CHAPTER XXVI 



THE DETERMINATION OF REDUCING SUGARS 



THE method adopted for the determination of reducing sugars is based 

 on the property possessed by these bodies of reducing cupric salts to cuprous. 

 This property was first used by Trommer 1 to distinguish grape sugar from 

 cane sugar, and established as an analytical method by Barreswil 2 . The 

 method was extended by Fehling 3 , whose name is connected with the process. 



Fehling himself concluded that one molecule of glucose reduced five 

 atoms of copper and, accordingly, he specified that the copper solution 

 should contain 34-56 grams of CuSO 4 5H 2 O in 1,000 c.c., since he found 

 that 10 c.c. of a solution of this strength was reduced completely by 0-05 

 gram, of anhydrous glucose. This strength of solution is retained in the 

 majority of the formulae since proposed. 



Much work has been done in connection with the process, and very rnany 

 routines and modifications have been proposed. The essential step forward 

 is due to Soxhlet 4 , who observed that the quantity of cupric salt reduced is 

 not a constant, but is dependent on the excess of copper which is present 

 during analysis ; other important points recognised are that the quantity of 

 copper reduced depends on the composition of the copper solution, on the 

 period over which the re-action extends, and on a number of minor points. 



There are a great number of sugars which reduce cupric salts, and their re- 

 ducing powers 5 differ one from another. These reducing powers have been 

 established by experiment and are, following Browne, conveniently referred to 

 glucose as unity (v. infra). The reducing sugars found in cane products are 

 mainly glucose and fructose, with occasionally small quantities of mannose 

 and glutose. The last two sugars are found as the result of the action of 

 alkalies on either the glucose or fructose which occurs naturally. Since the 

 glucose and fructose occur in quantities not far removed from equal, it is well 

 to calculate reducing sugar determinations in cane products as invert sugar. 



The methods of analysis in use do not separate the reducing sugars as 

 such, but indicate the reducing power calculated as dextrose, invert sugar, 

 etc. Hence all bodies adventitiously present and which possess the property 

 of reducing cupric salts are returned as reducing sugars. 



The methods in use for the determination of reducing sugars fall into 

 two classes : 



(a) A fixed quantity of copper solution of invariable composition is 

 reduced by a fixed volume of the solution containing reducing sugars under 

 fixed conditions. The quantity of reducing sugar used is insufficient to 

 effect complete reduction of the cupric salt. The quantity of cupric salt 

 reduced is obtained by one of many methods, whence the quantity of reduc- 

 ing sugar corresponding to the cupric salt reduced is obtained by reference 



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