Part 32

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.

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.

CHEMICAL PROPERTIES.

=314. Solubility in Alcohol.=—As has already been noted, the glycerids are freely soluble in ether, chloroform, carbon bisulfid, acetone, carbon tetrachlorid, and some other less commonly used solvents. Their solubility in absolute alcohol is variable and the determination of its degree may often be useful in analytical work.

The method used by Milliau for determining the degree of solubility is as follows:[267] The fatty matter is deprived of its free acids by shaking for half an hour with twice its volume of ninety-five per cent alcohol. After standing until the liquids are separated, the oil or fat is drawn off and washed three times with distilled water. The sample is deprived of water by filtering through a hot jacket filter and a given weight of the dry sample is well shaken with twice its weight of absolute alcohol. A weighed portion of the alcoholic solution obtained is evaporated to remove the alcohol and the weight of the residual fat determined. From the data obtained the percentage of solubility is calculated. Olive oils, when treated as described above, show a solubility of about forty-three parts per thousand of absolute alcohol, cotton oil sixty-two parts, sesamé forty-one parts, peanut sixty-six parts, colza twenty parts, and flaxseed seventy parts per thousand.

=315. Coloration Produced by Oxidants.=—When oils and fats are mixed with oxidizing reagents, such as sulfuric and nitric acids, the glycerids are partly decomposed with the production of colors which have some analytical significance. The most simple method of applying these tests is by the use of a thick porcelain plate provided with small cup-shaped depressions for holding the few drops of material required. Two or three drops of the oil under examination are placed in each of the cups, a like quantity of the oxidizing reagent added, and the mixture stirred with a small glass rod. The colors produced are carefully noted and the mixture is allowed to remain at room temperature for at least twelve hours in order that the final tint may be observed. The sulfuric acid used for this reaction should have a specific gravity of one and seven-tenths and the nitric acid should have the usual commercial strength of the strongest acid. Pure lard, when treated with sulfuric acid, as above described, shows but little change of color while the vegetable oils mostly turn brown or black. In addition to the reagents mentioned many others, including sulfuric and nitric acids, sulfuric acid and potassium bichromate, chlorin, ammonia, hydrogen peroxid, sodium hydroxid and aqua regia are used. Only a few of these tests seem to have sufficient analytical importance to merit any detailed description.[268]

=316. Coloration in Large Masses.=—Instead of applying the color test in the small way just described, larger quantities of the fat may be used, either in the natural state or after solution in petroleum or other solvent. For this purpose about ten cubic centimeters of the oil are shaken with a few drops of sulfuric acid or sulfuric and nitric acids. Lard, when thus treated (five drops of sulfuric acid to ten cubic centimeters of lard) shows practically no coloration. When treated with an equal volume of sulfuric acid and shaken, the lard on separating has a brown-red tint.[269]

Olive oil, with a few drops of sulfuric acid, gives a green color, while cottonseed, peanut and other vegetable oils, when thus treated with sulfuric and nitric acids, show brown to black coloration. The delicacy of the reaction may be increased by first dissolving the fat or oil in petroleum ether.

In the use of the coloration test with solvents, a convenient method is to dissolve about one cubic centimeter of the fat in a test tube in petroleum ether, add one drop of strong sulfuric acid and shake.

In the case of lard, the color does not change or becomes yellow or red. Cottonseed oil, similarly treated, shows a brown or black color.[270]

=317. Special Nitric Acid Test.=—A special nitric acid test for cottonseed oil is made with nitric acid of exactly 1.375 specific gravity at 15°. This test is especially valuable in detecting cottonseed in olive oil. The operation is conveniently conducted by shaking together equal volumes of the oil and acid in a test tube until an intimate mixture or emulsion is secured. When any considerable quantity of cottonseed oil is present an immediate brown coloration is produced, from the intensity of which the relative proportion of cottonseed oil in the case of a mixture may be roughly approximated. When only a little cottonseed oil is present in the mixture, the test tube containing the reagents should be set aside for several hours before the final observation is made.

=318. Coloration with Phosphomolybdic Acid.=—Among the color tests, one which we have found of use is the coloration produced in certain oils, mostly of a vegetable origin, by phosphomolybdic acid.[271]

The method of applying the test is extremely simple. A few cubic centimeters of the oil or melted lard are dissolved in an equal volume of chloroform, and a third volume of ten per cent phosphomolybdic acid added. The mouth of the test tube is closed with the thumb, and the whole is violently shaken. On being left in repose, the phosphomolybdic acid gathers at the top, and the coloration produced therein is easily observed. Cottonseed oil and peanut oil both give a beautiful green when treated in this way, which is turned to a blue on the addition of ammonia. Linseed oil gives a green color, but forms a kind of emulsion which obscures the color to some extent. The pure lards rendered in the laboratory give no coloration whatever to the reagent, but it retains its beautiful amber color in every case. Mixtures containing as little as ten per cent cottonseed oil and ninety per cent lard, show a distinct greenish tint, while twenty per cent cottonseed oil gives a distinct green. This reaction, therefore, may be considered of great value, and on account of its easy application it should come into wide use. But it is probable that different samples of cottonseed oil, refined to different degrees or in different ways, vary in their deportment with phosphomolybdic acid as they do with silver nitrate. In other words, there may be some samples of cottonseed oil which will not give the green color when treated as above, or so faintly as to have no diagnostic value in mixtures.

This reaction shows itself with nearly all vegetable oils but those which have been chemically treated either for the purpose of bleaching, or for the removal of the acidity, do not respond to the test at all, or else in a feeble manner, and that only after standing some time. Lard, goose fat, tallow, deer fat, butter fat, etc., show no change in color on being treated with this reagent, either with or without the addition of alkali. The presence of a small quantity of vegetable oil betrays itself by the appearance of the above mentioned coloration, the intensity of which forms an approximate measure of the amount of vegetable oil present in the sample. In experiments with suspected lards, which deviated in their iodin absorption numbers from those of genuine lard, the results were concordant, the color deepening as the iodin figure rose. The mineral fats (paraffin, vaselin) are without action on this reagent, and the only animal fat which reduces it is codliver oil.

In like manner some samples of lard may be found which exhibit a deportment with this reagent similar to that shown with vegetable oils, and tallow and lard oil have been shown to give more distinct reactions than some of the vegetable oils.[272]

The phosphomolybdic acid may be prepared by precipitating a solution of ammonium molybdate with sodium phosphate and dissolving the washed precipitate in a warm solution of sodium carbonate. The solution is evaporated to dryness and the dry residue subjected to heat. If a blue coloration be produced it may be discharged by adding a little nitric acid and reheating. The residue is dissolved in water, acidified with nitric and made of such a strength as to contain about ten per cent of the substance.