Part 63

=322. Coloration with Auric Chlorid.=—The use of auric chlorid for producing colorations in oils and fats was first proposed by Hirschsohn. One gram of auric chlorid is dissolved in 200 cubic centimeters of chloroform and about six drops of this reagent added to five cubic centimeters of the oil to be tested. In the case of cottonseed oil a beautiful red color is produced.

I have found that even pure lards give a trace of color sometimes with this reagent, and therefore the production of a slight red tint cannot in all cases be regarded as conclusive of the presence of cottonseed oil.

In general, it may be said that the color reactions with fats and oils have a certain qualitive and sorting value, and in any doubtful case they should not be omitted. Their value can only be established by comparison under identical conditions with a large number of fats and oils of known purity. The analyst must not depend too confidingly on the data found in books, but must patiently work out these reactions for himself.

=323. Thermal Reactions.=—The measurement of the heat produced by mixing glycerids with reagents which decompose them or excite other speedy chemical reactions, gives valuable analytical data. These measurements may be made in any convenient form of calorimeter. The containing vessel for the reagents should be made of platinum or some other good conducting metal not affected by them.

The heat produced is measured in the usual way by the increment in temperature noted in the mass of water surrounding the containing vessel. The determination of the heat produced in chemical reactions is a tedious and delicate operation requiring special forms of apparatus for different substances. The time element in these operations is a matter of importance, since it is necessary to work in rooms subject to slight changes of temperature and to leave the apparatus for some time at rest, in order to bring it and its contents to a uniform temperature. For these reasons the more elaborate methods of calorimetric examination are not well suited to ordinary analytical work, and the reader is referred to standard works on thermal chemistry for the details of such operations. For our purpose here a description of two simple thermal processes, easily and quickly conducted, will be sufficient, while a description of the method of determining the heat of combustion of foods will be given in another place.

=324. Heat of Sulfuric Saponification.=—Maumené was the first to utilize the production of heat caused by mixing sulfuric acid with a fat as an analytical process. In conducting the process a sulfuric acid of constant strength should be employed inasmuch as the rise of temperature produced by a strong acid is much greater than when a weaker acid is employed. The process is at best only comparative and it is evident that the total rise of temperature in any given case depends on the strength of the acid, the character, and purity of the fat or oil, the nature of the apparatus and its degree of insulation, the method of mixing and the initial temperature. For this reason the data given by different analysts vary greatly. For some of the methods of conducting the operation the reader may consult the work of Allen, cited above, or other authorities.

In this laboratory the process is conducted as follows: The initial temperature of the reagents should be a constant one. For oils 20° is a convenient starting point and for fats about 35°, at which temperature most of them are soft enough to be easily mixed with the reagent. The acid employed should be the pure monohydrated form, specific gravity at 20°, 1.845.

The apparatus used is shown in Fig. 98.

The test tube which holds the reagents is twenty-four centimeters in length and five in diameter. It is provided with a stopper having three perforations, one for a delicate thermometer, one for a bulb funnel for delivering the sulfuric acid, and one to guide a stirring rod bent into a spiral as shown. The thermometer is read with a magnifying glass. Fifty cubic centimeters of the fat are placed in the test tube and ten of sulfuric acid in the funnel and the apparatus is exposed at the temperature desired until all parts of it, together with the reagents, have reached the same degree. The test tube holding the oil should be placed in a vacuum-jacket tube, such as will be described in paragraph =316=. The oil is allowed to run in as rapidly as possible from the funnel and the stirring rod is moved up and down two or three times until the oil and acid are well mixed. Care must be exercised to stir no more than is necessary for good mixing. The mercury is observed as it ascends in the tube of the thermometer and its maximum height is noted. With the glass it is easy to read to tenths, when the thermometer is graduated in fifths of a degree. When oils are tested which produce a rise of temperature approaching 100°, in the above circumstances, (cottonseed, linseed and some others) either smaller quantities should be used or the oil diluted with some inert substance or dissolved in some inert solvent of high boiling point. For a study of the variations produced in the rise of temperature when varying proportions of oil and acid are used, the work of Munroe may be consulted.

The thermélaeometer described by Jean is a somewhat complicated piece of apparatus and does not possess any advantage over the simple form described above.

Instead of expressing the data obtained in thermal degrees showing the rise of temperature, Thompson and Ballentyne refer them to the heat produced in mixing sulfuric acid and water.

The observed thermal degree of the oil and acid divided by that of the water and acid is termed the specific temperature reaction. For convenience in writing, this quotient is multiplied by 100. The respective quantities of acid and water are ten and fifty cubic centimeters. This method of calculation has the advantage of eliminating to a certain degree the variations which arise in the use of sulfuric acid of differing specific gravities. In the following table are given the comparative data obtained for some common oils.

