82 THE CARBOHYDRATES 



Esti)}iation of Mixtures of Sugars. 



In many cases it is possible to isolate the different sugars 

 in solution, and estimate them separately by means of 

 Fehling's solution or by some other method, and this separa- 

 tion must be accomplished when their action on Fehling's 

 solution is similar. For example, it may be desired to esti- 

 mate the amount of levulose and dextrose in a solution. Add 

 to the dilute solution some ammoniacal lead acetate ; both 

 sugars are precipitated as lead compounds. Filter off and 

 wash the precipitate ; suspend the precipitate in water and 

 pass through it a current of carbon dioxide. The lead com- 

 pound of glucose alone is decomposed, and the glucose goes 

 into solution. Filter off and thoroughly wash the levulose 

 lead compound, and then suspend it in water and decompose 

 it with sulphuretted hydrogen. 



Similarly, should these two sugars be mixed with cane 

 sugar, the latter, on the addition of ammoniacal lead acetate, 

 remains in solution, and thus is easily separated. 



Inasmuch as this method is somewhat tiresome, the follow- 

 ing methods may be followed whenever possible : — 



GLUCOSE AND SUCROSE. 



1. Take lOO c.c. of the mixture and titrate with Fehling's 

 solution. 



2. Invert lOO c.c. of the mixture by the method given, 

 and titrate. 



The first operation gives the amount of glucose = a. 

 The second operation gives the original amount of glucose 

 together with that due to the inversion of the cane sugar = b. 

 .'. (6 - a) X •95= sucrose. 



GLUCOSE AND MALTOSE. 

 Pioceed exactly as for glucose and sucrose: — 



a = amount of sugar before inversion. 

 h = amount of sugar after inversion. 



From the reasons already given under maltose, it follows 

 that— 



(b-a) X 2-32 = maltose, 

 and a - (maltose x '62) = glucose. 



