Scale div. n_{D}. Δ. Scale div. n_{D}. Δ.
0 1.4220 8.0 50 1.4593 6.6
10 1.4300 7.7 60 1.4650 6.4
20 1.4377 7.5 70 1.4723 6.0
30 1.4452 7.2 80 1.4783 5.7
40 1.4524 6.9 90 1.4840 5.5
50 1.4593 100 1.4895
The process of observation is precisely the same as that already
described. In cases, however, where the critical line presents very
broad fringes (turpentine, linseed oil, etc.) it is advisable to repeat
the reading with the aid of a sodium flame.
=305. Viscosity.=—An important property of an oil, especially when its
lubricating qualities are considered, is the measure of the friction
which the particles exert on other bodies and among themselves, in
other words, its viscosity. In the measure of this property no definite
element can be considered, but the analyst must be content with
comparing the given sample with the properties of some other liquid
regarded as a standard. The usual method of procedure consists in
determining the time required for equal volumes of the two liquids to
pass through an orifice of given dimensions, under identical conditions
of temperature and pressure. In many instances the viscosity of oils is
determined by comparing them with water or rape oil, while, in other
cases, a solution of sugar is employed as the standard of measurement.
In case rape oil be taken as a standard and its viscosity represented
by 100 the number representing the viscosity of any other oil may be
found by multiplying the number of seconds required for the outflow of
fifty cubic centimeters by 100 and dividing by 535. If the specific
gravity vary from that of rape oil, _viz._, 0.915, at 15°, a correction
must be made by multiplying the result obtained above by the specific
gravity of the sample and dividing the product by 0.915. If _n_ be the
observed time of outflow in seconds and _s_ the specific gravity the
viscosity is expressed as follows:[255]
_n_ × 100 × _s_ _n_ × 100 × _s_
_V_ = ---------------- = ----------------.
535 × 0.195 489.525
[Illustration: FIG. 95.—DOOLITTLE’S VISCOSIMETER.]
It is important that the height of the oil in the cylinders from which
it is delivered be kept constant, and this is secured by supplying
additional quantities, on the principle of the mariotte bottle.
=306. The Torsion Viscosimeter.=—In this laboratory the torsion
viscosimeter, based on the principle described by Babcock is used.
The instrument employed is the one described by Doolittle.[256] The
construction of the apparatus is illustrated in Fig. 95.
A steel wire is suspended from a firm support and fastened to a stem
which passes through a graduated horizontal disk, thus permitting the
accurate measurement of the torsion of the wire. The disk is adjusted
so that the index point reads exactly _0_, thus showing that there is
no torsion in the wire. A brass cylinder seven centimeters long by
five in diameter, having a slender stem by which to suspend it, is
immersed in the oil and fastened by a thumbscrew to the lower part of
the stem of the disk. The oil cup is surrounded by a bath of water or
high fire-test oil, according to the temperature at which it is desired
to determine the viscosity. This temperature obtained, while the disk
is resting on its supports, the wire is twisted 360° by rotating the
milled head at the top. The disk being released, the cylinder rotates
in the oil by virtue of the torsion of the wire.
The action now observed is identical with that of the simple pendulum.
If there were no resistance to be overcome, the disk would return to
0, and the momentum thus acquired would carry it 360° in the opposite
direction. But the resistance of the oil to the rotation of the
cylinder causes the revolution to fall short of 360°, and the greater
the viscosity of the oil the greater will be the resistance, and also
the retardation. This retardation is found to be a very delicate
measure of the viscosity of the oil.
This retardation may be read in a number of ways, but the simplest
is to read directly the number of degrees of retardation between the
first and second complete arcs covered by the rotating pendulum. For
example, suppose the wire be twisted 360° and the disk released so that
rotation begins. In order to obtain an absolute reading to start from,
which shall be independent of any slight error in adjustment, ignore
the starting point and make the first reading of the index at the end
of the first swing. The disk is allowed to complete a vibration and
the needle is read again at its nearest approach to the first point
read. The difference in the two readings will measure the retardation
due to the viscosity of the liquid. In order to eliminate errors
duplicate determinations are made, the milled head being rotated in an
opposite direction in the second one. The mean of the two readings will
represent the true retardation. Each instrument is standardized in a
solution of pure cane sugar, as proposed by Babcock, and the viscosity,
in each case, is a number representing the number of grams of sugar in
100 cubic centimeters, which, at 22°, would produce the retardation
noted.
