FEELING'S SOLUTION 77 



when it is not known, the substance has to be classified, for 

 want of a better name, as a sugar-like polysaccharide. 



Gentianose and Lactosin are two such sugars, obtained 

 respectively from the roots of Gentiana lutea and of Silene 

 vulgaris. Both are dextro-rotatory, but the former on hydro- 

 lysis yields a laevo-rotatory mixture, while the latter yields a 

 mixture containing 50 per cent of galactose. 



ESTIMATION OF SUGARS. 

 A. VOLUMETRIC METHODS. 



I. ESTIMATION BY MEANS OF FEHLING'S SOLUTION. 



The principle of this method lies in the fact that certain 

 sugars are capable of reducing copper sulphate in hot alkaline 

 solutions to cuprous oxide, the presence of which is indicated 

 by a yellow-red precipitate. 



Fehling's solution is made up in two solutions : 



A, containing 69-28 grams of pure crystallized copper sul- 



phate in one litre of distilled water. 



B, containing 350 grams of Rochelle salt and 100 grams 



of caustic soda in one litre of distilled water. 



The solution A must be made up very accurately, whereas 

 the quantities required for solution B need only be roughly 

 weighed. 



For use, 5 c.c. of A are mixed with 5 c.c. of B ; the mix- 

 ture is a deep blue colour, and is known as Fehling's solution. 

 If correctly compounded, 10 c.c. of the solution contain *n 

 gram of cupric oxide, which is able to oxidize '05 gram of 

 glucose. 



This value is sufficiently correct for general purposes ; it 

 is, however, an approximation, and varies for different sugars, 

 the factor for levulose, for instance, is -05144, whilst that for 

 invert sugar is '0494 1 . If it be desired to obtain very accurate 

 results, it is better to standardize the solution by titrating 10 

 c.c. with a solution of glucose of known strength. Such a 

 solution may be obtained by dissolving -95 gram of pure 

 crystallized cane sugar in 500 c.c. of distilled water and boiling 

 for fifteen to twenty minutes with 2 c.c. of concentrated hydro- 



