TABLE 31.-Volume of milk corresponding to lactose double-normal weight tain copper in solution. Fill the flask to the 500-ml mark, mix, filter through a dry filter, and determine lactose in an aliquot of the filtrate by the Lane and Eynon or Munson and Walker method. 4. REFERENCES [1] U. S. Dept. Agri., Bur. Chem. Bul. 110 (1908) and 154 (1912); Z. Nahr. Genussm. 18, 625 (1909). [2] U. S. Dept. Agr., Bur. Chem. Bul. 110, 60 (1908). [3] U. S. Dept. Agri., Bur. Chem. Bul. 110 (1908) and 154 (1912). [4] U. S. Dept. Agri., Bur. Chem. Bul. 154, 15 (1912); J. Ind. Eng. Chem. 17, 612 (1925); J. Assn. Official Agr. Chem. 15, 78 (1932). [5] Z. Nahr. Genussm. 22, 412 (1911). [6] Z. Nahr. Genussm. 19, 72 (1910). [7] J. Hortvet, J. Am. Chem. Soc. 26, 1523 (1904); J. Assn. Official Agr. Chem. 15, 79 (1932); 16, 79 (1933); 17, 73 (1934); 18, 83 (1935). [8] A. E. Leach and A. L. Winton, Food Inspection and Analysis, 4th ed., p. 652 (John Wiley & Sons, Inc., N. Y., 1920). [9] J. F. Snell, J. Assn. Official Agr. Chem. 4, 437 (1921); 16, 80 (1933); 17, 74 (1934). [10] A. L. Winton and J. L. Kreider, J. Am. Chem. Soc. 28, 1204 (1906); J. Official Agr. Chem. 16, 80 (1933); 17, 74 (1934). [11] C. A. Browne, J. Am. Chem. Soc. 28, 435 (1906); J. Assn. Official Agr. Chem. 16, 80 (1933); 17, 74 (1934). [12] H. W. Cowles, Jr., J. Am. Chem. Soc. 30, 1285 (1908). [13] Analyst 55, 116 (1930). [14] Methods of Analysis of the Association of Official Agr. Chem., 5th ed. (1940). XII. DETERMINATION OF PENTOSANS 1. DESCRIPTIVE Pentosans are polysaccharides that yield pentoses upon hydrolysis. As their names imply, araban is thus related to arabinose, and xylan to xylose. The determination of pentosans depends upon their conversion into furfural and the subsequent determination of the furfural. The following reactions are assumed to take place: In carrying out pentosan determinations, one is therefore concerned with the quantitative conversion of the pentosan into furfural and the subsequent determination of the furfural. The method universally adopted for converting pentosans into furfural consists in boiling the substance with 12-percent hydro chloric acid and collecting the distillate at the rate of 30 ml per 10 minutes. The quantity of distillate collected in some procedures is arbitrarily defined, while in others the distillation is continued until furfural ceases to come over, as indicated by the aniline acetate test.11 Methylfurfural gives only a faint-yellow color with aniline acetate. A solution of aniline in alcohol is used to test for this substance. Furfural and hydroxyfurfural derivatives are very sensitive to the aniline alcohol reagent, giving a bright-red color. Jolles [1] employed steam distillation in order to remove the furfural from the acid solution more rapidly and reported quantitative conversion of arabinose and xylose into furfural. Pervier and Gortner [2] found that steam distillation from a 12-percent hydrochloric acid solution as used by Jolles, or from an 18- to 20-percent hydrochloric acid solution (the acid percentage being indicated by the boiling point), resulted in quantitative yields of furfural from arabinose and xylose. They also concluded that the rate of distillation did not affect the yield, and thus a slow current of steam was sufficient to sweep out the furfural as fast as it was formed from the pentose. When the usual method of distillation from 12-percent hydrochloric acid was used, these authors obtained a 95.3-percent yield of furfural from arabinose and 96.1-percent yield from xylose. These figures are approximately 7 percent higher than those generally obtained. Kline and Acree [3] concluded that steam distillation gave no better yields of furfural from xylose than did the usual method of distillation. Kullgren and Tydén [4] converted pentosans to furfural by distilling from a 13.15-percent hydrochloric acid solution saturated with sodium chloride. They thus had a constant acid concentration in the distilling flask. Hughes and Acree [5] reported that a rapid steam distillation from a 12-percent hydrochloric acid solution saturated with sodium chloride gave quantitative yields of furfural from xylose as compared with 96-percent yields when a slow current of steam was used, irrespective of whether or not the solution was saturated with sodium chloride. The gravimetric methods for the determination of furfural include the use of phloroglucin [6], barbituric acid [7], thiobarbituric acid [8], and diphenyl thiobarbituric acid [9]. The phloroglucin method has been