THE DETERMINATION OF REDUCING SUGARS. 



solution of glucose be used. With cane juices, &c., the solution after precipi- 

 tation of all the copper is yellow in colour. Fehling's solution, as sold in the 

 shops, is twice as concentrated as the solution obtained by mixing equal bulks 

 of the copper sulphate and the alkaline tartrate solutions, and 10 c.c. of the 

 one or 20 c.c. of the other will be completely precipitated by approximately 

 05 grm. of glucose. As Fehling's solution when mixed is apt to decompose, 

 it should be standardized before use, and even if its components are kept 

 separately it should still be standardized, as copper sulphate crystals are 

 difficult to obtain pure. The standardization is performed by titrating the 

 Fehling's solution with a solution of invert sugar of known composition until 

 all the copper is precipitated ; in subsequent analyses the conditions of the 

 standardization must be strictly adhered to. The manipulation of a deter- 

 mination, whether standardization or analysis, is as follows : 



10 c.c. of Fehling's solution as found in the druggists' shops or 20 c.c. 

 prepared as described above are placed in a small flask or large test tube and 

 about 50 c.c. of water added ; the exact quantity is immaterial so long as a 

 fixed measure is adhered to. The whole is brought to the boil and the glucose 

 solution added in quantities of 1 c.c. from a burette to the boiling Fehling's 

 solution until the addition of 1 c.c. causes the final discharge of the blue 

 colour. This preliminary experiment gives the amount of glucose solution 

 required to the nearest c.c. If the number of c.c. taken differs materially from 

 that of the solution used when standardizing, it should be diluted until of 

 approximately the same strength. A second portion of Fehling's solution is 

 now diluted and boiled and a quantity of glucose solution added, just a little 

 less than that which the preliminary experiment has shown necessary, and the 

 whole boiled for exactly two minutes ; the glucose solution is then added drop 

 by drop to the still boiling mixture until all the copper is reduced. Owing to 

 the yellow colour of cane juices, &c., in alkaline solution, the disappearance 

 of the blue colour is not sharp, and the critical point of the operation is deter- 

 mined as follows : A solution of copper and potassium ferrocyanide in acetic 

 acid solution gives an intense brown colouration. A drop of the nearly com- 

 pletely reduced Fehling's solution is removed in a Wiley or Knorr filtering 

 tube and placed on a porcelain tile ; to this is added a drop of potassium 

 ferrocyanide solution and a drop of acetic acid : the presence of copper in 

 solution is shown by a red colouration. This test is made for every drop of 

 glucose solution added, after the blue colour has become indistinct. 



Wiley's filter tube consists of a glass tube with a flange formed by 

 softening one end and pressing on a wooden block ; over this flange is tied a 

 piece of linen and an asbestos film is formed on this by suction. Knorr 

 modified this by sealing in the end of a glass tube a perforated platinum disc. 

 On placing one of the tubes in the turbid liquid and applying suction, a little 

 of the liquid is filtered through the asbestos free of copper oxide ; after using 

 each time the tube should be washed out. 



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