Part 46

Degree brix (total solids) 77.8 per cent. Sucrose 44.2 ” Solids, not sucrose 33.6 ” Apparent purity 56.8 ”

It is seen from the above that with a single weighing and a single polarization, and within from ten to fifteen minutes, all needful data in respect of the proper treatment of molasses for the practical control and direction of a factory can be obtained.

In case a laurent polariscope is used, five times the normal weight, viz., eighty-one grams of the raw material are used and the process conducted as above.

SUCROSE, DEXTROSE, INVERT SUGAR, LEVULOSE, MALTOSE, RAFFINOSE, DEXTRIN AND LACTOSE IN MIXTURES.

=237. Occurrence.=—Sucrose and invert sugar are found together in many commercial products, especially in raw sugars and molasses made from sugar cane, and in these products sucrose is usually predominant. They also form the principal saccharine contents of honey, the invert sugar, in this case, being the chief ingredient.

In commercial grape sugar, made from starch, dextrose is the important constituent, while in the hydrolysis of starch by a diastatic ferment, maltose is principally produced. In the manufacture of commercial glucose by the saccharification of starch with sulfuric acid, dextrin, maltose, and dextrose are the dominant products, while in the similar substance midzu ame, maltose and dextrose are chiefly found, and only a small quantity of dextrose. In honeys derived from the exudations of coniferous trees are found also polarizing bodies not enumerated above and presumably of a pentose character. In evaporated milks are usually found large quantities of sucrose in addition to the natural sugar therein contained. These mixtures of carbohydrates often present problems of great difficulty to the analyst, and the following paragraphs will be devoted to an elucidation of the best approved methods of solving them.

OPTICAL METHODS.

=238. Sucrose and Invert Sugar.=—The chemical methods of procedure to be followed in the case of a sample containing both sucrose and invert sugar have been given in sufficient detail in preceding paragraphs (=124, 171=). When, however, it is desirable to study further the composition of the mixture, important changes in the method are rendered imperative. While the estimation of the sucrose and the total invert sugar, or the sum of the dextrose and levulose, is easy of accomplishment the separate determination of the dextrose and levulose is not so readily secured. In the latter case the total quantity of the two sugars may be determined, and after the destruction or removal of one of them the other be estimated in the usual way; or in the mixture the levulose can be determined by the variation in its gyrodynat, caused by changes of temperature.

=239. Optical Neutrality of Invert Sugar.=—The gyrodynat of levulose decreases as the temperature rises (=107=) and at or near a temperature of 87°.2, it becomes equal to that of dextrose, and, therefore, pure invert sugar composed of equal molecules of levulose and dextrose is optically neutral to polarized light at that temperature. On this fact Chandler and Ricketts have based a method of analysis which excludes any interference in polarization due to invert sugar. To secure the polarization at approximately a temperature of 87°, a water-bath is placed between the nicols of an ordinary polariscope in such a way as to hold a tubulated observation tube in the optical axis of the instrument. The ends of the bath, in the prolongation of this axis, are provided with clear glass disks. The space between the cover glasses of the observation tube and the glass disks of the bath is occupied by the water of the bath. When this is kept at a constant temperature it does not interfere with the reading. The observation tube may be of glass, but preferably is constructed of metal plated with platinum on the inside. For the most exact work the length of the observation tube, at 87°, is determined by measurement or calculation. The bath is heated with alcohol lamps or other convenient means. The arrangement of the apparatus is shown in Fig. 75.

In a mixture of sucrose and invert sugar any rotation of the plane of polarized light at 87° is due to the sucrose alone. In a mixture of dextrose and sucrose the polarization is determined, and, after inversion, again determined at 87°. The latter number is due to dextrose alone, and the difference between the two gives the rotation due to sucrose.

=240. Sucrose and Raffinose.=—In raw sugars made from beet molasses considerable quantities of raffinose are found. The method of inversion and polarization in such cases is described in paragraph =100=. In making the inversion by the method proposed by Lindet (=95=), and conducting the polarization on a laurent instrument, a slightly different formula, given below, is used; viz.:

C - 0.4891A S = --------------- 0.810

A - S and R = ----------, 1.54

in which the several letters refer to the same factors as are indicated by them in the formula of Creydt. In the application of the formula just given the normal weight of the mixed raw sugars used is 16.2 grams.

=241. Optical Determination of Levulose.=—The determination of levulose by optical methods alone is made possible by reason of the fact that the gyrodynats of the sugars with which it is associated are not sensibly affected by changes of temperature. The principle of the process, as developed by the author, rests on the ascertainment of the change in the gyrodynat of levulose when its rotation is observed at widely separated temperatures. The observation tube employed for reading at low temperatures is provided with desiccating end tubes, which prevent the deposition of moisture on the cover glasses. The relations of this device to the optical parts of the apparatus are illustrated in Fig. 76.

