THE DETERMINATION OF REDUCING SUGARS. 



Peskds Method. Peska's modification of Pavy's method consists in 

 covering the copper solution with a layer of pure paraffin and conducting the 

 experiment at a temperature of 80 C., the copper solution not heing allowed to 

 boil. The solution employed by him is 6-9 grms. pure copper sulphate and 

 160 c.c. of 25 per cent, ammonia in 500 c.c. mixed immediately before use with 

 an equal volume of 34*5 grms. Rochelle salt and 10 grms. of caustic soda, also 

 in 500 c.c. ; 100 c.c. of the mixed reagent are reduced by 80-2 mgrms. of 

 dextrose in 1 per cent, solution, or by 84-0 mgrms. of invert sugar in 1 per 

 cent, solution. 



Soldaintfs Method. 1 ' 3 This process is not generally used in estimating 

 reducing sugars, but is useful for the detection of small quantities of reducing 

 sugars, and of cane sugar after inversion. A formula for its preparation is 

 copper sulphate, 3'464 grins. ; potassium bicarbonate, 297 grms. ; dissolve in 

 1000 c.c. ; equal volumes of this reagent and of the solution under inspection 

 are boiled ; in the presence of reducing sugars the cuprous oxide separates in 

 an intensely red condition ; as little as *005 grm. sugar per 100 c.c. may be 

 detected. 



General Considerations of Volumetric Methods. In the 



volumetric assay of glucose the Fehling solution is standardized against a solu- 

 tion of dextrose, invert or other sugar of known composition ; the amount of 

 copper reduced is, however, influenced by the time of boiling, by the concen- 

 tration of the sugars, and by the simultaneous presence of cane sugar. The 

 first two factors are capable of control, but the latter is not ; the last factor is 

 of greatest influence when the cane sugar is in great excess, as happens in the 

 analysis of sugars ; the volumetric process then cannot lay claim to great 

 accuracy unless the Fehling solution be standardized by a preliminary assay 

 with cane sugar and reducing sugars in that proportion approximately in which 

 they occur in the materials under analysis. 



Ling and Rendle, 14 however, consider that the volumetric method, when 

 properly conducted, gives results equally accurate with the gravimetric method, 

 and at a considerable saving of time. 



Preparation of Sugar Solutions for Reducing Sugar 



Assay. The material used for the assay of reducing sugar may be submitted 

 to the action of a clarifying agent or not, and very great differences in methods 

 of working obtain. Pellet has repeatedly protested against the use of basic 

 lead acetate, and it has been shown by Geerligs 15 and himself that this material, 

 in the presence of chlorides, sulphates and other bodies carries down both 

 dextrose and levulose ; on the other hand, with the use of no clarifying agent, 

 bodies other than reducing sugars which act on the copper solution may be 

 present ; for this reason it is customary with many chemists to use neutral 

 acetate of lead as a clarifying agent. Zerban and Naquin 9 have shown that 

 with cane molasses a little more copper is precipitated with unclarified solutions 



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