Part 61

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:

n × 100 × s n × 100 × s V = ---------------- = ----------------. 535 × 0.195 489.525

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. 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.

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.

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. 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. 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, 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.

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.