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
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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