The protecting tubes are made of hard rubber and the desiccation is secured by surrounding the space between the rubber and the perforated metal axis with fragments of potash or calcium chlorid.

The details of the construction are shown in a horizontal section through the center of the observation tube in Fig. 77. In this figure the observation tube, made of glass or metal, is represented by i, the metal jacket, open at the top in the =V= shape as described, by k. The observation tube is closed by the heavy disk b made of non-polarizing glass. This disk is pressed against the end of the observation tube by the rubber washer a, when the drying system about to be described is screwed on to k. The apparatus for keeping the cover glass dry is contained in the hard rubber tube m and consists of a perforated cylinder of brass e, supported at one end by the perforated disk c and at the outer ends by the arms d. It is closed by a cover glass of non-polarizing glass s and can be screwed on to the system h at n. The space p is filled with coarse fragments of caustic soda, potash, or calcium chlorid by removing the cover glass s. The perforated disk c prevents any of the fragments from entering the axis of observation. When the cover glass s is replaced, it just touches the free end of the perforated metal tube preventing any of the fragments of the drying material from falling into the center at the outer end. When this drying tube is placed in position, the contents of the observation tube i can be kept at the temperature of zero for an indefinite time without the deposition of a particle of moisture either upon the glass b or s.

For determining the rotation at a high temperature the apparatus of Chandler and Ricketts (=238=) may be used or the following device: The polarizing apparatus shown above, Fig. 76, may be used after the =V= shape box is removed from the stand, which is so constructed as to receive a large box covered with asbestos felt an inch thick. The observation tube is held within this box in the same way as in the one just described so that the hot water extends not only the entire length of the tube but also covers the cover glasses. In both cases the cover glasses are made of heavier glass and are much larger in diameter than found in the ordinary tubes for polariscopes. The protecting cylinders of hard rubber are not needed at high temperatures but can be left on without detriment.

The illustration, Fig. 78, shows the arrangement of the apparatus with a silver tube in position, which can be filled and emptied without removing it.

In practice the water is heated with a jet of steam and an even temperature is secured by a mechanical stirrer kept slowly in motion. With such a box it is easy to maintain a temperature for several hours which will not vary more than half a degree. The temperature for reading the hot solutions was fixed at 88°, this being nearly the temperature at which a mixture of equal molecules of levulose and dextrose is optically inactive. In every case the sugar solutions were made up to the standard volume at the temperatures at which they were to be read and thus the variations due to expansion or contraction were avoided. When solutions are read at a high temperature, they must be made with freshly boiled water so as to avoid the evolution of air bubbles which may otherwise obscure the field of vision.

By means of the apparatus described it is easy for the analyst to make a polarimetric reading at any temperature desired. In all cases the observation tube should be left at least a half an hour and sometimes longer in contact with the temperature control media before the reading is made.

The appearance of the field of vision is usually a pretty fair index of the point of time at which a constant temperature is established throughout all parts of the system. Any variation in temperature produces a distortion of the field of vision while a constant fixed temperature will disclose the field of vision in its true shape and distinctness of outline.

Principles of the Calculation.—If 26.048 grams of pure sucrose be dissolved in water and the volume made 100 cubic centimeters, it will produce an angular rotation of 34°.68 when examined in a 200 millimeter tube with polarized sodium monochromatic light. Upon the cane sugar scale of an accurately graduated shadow instrument the reading will be 100 divisions corresponding to 100 per cent of pure sucrose.

In the complete inversion of the cane sugar the reaction which takes place is represented by the following formula:

— + C₁₂H₂₂O₁₁ + H₂0 = C₆H₁₂O₆ + C₆H₁₂O₆.

The minus and plus signs indicate that the resulting invert sugar is a mixture of equal parts of levulose (d fructose) and dextrose (d glucose). We are not concerned here with the fact that a complete inversion of cane sugar is a matter of great difficulty nor with the danger which is always experienced of destroying a part of one of the products of inversion. They are matters which may cause a variation in the analytical data afterward, but do not affect the principles on which the process is based.

In the inversion of 26.048 grams of cane sugar there are therefore produced 13.71 grams of levulose and 13.71 grams of dextrose or, in all, 27.42 grams of the mixed sugars.

The angular rotation which would be produced by 13.71 grams of dextrose in a volume of 100 cubic centimeters and through a column 200 millimeters in length is, with sodium light, 14°.53 equivalent to 41.89 divisions of the cane sugar scale. The specific rotatory power of a dextrose solution of the density given is almost exactly 53, and this number is used in the calculations.