CHAPTER XXIII. 



THE DETERMINATION OP REDUCING SUGARS. 



Solutions of dextrose and other hexose sugars when boiled with an 

 alkaline solution of a cupric salt reduce the latter to the cuprous state in the 

 form of cuprous oxide. Approximately two molecules of dextrose reduce five 

 molecules of a cupric salt, but the quantities reduced vary with the conditions 

 of experiment. Tables connecting the amount of copper contained and the 

 amount of reducing sugar have been constructed by a number of experimenters. 

 The method of Allihn 1 is that generally adopted, and is as follows :* 



Allihn's Method. The solutions used are : 



A. 34-64 grms. copper sulphate in 500 c.c. 



. 173 grms. potassium sodium tartrate and 125 grms. caustic potash in 

 500 c.c. 



Thirty c.c. of each solution are mixed, 60 cc. of water added, and the 

 whole brought to the boiling point over a direct flame ; twenty-five cc. of the 

 reducing sugar solution are then run in, the whole again brought to the boiling 

 point, and the boiling continued for two minutes ; an equal volume of cold 

 water is then added and the reduced copper determined by one or other of the 

 methods given below, whence the amount of reducing sugar is obtained from 

 reference to the tables in the Appendix. 



The Filtration of the Cuprous Oxide. A. If filtration over 

 reduced pressure is not available paper must be used ; the reduced copper oxide 

 passes very readily through paper, and only paper of close texture can be used. 

 The paper always absorbs and retains copper sulphate, and the amount must 

 be determined by a blank experiment. 



B. The Soxhlet tube, Fig. 255, very often used, consists of a glass 

 tube about six inches long in all ; the upper portion is about three inches long 

 and half an inch in diameter, and terminates in a concave bottom to which is 

 attached a short capillary of about J% in. bore ; the lower half is about three 

 inches long and in diameter tapers from half to T 3 ^ in. It is prepared for use 



* Munson and Walker's method substitutes caustic soda for potash and a two for a four 

 minute boiling ; I give Allihn's method since it still remains official in the U.S. : it is moreover 

 used by Browne whose scheme is the only one completely accounting for all the factors in the 

 assay of complex sugar mixtures ; Browne's check analyses by Allihn's method leave nothing to 

 be desired on the grounds of accuracy. (N.D.) 



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