Each instrument is accompanied by a table which contains the
necessary corrections for it and the number expressing the viscosity,
corresponding to the different degrees of retardation, as read on the
index. The following numbers, representing the viscosity of some oils
as determined by the method of Doolittle, were obtained by Krug.[257]
Peanut oil 48.50
Olive ” 53.00
Cottonseed ” 46.25
Linseed ” 33.50
=307. Microscopic Appearance.=—When fats are allowed to slowly
crystallize from an ethereal solution they may afford crystalline
forms, which, when examined with a magnifying glass, yield valuable
indications of the nature and origin of the substance under
examination.[258]
The method of securing fat crystals for microscopic examination, which
has been used in this laboratory, is as follows: From two to five
grams of the fat are placed in a test tube and dissolved in from ten
to twenty cubic centimeters of ether. The tube is loosely stoppered
with cotton and allowed to stand, for fifteen hours or longer, in a
moderately warm room where no sudden changes of temperature are likely
to take place. It is advisable to prepare several solutions of the same
substance with varying properties of solvent, for it is not possible
to secure in a given instance those conditions which produce the most
characteristic crystals. The rate and time of the crystallization
should be such that the microscopic examination can take place when
only a small portion of the fat has separated in a crystalline
condition. A drop of the mass containing the crystals is removed by
means of a pipette, placed on a slide, a drop of cotton or olive
oil added, a cover glass gently pressed down on the mixture and the
preparation subjected to microscopic examination. Several slides should
be prepared from the same or different crystallizations. Sometimes the
results of an examination made in this way are very definite, but the
analyst must be warned not to expect definite data in all cases. Often
the microscopic investigations result in the production of negative or
misleading observations, and, at best, this method of procedure must be
regarded only as helpful and confirmatory.
A modification of the method of preparation described above has been
suggested by Gladding.[259] About five grams of the melted fat are
placed in a small erlenmeyer, dissolved in a mixture of ten cubic
centimeters of absolute alcohol mixed with half that quantity of ether.
The flask is stoppered with a plug of cotton and allowed to stand in a
cool place for about half an hour. By this treatment the more easily
crystallizable portions of the fat separate in a crystalline form,
while the triolein and its nearly related glycerids remain in solution.
The crystalline product is separated by filtration through paper wet
with alcohol and washed once with the solvent mentioned above. After
drying in the air for some time the crystals are removed from the paper
and dissolved in twenty-five cubic centimeters of ether, the cotton
plug inserted, and the erlenmeyer placed, in a standing position, in
a large beaker containing water. The water jacket prevents any sudden
changes of temperature and affords an opportunity for the uniform
evaporation of the ether which should continue for fifteen hours or
longer in a cool place.
Other solvents, _viz._, alcohol, chloroform, carbon disulfid, carbon
tetrachlorid, petroleum and petroleum ether have been extensively used
in the preparation of fat crystals for microscopic examination, but in
our experience none of these is equal to ether when used as already
described.
=308. Microscopic Appearance of Crystals of Fats.=—For an extended
study and illustration of the characteristics of fat crystals the
bulletin of the Division of Chemistry, already cited, may be consulted.
In the case of lard, there is a tendency, more or less pronounced, to
form prismatic crystals with rhombic ends. Beef fat on the other hand
shows a tendency to form fan-shaped crystals in which the radii are
often curved.