carefully studied and generally adopted. The volumetric methods include the use of phenylhydrazine [10], potassium bisulfite [1], and potassium bromate-bromide solution. The latter was studied by Pervier and Gortner [2]. They used an electrometric titration method and found that under their experimental conditions the potassium bromate and furfural reacted in the molecular ratio 1:3. Powell and Whittaker [11] determined the amount of bromine absorbed by a back-titration method, using potassium iodide and sodium thiosulfate. Under their experimental conditions they found that 4 atoms of bromine reacted with 1 molecule of furfural. În 1933, Magistad [12] pointed out that the furfuralbromate reaction is influenced greatly by temperature. Hughes and Acree [13] carried out the reaction at 0° C. By carefully controlling the temperature and time of reaction, they obtained reproduc The aniline acetate reagent is prepared by shaking and adding glacial acetic acid until the solution is clear. equal volumes of aniline and water in a test tube Place a drop of the reagent on a filter paper and allow a drop of the distillate to spread into the reagent. If furfural is present, a red color will appear where the circles intersect. Toward the end of the distillation the red line will appear only after drying. ible results in which 1 mole of bromine reacted with 1 mole of furfural. Kullgren and Tydén employed a potassium bromate-bromide solution, which they added at a temperature of 18° to 19° C, together with 10 ml. of ammonium molybdate. In 0 to 4 minutes a yellow color developed, and the reaction was allowed to run exactly 4 minutes after this coloration appeared. It will be noted that the bromine reaction with furfural takes place in two distinct stages. One mole of bromine reacts with furfural very rapidly and then another mole is slowly absorbed. Powell and Whittaker allowed their reaction mixture to stand 1 hour, under which condition more than 1 mole of bromine reacts with a mole of furfural. During the distillation of plant products with hydrochloric acid, along with the furfural are formed other substances which react with phloroglucin and with potassium bromide-bromate. These include methylfurfural (from rhamnosan), hydroxymethylfurfural (from hexosans), and formaldehyde (from lignin). The separation of the phloroglucin compounds is dependent upon the assumption that furfural forms with phloroglucin, a product which is insoluble in alcohol, while the other phloroglucin compounds are soluble. This is not entirely true. The Kullgren process employs a second distillation, which destroys 33 percent of the methylfurfural and all of the hydroxymethylfurfural. Hughes and Acree [14] studied the reaction of bromine with furfural and methylfurfural at 0°C and found that by determining the amount of bromine consumed during two periods of time it was possible to calculate the quantities of furfural and methylfurfural present. They expressed the consumption of bromine at two periods of time, based upon the molar consumption of bromine for each aldehyde, by the following equations: f=grams of furfural. m=grams of methylfurfural. r1 and x2=milliequivalents of bromine consumed by aliquots at times t1 and t2. a, and a molar consumption of bromine by furfural at t, and t2. b1 and b2=molar consumption of bromine by methylfurfural at t1 and t2. For 5- and 10-minute periods these equations become Reagents [6]-Hydrochloric acid.-Contains 12 percent by weight of hydrogen chloride. To 1 volume of concentrated hydrochloric acid add 2 volumes of water. Determine the percentage of acid by titration against standard alkali and adjust to proper strength by dilution or addition of acid, as may be necessary. Phloroglucin. Dissolve a small quantity of phloroglucin in a few drops of acetic anhydride, heat almost to boiling, and add a few drops of sulfuric acid. A violet color indicates the presence of diresorcin. A phloroglucin which gives more than a faint coloration may be purified by the following method. Heat in a beaker about 300 ml of the dilute hydrochloric acid and 11 g of commercial phloroglucin, added in small quantities at a time, stirring constantly until it is nearly dissolved. Pour the hot solution into a sufficient quantity of the same hydrochloric acid (cold) to make the volume 1,500 ml. Allow to stand at least overnight, preferably several days, to permit the diresorcin to crystallize. Filter immediately before using. yellow tint does not interfere with its usefulness. In using, add the volume containing the required quantity of phloroglucin to the distillate. A Determination.