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174

QUANTITATIVE AGRICULTURAL ANALYSIS

The specific gravity at any temperature other than 20 may be
determined by the method outlined for this temperature or it may
be calculated from the determination at this temperature, using
the formula given above. It should be understood that the figure
desired for purposes of identification is the specific gravity at
the temperature for which data may be found in the literature.

Index of Refraction.—A discussion of the underlying theory
and of the determination of index of refraction is found on pages
113 to 120, Part II.

The measurement of index of refraction is a valuable addition
to the list of tests for oils. While not in all cases characteristic
it will frequently serve to distinguish between certain possi-
bilities when other tests, are not conclusive. The refractive
index increases with (a) increasing molecular weight of the
combined acids and (6) increasing unsaturation. If free fatty
acids are present in an oil the refractive index will be lower than
the normal value for the oil. In consequence of the latter fact
one may expect to find abnormally low indices for old or rancid
fats or oils.

The selection of standard temperatures for the determination
is highly desirable in order to make comparison data useful.
Temperatures of 20° for oils and 40° or 60° for fats and waxes
are suitable in most cases. For calculating the index of refrac-
tion at any temperature from experimental results at another
temperature the following formula may be used:

R = R' + 0.000365 (f - f),

where R and R' indicate indices of refraction at temperatures
t and t', respectively. The coefficient 0.000365 is the average
change of index for 1° for a large number of common oils.

The index of refraction of oils is conveniently determined by
use of any of the standard instruments, such as the Abb£,
Pulfrich, Zeiss butyro-refractometer or the immersion refrac-
tometer. Of those named the Abb6 instrument is probably the
most generally useful because it may be used with liquids cover-
ing a wide range of refractive indices and because it does not
require the use of monochromatic light. The principles under-
lying the use of this and other instruments are discussed on pages
115 to 120, Part II.

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