268 



QUANTITATIVE ESTIMATION OF SUGAR. 



QUANTITATIVE ESTIMATION OF SUGAR. 



By Fermentation. (An illustration of yeast is given in Fig. 140.) For this 

 purpose the apparatus illustrated in Fig. 97 is employed. In the glass flask 

 a is measured a quantity (as, for example, 20 cu. cm.) of fluid containing sugar, 

 to which yeast is added. The flask b contains concentrated sulphuric acid. The 

 entire apparatus is weighed immediately after being filled. At ordinary tem- 

 perature (between 10 and 40 C.), most energetically at 25 C., the sugar breaks 

 up into 2 molecules of alcohol and 2 molecules of carbon dioxid : 



C 6 H 12 6 = 2 (C 2 H 6 0) 



Sugar = 2 Al 



2(C0 2 ) 



2 Carbon dioxid. 



FIG. 97. A 



In addition some glycerin and succinic acid are formed. The carbon dioxid 

 escapes through the flask b, and returns to the sulphuric acid any water that it may 

 have taken with it. If the decomposition is concluded in the course of about two 

 days, the apparatus is again weighed. From the loss in weight the amount of sugar 

 that was contained in the 20 cu. cm. of fluid is estimated, in accordance with the 

 fact that 100 parts by weight of sugar free from water are equal to 48.89 parts of 

 carbon dioxid, or that 100 parts of carbon dioxid by weight correspond to 204.54 

 parts of sugar. 



By Titration, with Fehling's alkaline cupric-oxid solution based on Trommer's 

 test. The deep-blue titration-fluid, composed of cupric sulphate, potassium ace- 

 tate, sodium hydrate and water, is so prepared that all of the cupric oxid in 10 

 cu. cm. of the solution will be reduced to yellowish-red cuprous oxid by just 0.05 

 gram of grape-sugar. For example, as in determining the amount of sugar in 

 urine, 10 cu. cm. of Fehling's solution are placed in a porcelain dish, and gradually 

 IPil diluted with 40 cu. cm. of water and heat 



> ^ ^. applied almost up to the boiling-point. The 



urine, previously diluted to from 10 to 20 

 times its volume, is dropped from a burette into 

 the hot titration-solution, and stirred until 

 every trace of blue color has disappeared or until 

 one drop of the fluid no longer produces a red 

 color on blotting-paper saturated with acetic 

 acid and potassium ferrocyanid. The amount 

 of urine needed is now read from the scale of the 

 burette, making allowance for the dilution, and 

 it will then be known that the amount of urine 

 used for reduction contained 0.05 gram of 



grape-sugar. From this the amount of sugar in the entire quantity of urine 

 excreted can be readily estimated. 



By Polarization. Sugar possesses the peculiarity of turning the plane of polar- 

 ized light to the right, just as albumin turns it to the left. Specific polarizing 

 power is the term applied to the degree of rotation that i gram of the substance 

 in question, dissolved in i cu. cm. of water, forming a layer 10 cm. thick, the 

 length of the tube of the apparatus, effects with yellow "light. For dextrose 

 this is +56. As the rotatory power is directly proportional to the quantity 

 of the substance dissolved in the fluid, the degree of deflection affords informa- 

 tion as to the amount of the optically active substance contained in the fluid. 

 In making the observation, the Soleil-Ventzke polarization-apparatus (Fig. 98) 

 shows on its scale to the right directly the percentage of sugar; to the left, that 

 of albumin. 



The light derived from the lamp encounters a crystal of calcspar at a. Two 

 Nicol's prisms are placed at v and s; that at v can be rotated about the visual 

 axis, while the other is fixed. The Soleil double plate of quartz is attached at m; 

 one-half of this deflects the plane of polarized light as far to the right as the other 

 deflects it to the left. At c the field of vision is covered by a plate of levorotatory 

 quartz. At b c is placed a compensator formed of two dextrorotatory prisms of 

 quartz, which can be moved laterally by means of the screw g in such a way 

 that the polarized light sent through the apparatus must pass through a thinner 

 or thicker layer of the dextrorotatory quartz in accordance with the degree of 

 rotation. With these dextrorotatory prisms in a certain position, the deflection 

 of the levorotatory quartz at n is exactly neutralized. In this position the 

 scale and vernier placed upon the compensator will be exactly at O, and both 



atus for the Quantitative 

 imation of Sugar. 