Acid of 95.4 Acid of 96.8 Acid of 99 per cent. per cent. per cent. /-----------------\ /----------------\ /--------------\ Rise of Specific Rise of Specific Rise of Specific temp. temp. temp. temp. temp. temp. with reaction. with reaction with reaction. Kind of oil. the oil. the oil. the oil. 0° 0° 0° 0° 0° 0°

Olive oil 36.5 95 39.4 85 44.8 96 Rapeseed oil 49.0 127 37.0 89 58.0 124 Castor oil 34.0 88 Linseed oil 104.5 270 125.2 269

=325. Method of Richmond.=—The rise of temperature produced by mixing an oil and sulfuric acid is determined by Richmond in a simple calorimeter, which is constructed by fitting a small deep beaker inside a larger one with a packing of cotton. The heat capacity of the system is determined by adding to ten grams of water, in the inner beaker, at room temperature, twenty-five grams of water of a noted higher temperature and observing the temperature of the mixture. The cooling of the system, during the time required for one determination of heat of sulfuric saponification, does not exceed one per cent of the whole number of calories produced. Between the limits of ninety-two per cent and one hundred per cent the rise of temperature observed is directly proportional to the strength of the acid.

Relative Maumené Figure.—The total heat evolved per mean molecule is called by Richmond the relative maumené figure. It is calculated as follows:

Let x = percentage of sulfuric acid in the acid employed; h = heat capacity of calorimeter in grams of water; R = observed rise of temperature (twenty-five grams of oil, five cubic centimeters sulfuric acid); K = potash absorbed for saponification (19.5 per cent of potassium hydroxid, standard of comparison); M = relative maumené figure:

21.5 20 + h 19.5 Then M = R × -----------× --------- × -----. x - 78.5 20 K

=326. Heat of Bromination.=—The rise of temperature caused by mixing fats with sulfuric acid has long been used to discriminate between different fats and oils. Hehner and Mitchell propose a similar reaction based upon the rise of temperature produced by mixing bromin with the sample. The action of bromin on unsaturated fatty bodies is instantaneous and is attended with a considerable evolution of heat. Since the action of bromin on many of the oils is very violent it is necessary to dilute the reagent with chloroform or glacial acetic acid. Owing to its high boiling point the acetic acid has some advantage over chloroform for this purpose. The tests are conveniently made in a vacuum-jacket tube. In such a tube there is no loss of heat by radiation. The bromin is measured in a pipette having at its upper end a tube filled with caustic lime held between plugs of asbestos. The bromin sample to be tested and the diluent employed are kept at the same temperature before beginning the trial. They are quickly mixed and the rise of temperature noted. The oil is first dissolved in the chloroform and the bromin then added.

A somewhat constant relation is noticed between the rise of temperature and the iodin number when one gram of oil, ten cubic centimeters of chloroform and one cubic centimeter of bromin are used.

If the rise in temperature in degrees be multiplied by 5.5 the product is approximately the iodin number of the sample. Thus a sample of lard gave a rise in temperature of 10°.6 and an iodin number of 57.15. The number obtained by multiplying 10.6 by 5.5 is 58.3.

In like manner the numbers obtained for some common oils are as follows:

Material. Rise of temperature Iodin No. Calculated with bromin. Iodin No.

Butter fat 6.6 37.1 36.3 Olive oil 15.0 80.8 82.5 Maize ” 21.5 122.0 118.2 Cotton ” 19.4 107.1 106.7 Castor ” 15.0 83.8 82.5 Linseed oil 30.4 160.7 167.2 Codliver ” 28.0 144.0 140.0

=327. Modification of the Heat of Bromination Method.=—The method described above by Hehner and Mitchell presents many grave difficulties in manipulation, on account of the inconvenience of handling liquid bromin. The process is made practicable by dissolving both the oil or fat and the bromin in chloroform, or better in carbon tetrachlorid, in which condition the bromin solution is easily handled by means of a special pipette.

In order to make a number of analyses of the same sample ten grams of the fat may be dissolved in chloroform or carbon tetrachlorid and the volume completed with the same solvent to fifty cubic centimeters. In like manner twenty cubic centimeters of the bromin are dissolved in one of the solvents named and the volume completed to 100 cubic centimeters therewith.

For convenience of manipulation the solutions are thus made of such a strength that five cubic centimeters of each represent one gram of the fat and one cubic centimeter of the liquid bromin respectively.

The apparatus used for the work is shown in the accompanying figure. The pipette for handling the bromin solution is so arranged as to be filled by the pressure of a rubber bulb, thus avoiding the danger of sucking the bromin vapor into the mouth. The filling is secured by keeping the bromin solution in a heavy erlenmeyer with a side tubulure such as is used for filtering under pressure. The solutions are mixed in a long tube, held in a larger vessel, from which the air is exhausted to secure a minimum radiation of heat. A delicate thermometer graduated in tenths serves to register the rise of temperature. The fat solution is first placed in the test tube, with care not to pour it down the sides of the tube but to add it by means of a pipette reaching nearly to the bottom. The whole apparatus having been allowed to come to a standard temperature the bromin solution is allowed to run in quickly from the pipette. No stirring is required as the liquids are sufficiently mixed by the addition of the bromin solution. The mercury in the thermometer rapidly rises and is read at its maximum point by means of a magnifying glass. With a thermometer graduated in tenths, it is easy to read to twentieths of a degree.