THE DETERMINATION OF REDUCING SUGARS. 



of original solution -96 x '4 = -384 ; mgrms. dextrose + 40 = 83-2 ; 

 384 -r- 83-2 = -0046 grm. ; i.e., 4'6 mgrms. dextrose are to be deducted 

 from the amount found, which now appears as 43-2 4'6 = 38-6 mgrms. 

 dextrose = 40*3 mgrms. invert sugar, or 1*07 per cent, as the correct amount 

 of reducing sugars calculated as invert sugars. 



The invert sugar due to the cane sugar is then 102-32 1-07 = 101-25 

 per cent. = 96'18 per cent, as the true amount of cane sugar present. 



Optical Assay of Levulose. 16 The rotation of levulose falls 

 rapidly with rise of temperature ; hence by observations of the optical activity 

 at different temperatures the amount of levulose can be estimated ; for each 

 1 C. rise in temperature and for 1 grm. levulose in 100 c.c., the rotation falls 

 0357 Yentzke degree ; assuming the other sugars present are not affected, the 

 amount of levulose present follows directly. 



Identification of Levulose. Pierarts 17 uses the folio wing solution: 

 per 1000 c.c. 12 grms. of glycocoll, 6 grms. freshly prepared cupric hydroxide, 

 50 grms. potassium carbonate ; in the cold this solution is reduced by levulose 

 alone and by no other sugars which occur in analysis. 



Direct Assay of Dextrose in the Presence of other 

 Reducing Sugars. Romijn's 18 method is based on the observation that 

 dextrose and other aldose sugars readily oxidize iodine, levulose and ketose 

 sugars having a very small oxidizing action at low temperatures. The assay 

 is made as under. The iodine solution contains 40 grms. of borax and about 

 10 grms. of iodine per 1000 c.c. ; of this solution 25 c.c. are mixed with 25 

 c.c. of the solution under analysis which should contain about -15 grms. sugar ; 

 the mixed solutions are contained in a narrow necked flask with a long glass 

 stopper ; the flask and its contents are then placed in a thermostat for 16 22 

 hours ; the stopper is wired down and the gutter between the stopper and 

 neck of the flask is sealed with water. On removal from the thermostat the 

 contents of the flasks are acidified with hydrochloric acid and the excess of 

 iodine determined with thiosulphate. For 2 atoms of iodine I molecule of 

 glucose is calculated. The reaction involved is given on page 462. 



Separation of Dextrose and Levulose from Sucrose. 19 



A.mmoniacal lead acetate prepared by adding ammonia to lead acetate until the 

 opalescence which forms just disappears precipitates dextrose and levulose 

 from solution ; the sucrose remains in solution as a soluble lead compound ; the 

 precipitated lead-sugar compounds are suspended in water through which is 

 passed a current of carbon dioxide ; the lead dextrose compound is decomposed 

 and is removed by filtration ; the lead levulose compound may then be decom- 

 posed by hydrogen sulphide ; this method was used by Winter in pioneer work 

 on the nature of the sugars of the cane, but is unsuited for ordinary laboratory 

 routine. 



469 



