The determination of the index of refraction nD has been recommended repeatedly for the examination of the volatile oils. As is well known, certain relations exist between chemical constitution and refraction and in many cases conclusions as to the position and number of double bonds may be drawn from the molecular refraction. Chemical individuals carefully purified are, however, necessary in order to obtain useful results. As the refractive coefficients of the constituents of the volatile oils are on the whole only slightly different from each other, they are not so well suited for the detection of adulterations as are other methods of examination. The addition of turpentine oil, for instance, influences the refraction of lemon oil only slightly, but changes the rotation to a marked degree.1)
1) As to the influence of the solvent on the angle of rotation compare Landolt, Liebig's Annalen 189 (1877), 311; Rimbach, Zeitschr. f. physik. Chem. 9 (1892), 701.
In isolated cases, the index of refraction may nevertheless serve as a valuable supplement to the other constants and should, therefore, not be omitted on principle. Hence, in the discussion of the individual oils, reference will frequently be made to these constants and limit values will be given.
For this purpose the Pulfrich refractometer can be especially recommended. The observations are made with sodium light.
The variations in the index of refraction due to differences in temperature vary somewhat for different oils and average about 0,00035'-) for each degree of temperature. These variations are noteworthy in as much as the indices of refraction of the different volatile oils vary but little among themselves, viz., from 1,43 (oil of rue and cognac oil) and 1,61 (cassia oil). Great care with reference to temperature should, therefore, be exercised in the determination. As a rule, the observations are made at 20°. Only in such cases, as that of oil of rose, in which the consistence of the oil does not admit of the determination, a higher temperature is chosen. Hence nD20o signifies the index of refraction for sodium light at 20° C. A recomputation with the aid of the above mentioned factor yields approximate values only and is not admissible for accurate values.
As a result of the formation of oxidation and polymerization products, the index of refraction increases with age. An exception is found in the behavior of anethol.3)
Mention should yet be made of the specific and molecular index of refraction although they apply to chemical individuals only.
The specific index of refraction finds expression in the formula n2 - 1 1 n2 + 2 d 4 in which d is the specific gravity determined at the same temperature at which the index of refraction is determined. The molecular index of refraction or the molecular refraction is obtained by multiplying the specific index of refraction with the molecular weight.
1) Report of Schimmel & Co. October 1893, 55.
2) According to observations by Schimmel & Co.
3) Comp. Report of Schimmel & Co. October 1904, 42.
4) The above formula has been proposed by Lorenz and Lorentz. The older formula by Gladstone, viz. --- does not always yield constant values.