Rape and flaxseed oils absorb a part of the spectrum but do not affect
the rest of it. The spectroscope is of little practical utility in oil
analysis.[263]
=311. Critical Temperature of Solution.=—The study of the critical
temperature of solution of oils has been made by Crismer, who finds it
of value in analytical work.[264] If a fatty substance be heated under
pressure, with a solvent, _e. g._, alcohol, it will be noticed that as
the temperature rises the meniscus of separation of the two liquids
tends to become a horizontal plane. If at this point the contents of
the tube be thoroughly mixed by shaking and then be left at rest, a
point will soon be reached at which the two liquids again separate,
and this point is distinctly a function of temperature. Following
is a description of a convenient method of determining the critical
temperature of the solution of fats and oils for experimental purposes.
Tubes are prepared for holding the reagents in such a way that, after
the introduction of the fat and alcohol, they can be easily sealed. The
capacity of these tubes should be about five cubic centimeters. They
should be charged with about one cubic centimeter of the dry filtered
fat and about twice that quantity of ninety-five per cent alcohol.
Care should be exercised to avoid touching the upper sides of the tube
with the reagents. When charged the tubes are sealed in the flame of
a lamp and attached to the bulb of a delicate thermometer in such a
manner as to have the surface of its liquid contents even with the top
of the bulb. The tube is conveniently fastened to the thermometer by a
platinum wire. For duplicate determinations two tubes may be fastened
to the same thermometric bulb. The apparatus thus prepared is placed in
a large vessel filled with strong sulfuric acid. The operator should
be careful to protect himself from the danger which might arise from
an explosion of the sealed tubes during heating. It is advisable in
all cases to observe the reaction through a large pane of clear glass.
The bath of sulfuric acid is heated by any convenient means and an
even temperature throughout the mass is secured by stirring with the
thermometer and its attachments. When the meniscus which separates
the two liquids becomes a horizontal plane the thermometer is removed
and the liquid in the tubes well mixed until it appear homogeneous.
The thermometer is replaced in the bath, which is allowed to cool
slowly, and the phenomena which take place in the sealed tubes are
carefully noted. The critical temperature of solution is that at which
the two liquids begin to separate. This moment is easily noted. It
is, moreover, preceded by a similar phenomenon taking place in the
capillary part of the tube which retains a drop of the mixture on
shaking. In this droplet an opalescence is first noted. In the mass of
the liquid this opalescence, a few seconds afterwards, is observed to
permeate the whole, followed by the formation of zones and finally of
the reappearance of the meniscus of separation between the two liquids.
The temperature at this moment of opalescence preceding the separation
of the liquid is the critical temperature of solution. With alcohol of
0.8195 specific gravity, at 15°.5 (ninety-five per cent), the observed
critical temperatures for some of the more common fats and oils are as
given below:
Butter fat 100°.0
Oleomargarin 125°.0
Peanut oil 123°.0
Cotton ” 116°.0
Olive ” 123°.0
Sesamé ” 121°.0
Colza ” 132°.5
Mutton tallow 116°.0
Beef marrow 125°.0
Nut oil 100°.5
When the alcohol is not pure or if it be of a different density from
that named, the numbers expressing the critical temperature of solution
will vary from those given above.
=312. Polarization.=—The pure glycerids are generally neutral to
polarized light. In oils the degree of polarization obtained is often
variable, sometimes to the right and sometimes to the left. Olive oil,
as a rule, shows a slight right hand polarization. Peanut, sesamé, and
cottonseed oils vary in polarizing power, but in no case is it very
marked. Castor oil polarizes slightly to the right.
In determining the polarizing power of an oil it should be obtained in
a perfectly limpid state by filtration and observed through a tube of
convenient length, as a rule, 200 millimeters. The deviation obtained
may be expressed in divisions of the sugar scale of the instrument or
in degrees of angular rotation.
=313. Turbidity Temperature.=—The turbidity temperature of a fat, when
dissolved in glacial acetic acid, as suggested by Valenta, may prove
of some diagnostic value.[265] The fats are dissolved, with the aid of
heat, in glacial acetic acid and, on slowly cooling, the temperature at
which they become turbid is observed. The following data observed by
Jones are given for comparison.[266]
The numbers represent the turbidity temperature of the fat when treated
with the glacial acetic acid, and allowed to cool slowly. Butter fat,
from 40° to 70°, mostly from 52° to 65°; oleomargarin, 95° to 106°;
rape oil, 101°; sesamé oil, 77°; linseed oil, 53° to 57°; lard oil,
96°; olive oil, 89°; peanut oil, 61° to 88°.
It is important in this test that the acetic acid be absolutely
glacial. About three cubic centimeters of the glacial acetic acid, and
three of the fat, should be used.