92 THE CARBOHYDRATES 



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 'II 

 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 -04941. 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- 

 chloric acid. The solution must then be neutralized by the 

 addition of solid sodium carbonate, and made up to I litre ; 

 50 c.c. of this solution contain '05 gram of glucose, and should 

 reduce exactly 10 c.c. of Fehling's solution. 



It frequently happens in titrating liquid extracts from 

 plants, etc., that the cuprous oxide will not settle down, but 

 remains suspended in the solution as a fine turbidity. In such 

 cases the addition of a few drops of aluminium sulphate may 