Typical crystals of swine and beef fat are shown in the accompanying
figures, 96 and 97.[260] In mixtures of swine and beef fats the typical
crystals are not always developed, but in most cases the fan-shaped
crystals of the beef fat will appear more or less modified when that
fat forms twenty per cent or more of the mixture. When only five or
ten per cent of the beef fat on the one hand or a like amount of
swine fat on the other are present the expectation of developing any
characteristic crystals of the minimum constituent is not likely to be
realized.
The typical crystals of lard are thought by some experts to be palmitin
and those of beef fat stearin, but no direct evidence has been adduced
in support of these _a priori_ theories.
In the experience of this laboratory, as described by Crampton,[261]
the differences between the typical crystallization of beef and swine
fats are plainly shown. In mixed fats, on the contrary, confusing
observations are often made. In a mixture of ten per cent of beef
and ninety per cent of swine fats a uniform kind of crystallization
is observed, not distinctly typical, but the characteristics of the
lard crystals predominate. In many cases a positive identification
of the crystals is only made possible by repeated crystallizations.
In the examination of so-called refined lards, which are mixtures of
lard and beef fat, the form of aggregation of the crystals is found
to resemble the fan-shaped typical forms of beef fat. When the single
crystals, however, are examined with a higher magnifying power, they
are not found to be pointed but blunt, and some present the appearance
of plates with oblique terminations, but not so characteristic as
those obtained from pure lard. In other cases in compound lards no
beef fat crystals are observed and these lards may have been made
partly of cotton oil stearin. When a lard crystal presents its edge
to observation it may readily escape identification, or may even be
mistaken for a crystal of beef fat. In order to insure a side view the
cover glass should be pressed down with a slight rotatory movement,
whereby some of the lard crystals at least may be made to present a
side view.
=309. Observation of Fat Crystals with Polarized Light.=—The
appearance of fat crystals, when observed by means of polarized light
alone or with the adjunct of a selenite plate, is often of value in
distinguishing the nature and origin of the sample.[262]
Every fat and oil which is amorphous will present the same set of
phenomena when observed with polarized light through a selenite plate,
but when a fat has been melted and allowed to cool slowly the field of
vision will appear mottled and particolored when thus examined. This
method has been largely used in the technical examination of butter
for adulterants, and the microscope is extensively employed by the
chemists of the Bureau of Internal Revenue for this purpose. In the
examination of the crystals of butter fat by polarized light a cross is
usually observed when the nicols are turned at the proper angle, but
the cross, while almost uniformly seen with butter, is not distinctive,
since other fats often show it. These forms of crystals are best
obtained by heating the butter fat to the boiling-point of water for
about a minute and then allowing it to slowly solidify, and stand for
twenty-four hours.
Pure butter, properly made, is never subjected to fusion, and hence,
when examined through a selenite plate, presents a uniform field of
vision similarly illuminated and tinted throughout. In oleomargarin,
the fats are sometimes, during their preparation, in a fused condition.
The field of vision is therefore filled to a greater or less extent
with crystals more or less perfect in form. Some of these crystals,
being doubly refracting, will impart to a selenite field a mottled
appearance. Such a phenomenon is therefore indicative of a fraudulent
butter or of one which has been at some time subjected to a temperature
at or above its fusing point.
=310. Spectroscopic Examination of Oils.=—The presence of chlorophyll
or of its alteration products is a characteristic of crude oils of
vegetable origin. In refined oils, even when of a vegetable origin,
all traces of the chlorophyll products may disappear. The absorption
bands given by oils are not all alike and in doubtful cases a suspected
sample should be compared with one of known origin.
In conducting the examination, the oil in a glass vessel with parallel
sides, is placed in front of the slit of the spectroscope and any
absorption band is located by means of the common divided scale and by
the color of the spectrum on which it falls. Olive and linseed oils
give three sharply defined absorption bands, a very dark one in the
red, a faint one on the orange and a well marked one in the green.
Sesame, arachis, poppyseed and cottonseed oils also show absorption
bands. Castor and almond oils do not affect the spectrum.
[Illustration: Fig. 96. LARD CRYSTALS × 65.]
[Illustration: Fig. 97. REFINED LARD (BEEF FAT) CRYSTALS × 65.