(e) METHOD OF DETERMINING REFRACTIVE INDEX Place a few drops of the solution on the polished face of the fixed prism and slowly bring the two prisms together and clamp them. To insure a sharp, clear image use sufficient liquid to fill the space between the prisms. Swing the instrument to an upright position and adjust the illuminating mirror so that the light source is reflected into the lower prism of the instrument. Rotate the compensating prisms to obtain a sharp colorless dividing line. Circulate water at a constant temperature, preferably 20° C, through the jackets of the prisms long enough to allow the temperature of the prisms and of the sample to reach an equilibrium, continuing the circulation during observations and taking care that constant temperature is maintained. If the determination is made at a temperature other than 20° C, correct the reading to the standard temperature of 20° C by means of tables 123 or 127, pp. 657, 664. Caution must be observed if the humidity causes condensation of moisture on the exposed faces of the prisms. If such a condition exists, make the measurements at room temperature and correct the readings to 20° C by means of table 123 or 126. Dark-colored solutions, such as molasses, are frequently difficult or even impossible to read on the refractometer. In such cases it is necessary to follow the procedure of Tischtschenko [9], who diluted the dark-colored solution with a pure sucrose solution of about the same concentration. Water should never be used, since such dilution introduces errors due to contraction in volume. The method employed is as follows: Mix thoroughly a weighed quantity of the solution under examination (A) and a weighed quantity of a solution of pure sucrose of about the same concentration (B) whose sugar content has been previously determined by the refractometer. Obtain the refractive index of this mixture and, by means of table 122, p. 652, convert to percentage of dry substance. The percentage of dry substance in the sample in question is calculated by the formula: X = (A+B) C-BD (61) where X=percentage of dry substance to be found, A weight in grams of the sample mixed with B, B=weight in grams of pure sucrose solution used in the dilution, C=percentage of dry substance in the mixture A+B obtained from the refractive index, D=Percentage of dry substance in the pure sucrose solution obtained from its refractive index. In using the refractometer for determining the percentage of dry substance in solution, it is well to consider the effect of impurities, such as salts and organic nonsugars. Tolman and Smith [3] and others have shown that the instrument is applicable for determining the soluble carbohydrates in solution. The various sugars have approximately the same refractive index for equal concentrations. Stanek [10] has shown the effect of organic salts of sodium and potassium on the refractometric estimation of dissolved solids. 3. IMMERSION REFRACTOMETER This instrument is designed for the measurement of solutions in bulk, but, with accessory prisms, may be used where only small quantities of solution are available. Originally the immersion instrument of Zeiss [11] was designed with a single prism having a range of refractive indices from 1.32 to 1.36. The present design permits the use of interchangeable prisms, usually six in number, extending the range to 1.54. The arbitrary scale of the instrument, graduated in equal divisions from 5 to +105, is engraved on the plane side of the collecting lens of the eyepiece. The image of the border line is coincident with the scale. By means of the micrometer drum, readings may be made to tenths of a scale division. Each instrument is furnished with tables for converting the readings on the arbitrary scale to refractive indices. Since the relation between the arbitrary scales and refractive indices. is not the same for all immersion refractometers, it is necessary to make the conversons by means of the tables furnished with the individual instrument. The refractive indices corresponding to the scale divisions of the original single prism Zeiss immersion refractometer are given in table 32. TABLE 32.—Refractive indices corresponding to the scale divisions of the original Zeiss immersion refractometer In the Bausch & Lomb immersion refractometer the six interchangeable prisms were furnished and have the following index ranges: A, 1.32539 to 1.36639. B, 1.36428 to 1.40608. C, 1.39860 to 1.43830. E, 1.47320 to 1.51335. The adjustment of the scale of the instrument is made for each prism by means of a test solution or test plate. With prism A the adjustment is made with distilled water; B, with a standard sodium chloride solution; C, with a test piece of fluorite; and D, E, and F, with glass test pieces. The observations regarding illumination and temperature control of the Abbe refractometer also apply to the immersion instrument. The use of the immersion refractometer has been found advantageous by Bachler, [12], who devised a "one-solution method of analysis of sugar products," in which a sufficient quantity of a normal weight solution of the product is prepared and all or part of the necessary analytical data are obtained on this one solution. Certain changes were necessary in the immersion refractometer. At the suggestion of Bachler, the firm of Carl Zeiss, of Jena, produced an immersion refractometer with a single prism having a range of from no 1.331 to 1.372 and adjusted with distilled water at scale division 0. With the use of this instrument and the Goldbach flow-through cell, the method has been shown to give satisfactory results for factory control work. 4. SPECIAL REFRACTOMETERS For special purposes there are available other types of refractometers. A pocket-type instrument is used for the estimation of the sugar present in the juice of the cane. Another special type is the factory refractometer, which is mounted on the vacuum pan and permits readings to be taken during the boiling process. A new type of laboratory instrument is now in the course of manufacture by the Bausch & Lomb Optical Co. It is mounted in a horizontal position and illuminated by a sodium lamp. It combines features of both the Abbe and the immersion instruments and is capable of readings to a few units in the fifth decimal place. 