THE DETERMINATION OF REDUCING SUGARS. 



paper and ignited in a porcelain crucible ; the paper and adhering cuprous 

 oxide are burnt separately, the cuprous oxide being partly reduced to copper ; 

 the ash and reduced copper are placed in the crucible, a few drops of nitric 

 acid 'added, evaporated to dryness and cautiously ignited. If collected in a 

 Soxhlet tube the narrow end of the tube is connected by rubber tubing to a 

 vacuum pump and a current of air is sucked through the layer of cuprous 

 oxide ; at the same time the tube is heated over a small flame ; the cuprous 

 oxide is seen to glow and is rapidly converted into cupric oxide. If a Gooch 

 crucible has been used, it and its contents are heated at a low red heat, care 

 being taken to prevent the reducing gases of the flame entering the crucible, 

 an end which is best obtained by placing the crucible containing the cuprous 

 oxide inside a second one. 



C. As Copper, by Reduction in Hydrogen. The precipitate of cuprous 

 oxide conveniently collected in a Soxhlet tube is attached to an apparatus 

 generating hydrogen, and a current of hydrogen is passed through the tube. 

 On gently heating the tube the cuprous oxide is rapidly reduced to metallic 

 copper. 



According to Perrault 2 the hydrogen should be purified by being passed 

 through towers containing 



a. Crystals of iodine, mixed with pumice stone. 



b. Caustic soda. 



c. Potassium permanganate 5 per cent., in caustic soda of density 1*32. 



d. Potassium bichromate in concentrated sulphuric acid. 



D. By Electrolytic Deposition. In the United States 

 Agricultural Department's laboratory the copper is obtained 

 by electric deposition ; the cuprous oxide is dissolved in 

 nitric acid and collected in a platinum basin of about 175 

 c.c. capacity ; after the addition of 3-4 c.c. sulphuric acid the 

 copper is ready for deposition, which is thus effected by 

 Spencer. 3 



" Where a direct current is used in lighting the 

 sugar-house, it is the most convenient source of electricity 

 for depositing the copper. The current must be passed 

 through a resistance or regulator in addition to the lamp. A 

 convenient and durable regulator is shown in Fig. 257 ; c 

 is a glass tube partly filled with water slightly acidulated 

 with sulphuric acid ; the wire a connects with a platinum 

 sealed into the tube ; b s a glass tube through which a 

 copper wire extends and connects with a platinum wire 

 e sealed into this tube. The tube b may be slipped up and down, thus 

 regulating the distance between the wires e and a and regulating the current. 

 The twin wire m is separated, severed, and one end d connected with the 

 platinum dish in which the copper is to be deposited and the other with the 



FIG. 257. 



461 