-Place such a quantity of the sample, 2 to 5 g, that the weight of phloroglucide obtained shall not exceed 0.300 g, in a 300-ml distillation flask, together with 100 ml of the dilute hydrochloric acid and several pieces of recently ignited pumice stone. Place the flask on a wire gauze, connect with a condenser, and heat, rather gently at first, and then regulating so as to distill over 30 ml in about 10 minutes. Pass the distillate through a small filter paper. Replace the 30 ml distilled by a like quantity of the dilute acid, added by means of a separatory funnel in such a manner as to wash down the particles adhering to the sides of the flask, and continue the process until the distillate amounts to 360 ml. To the total distillate add gradually a quantity of phloroglucin dissolved in the dilute hydrochloric acid and thoroughly stir the resulting mixture. (The quantity of phloroglucin used should be about double that of the furfural expected. The solution turns yellow, then green, and very soon there appears an amorphous greenish precipitate that grows darker rapidly, until it becomes almost black.) Make the solution. up to 400 ml with the dilute hydrochloric acid and allow to stand overnight. Collect the amorphous black precipitate in a weighed Gooch crucible having an asbestos mat, wash carefully with 150 ml of water so that the water is not entirely removed from the crucible until the very last, and dry for 4 hours at the temperature of boiling water. Cool, and weigh in a weighing bottle. The increase in weight is taken to be furfural phloroglucide. To calculate the furfural, pentoses, or pentosans from the phloroglucide, use the following formulas given by Kröber: (1) For a weight of phloroglucide, designated by a in the following formulas, under 0.03 g: Furfural (a+0.0052)×0.5170. Pentoses=(a+0.0052) X1.0170. In the above and also in the following formulas, the factor 0.0052 represents the weight of the phloroglucide that remains dissolved in the 400 ml of acid solution. (2) For a weight of phloroglucide a between 0.03 and 0.300 g, use Kröber's table (table 105, p. 610), or the following formulas: Furfural (a+0.0052) X0.5185. (3) For a weight of phloroglucide a over 0.300 g, use the following formulas: 3. METHOD FOR THE ESTIMATION OF FURFURAL ACCORDING TO KULLGREN AND TYDÉN METHOD Reagents. Sodium hydroxide solution, 1.58 N. A bromate-bromide solution containing 1.392 g of potassium bromate and 10 g of potassium bromide per liter, representing 0.05 N bromate solution. Sodium thiosulfate solution, 0.05 N. Ammonium molybdate solution containing 25 g of the salt per liter. Hydrochloric acid (density, 1.065) which contains 13.15 percent of hydrochloric acid. Sodium chloride and potassium iodide. (a) PROCEDURE APPLICABLE TO THE DETERMINATION OF PENTOSANS OR METHYLPENTOSANS Place a quantity of material corresponding to 0.15 to 0.2 g of xylose or rhamnose, or 0.20 to 0.30 g of arabinose, in a 300-ml distilling flask with 100 ml of 13.15-percent hydrochloric acid and 19 to 20 g of sodium chloride. Continue the distillation for 100 minutes for xylose and 140 minutes for arabinose and rhamnose at the rate of 25 ml per 10 minutes. After the distillation of each 25 ml add 25 ml of acid. For xylose 250 ml of distillate is obtained and for arabinose about 350 ml. Make the solution up to 250 ml in the first case, and 400 ml in the second, with hydrochloric acid. Place a 100-ml portion of this solution in a 500-ml flask fitted with a cork. While the liquid is cooling, add 200 ml of 1.58 N sodium hydroxide solution and cool to 18° to 19° C. Add at once 10 ml of ammonium molybdate solution and 25 ml of bromide-bromate solution. Place the flask over a white surface so as to be able to recognize the appearance of a yellow color. This usually occurs in zero to 2 minutes' time. Allow the flask to stand 4 minutes after the appearance of the yellow color, and add 1 g of solid potassium iodide and shake. Let stand for 5 to 10 minutes and titrate the liberated iodine with 0.05 N thiosulfate. The bromate consumed is equal to the volume of solution added minus the volume of thiosulfate required. The total bromate is obtained by multiplying the volume of bromate consumed by 2.5 for xylose and by 4 for arabinose and rhamnose. Calling this volume n ml of 0.05 N potassium bromate, the quantities obtained in grams are Furfural=nX0.00240. Xylose=nX0.00425. Xylan=n×0.00374. Arabinose n×0.00506. Araban=nX0.00445. |