REFRACTIVE INDEX-DETERMINATION OF PHYSICAL PROPERTIES

3. REFRACTIVE INDEX

When a ray of light passes from a less dense to a more dense medium, it is bent or "refracted" toward the normal. If e represents the angle of reffaction, and i the angle of incidence, according to the law of refraction,
Sini/Sine = N/n
where n is the index of refraction of the less dense, and Nt the index of refraction of the more dense medium.
Refractometers offer a rapid and convenient method for the determination of this physical constant. Of the various types, the Pulfrich or the Abbé refractometer proves very satisfactory.22 The Abb6 type, with a range of 1.3 to 1.7, is recommended for the routine analyses of essential oils, the accuracy of this instrument being sufficient for all practical work. The readings may be made directly from the scale without consulting conversion tables; only one or two drops of the oil are required for a determination; the temperature at which the reading is taken may be adjusted conveniently.
Procedure: Place the instrument in such a position that diffused daylight or some form of artificial light can readily be obtained for illumination. Circulate through the prisms a stream of water at 20o. Carefully clean the prisms of the instrument with alcohol and then with ether. To charge the instrument, open the double prism by means of the screw head and place a few drops of the sample on the prism, or, if preferred, open the prisms slightly by turning the screw head and pour a few drops of sample into the funnel-shaped aperture between the prisms. Close the prisms firmly by tightening the screw head. Allow the instrument to stand for a few minutes before the reading is made so that the sample and instrument will be at the same temperature. Move the alidade backward or forward until the field of vision is divided into a light and dark portion. The line dividing these portions is the "border line,” and, as a rule, will not be a sharp line but a band of color. The colors are eliminated by rotating the screw head of the compensator until a sharp, colorless line is obtained. Adjust the border line so that it falls on the point of intersection of the cross hairs. Head the refractive index of the substance directly on the scale of the sector. A second reading should be taken a few minutes later to assure that temperature equilibrium has been attained.
Occasionally, the instrument should be checked by means of the quartz plate that accompanies it, using monobromnaphthalene, or if such a plate is not available, by means of distilled water at 20; the refractive index of pure water at this temperature is 1.3330.
Great care should be exercised when determining refractive indexes during hot, humid weather, since moisture in the air may condense on the cooled prisms. This will result in a blurred and indistinct line of separation between the light and dark fields if the oil between the prisms does not dissolve the condensed moisture ; if the oil dissolves the moisture, the dividing line will be sharp, but the observed index will be low.
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22 For a discussion of the theory involved and for a description of the instruments, thereader is referred to a standard text on physical chemistry, e.g., Findlay "Introduction to Physical Chemistry," Longmans, Green <fe Co. (1033), 103; Daniels, Mathews, an Williams, "Experimental Physical Chemistry," McGraw-Hill Book Co., New York (1941), 44.

It has become the accepted procedure to report refractive indexes for essential oils at 20o, using a monochromatic sodium light source,23 unless the material is a solid at that temperature. Thus, in the case of rose oil the refractive index is often given at 30o;24 in the case of anethole, at 25o.25
Whenever possible, however, all observations should be made at 20o. The use of factors to reduce readings to 20o is not recommended. Various investigators, notably Bosart, have reported the change of refractive index with temperature for numerous oils. According to the findings of Bosart,26 the values for the fifty-four oils examined lie between the limits of 0.00039 and 0.00049 per degree centigrade, and for the forty-seven synthetics and isolates between the limits of 0.00038 and 0.00054. A summary of Bosart's work is given in Table 4.4 and Table 4.5. These tables may be used con-
TABLE 4.4. CHANGE IN REFRACTIVE INDEX OF ESSENTIAL OILS 
CHANGE IN REFRACTIVE INDEX OF ESSENTIAL OILS

TABLE 4.5. CHANGE IN REFRACTIVE INDEX OF SYNTHETICS AND ISOLATES 
CHANGE IN REFRACTIVE INDEX OF SYNTHETICS AND ISOLATES

voniontly to convert values reported in the literature at other than 20o. If an oil is encountered which is not listed in the table, the use of a correction factor of 0.00045 per degree will give approximately correct results. If the refractive index is reported at a temperature above 20, the proper correction must be added; conversely, if reported at below 20, the correction must be subtracted.

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