ESTIMATION 127 



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 -0475, 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. 



Plant extracts frequently contain tannins and other 

 substances which may interfere with Fehling's solution ; 

 in such cases, and likewise when the solution to be titrated 

 is coloured, a preliminary treatment or " clarification " is 

 necessary. 



For this purpose 50 c.c. of the solution contained in a 

 100 c.c. graduated flask should be treated with the minimum 

 possible amount of basic lead acetate * (carefully added until 

 no further precipitate is formed) and then a little alumina 

 cream. t The amounts of these reagents to be employed will 

 naturally vary in different circumstances, though in the 



* Excess of basic lead acetate is to be avoided, since a loss of levulose 

 is liable to occur especially if this sugar is left in contact with the basic 

 acetate for some time ; there is, however, little danger of loss from this 

 cause if excess is avoided. 



t Prepared by adding a slight excess of dilute ammonia to a saturated 

 solution of alum, and then adding more alum until the reaction becomes 

 slightly acid. 