5. REFERENCES [1] F. Strohmer, Oesterr-ungar. Z. Zuckerind. Landw. 12, 925 (1883); 13, 185 (1884). [2] F. Stolle, Z. Ver. deut. Zucker-Ind. 51, 469 (1901). [3] L. M. Tolman and W. B. Smith, J. Am. Chem. Soc. 28, 1476 (1906). [4] H. C. P. Geerligs, Arch. Suikerind. 15, 487 (1907). [5] H. Main, Int. Sugar J. 9, 481 (1907). [6] O. Schönrock, Z. Ver. deut. Zucker-Ind. 61, 424 (1911). [7] E. Landt, Z. Ver. deut. Zucker-Ind. 83, 692 (1933). [8] E. Landt, Int. Sugar J. 39, 225 (1937). [9] J. Tischtschenko, Z. Ver. deut. Zucker-Ind. 59, 103 (1909). [10] V. Stanek, Z. Zucker Ind. Böhmen 34, 5 (1909). [11] C. Pulfrich, Z. angew. Chem. p. 1168 (1899). [12] F. R. Bachler, Facts About Sugar 28, 420 (1933). XV. DETERMINATION OF MOISTURE 1. GENERAL An accurate determination of the moisture in sugar products is frequently a matter of great difficulty, and the proper procedure has not been definitely established. Moist sugar or sugar products, sirups, and molasses resist drying with great obstinacy. Sometimes the material is hygroscopic. At ordinary pressures, moisture can be removed only by prolonged heating at a high temperature. But a high temperature frequently exerts a destructive effect on the solid sugars. The destructive action of a temperature of 100° C. is a particularly important consideration in the case of levulose and, to a less extent, in the cases of dextrose and sucrose. Consequently, the best drying methods are those which combine mild temperature and high vacuum. If the substance is in the form of a sirup, drying is facilitated by mixing it thoroughly with dry quartz sand or pumice. The action of the sand is to cause a larger area to be exposed and prevent the formation of a crust. Flaked asbestos or, for fluid substances, a roll of filter paper may be used. Below are given various methods in common use for determining dry substance. The method to be adopted depends somewhat upon the material. If levulose is present in considerable quantity, the temperature of drying should not exceed 70° C. 2. METHOD OF THE UNITED STATES BUREAU OF CUSTOMS For control analysis of raw sugars, the Bureau of Customs, Treasury Department, has adopted an arbitrary method which yields readily reproducible results. The procedure is as follows [1]: For the determination of moisture in sugars, dry approximately 4 g in a metal dish 55 mm in diameter and 15 mm in height. Subject each sample to a temperature of 100° C. for 2 hours, care being exercised that the dishes are not in close proximity to the heaters. In making the determination, dishes of polished aluminum with tightly fitting covers are used. Upon removing from the drying oven, the dish is immediately covered and placed in a desiccator. As soon as the dishes and contents have cooled, they are weighed. The loss of weight is expressed as moisture. 3. METHODS OF THE ASSOCIATION OF OFFICIAL AGRICULTURAL CHEMISTS (a) DIRECT DRYING [2] (Applicable to Cane and Beet Raw and Refined Sugar) Dry 2 to 5 g of the prepared sample in a flat dish (Ni, Pt, or Al), at the temperature of boiling water, for 10 hours; cool in a desiccator and weigh. Dry again for an hour or until the change in weight is not more than 2 mg. For sugars of large grain, heat at 105° to 110° C to expel the last traces of occluded water. Report the loss as moisture. (b) VACUUM DRYING [3] Dry 2 to 5 g of the prepared sample in a flat dish (Ni, Pt, or Al, and with a tightly fitting cover) at a temperature not exceeding 70° C (preferably at 60° C), under a pressure not exceeding 50 mm of Hg, and for 2 hours. Remove from the oven, put cover in place, cool in desiccator, and weigh. Redry for an hour and repeat until the change in weight is not more than 2 mg between successive weighings at 1-hour intervals. The oven should be bled with a current of dry air during drying to insure removal of water vapors. (c) DRYING ON QUARTZ SAND [2] (Applicable to massecuites, molasses, and other liquid and semiliquid products) Digest pure quartz sand that will pass a 40-mesh but not a 60-mesh sieve with hydrochloric acid, wash free from acid, dry, and ignite. Preserve in a stoppered bottle. Place 25 to 30 g of the prepared sand and a short stirring rod in a dish approximately 55 mm in diameter and 40 mm in depth, fitted with a cover. Dry thoroughly, cover dish, cool in a desiccator, and weigh immediately. Then add sufficient diluted sample of known weight to yield approximately 1 g of dry matter and mix thoroughly with the sand. Heat on a steam bath for 15 to 20 minutes and stir at intervals of 2 to 3 minutes, or until the mass becomes too stiff to manipulate readily. Dry at 70° C under a pressure of not to exceed 50 mm of mercury. Make trial weighings at 2-hour intervals toward the end of the drying period (about 18 hours) until the change in weight does not exceed 2 mg. For materials containing no levulose or other readily decomposable substance, the material may be dried at atmospheric pressure by heating 8 to 10 hours in a water oven at the temperature of boiling water. The sample is cooled in a desiccator and weighed; the heating and weighing are repeated until the loss in 1 hour does not exceed 2 mg. The loss of weight is reported as moisture. (Dry sand, as well as the dried sample, will absorb an appreciable quantity of moisture after standing over most desiccating agents, therefore all weighings should be made as quickly as possible after cooling in the desiccator.) 4. ADDITIONAL METHODS The length of time required to determine moisture in molasses and other low-grade products by drying on sand has led to the introduction of a number of special methods, such as the method of Spencer |