent of this variation is so different in the individual oils that no general rules can be formulated. With normal bergamot oil, for instance, the specific gravity lies between 0,881 and 0,886. The difference between the highest and the lowest density, therefore, amounts in this case to only five places in the third decimal.
As a rule, however, the limit values lie much further apart.
Most oils are lighter, some, however, heavier than water, especially those which contain larger amounts of oxygenated constituents of the aromatic series (e. g. wintergreen oil, clove oil, sassafras oil). The lowest specific gravity of all volatile oils is that of Pin us Sabiniana with 0,6962; the highest is that of wintergreen oil with 1,188.
The determination is conveniently performed with an hydrostatic balance according to Mohr or Westphal1), as the accuracy attained with this instrument, if rightly handled, is sufficient. The values obtained thereby are reliable to the third decimal. For the more accurate determination, or when only small quantities of an oil are at disposal, a pycnometer is used. The modifications of Sprengel (Fig. 66) and of Ostwald (Fig. 67) are especially to be recommended. For practical reasons it is expedient to mark the capillaries at a and b and to bend these slightly upward as shown in the accompanying illustrations. The instrument is suspended from the balance by means of the platinum wire c. The capillaries can be sealed by means of the glass caps d and e. The pycnometer likewise yields more accurate results with viscid oils in which the thermometer body of the Mohr-Westphal balance does not move up and down with sufficient ease. For viscid oils the capillaries should be relatively large otherwise the filling and emptying of the instrument is accompanied with difficulties. For the determination of the specific gravity, the weight of the completely dry instrument should be known, also its water value. For the determination of the latter, the pycnometer, previously filled with distilled water, is placed into a water bath of constant temperature (usually + 15° C.) for about a quarter of an hour or until it has assumed the temperature of the water bath. In the mean time distilled water is either added or withdrawn so as to bring the contents to the marks on the capillaries, when the weight is determined accurately. These two values, viz., P1 (weight of the pycnometer) and P2 (weight of the pycnometer plus water) are determined once and for all for a given instrument.
1) The hydrostatic balance with steel axes by F. Sartorius, Gottingen, may be especially recommended.
P2 - P1 is the water value W of the pycnometer. If the specific gravity of an oil is to be ascertained, the pycnometer is -filled with the oil in question, the same conditions as to temperature being observed as before, and weighed. If the pycnometer filled with oil weighs P3, then the specific gravity of the oil is s = P3 - P1 / W
In order to enable the convenient filling and evacuation of the instrument, a tube with glass ground connection with the capillary, as illustrated in fig. 68, is employed. In the case of viscid oils these operations are facilitated with the aid of gentle heat.
The specific gravity is naturally dependent on the temperature. For its determination + 15° C. has been chosen as a matter of expediency. The water value is likewise determined at this temperature.1) The instruments used in Germany are calibrated in accordance with this rule, including the Mohr-Westphal balance. Only the specific gravity of such oils as are too viscid at 15° C. or congeal either entirely or in part, is taken at higher temperatures, however, with reference to water at 15°, so that the same instruments can be used for these determinations at higher temperatures.
Experiments have shown'2) that for the temperature of water at 15° C. the specific gravity of the volatile oils varies from 0,0007 to 0,0008 for each degree centigrade, /'. e. on an average 0,00075. With the aid of this factor, the specific gravity of an oil can be recomputed for any temperature. If the temperature at which the specific gravity was determined was higher, 0,00075 should be added for each degree difference, if lower, the corresponding value should be deducted.
If, however, the specific gravities have been determined, not with reference to water at 15° but with reference to water at the same temperature at which the density of the oil was ascertained, corrections can be made in like manner. According to Schreiner and Downer1) the correction factor for observations between 15 and 25° is 0,00064 for each degree.
1) Unless otherwise specified, the specific gravities recorded in this book are for the temperature of +15° C. with reference to water at 15°. 2) Report of Schimmel & Co. October 1905, 87.
As a matter of course such computed values can be regarded as approximations only. For accurate data the specific gravity should be actually determined at the temperature in question.