THE DETERMINATION OF REDUCING SUGARS 535 



or porcelain Gooch crucible. After ignition the mat of spongy platinum 

 is pressed down carefuUy %\ath a glass rod and manipulated untU a satisfac- 

 tory' filtering surface is obtained. 



Determination of the Reduced Copper. -4 s Cuprons\ Oxide. — The cuprous 

 oxide after collection by one or other of the above methods is dried to constant 

 weight. The drjdng is materially accelerated by washing the precipitate 

 first with alcohol and then with ether. 



As Cupric Oxide. — If the cuprous oxide has been collected on paper 

 the precipitate is, after drying, detached as completely as possible from the 

 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, when 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. 



.4 s Copper, by Reduction in Hydrogen. — The precipitate of cuprous oxide 

 conveniently collected in a Soxhlet tube is attached to an apparatus generat- 

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

 gently heating the tube, the cuprous oxide is rapidly reduced to metaUic 

 copper. 



According to Perrault^* the hydrogen should be purified by being passed 

 through towers containing : — 



a. Crystals of iodine, mixed with pmnice stone. 



h. Caustic soda. 



c. Potassimn permanganate 5 per cent., in caustic soda of density i'32 



d. Potassiimi bichromate in concentrated sulphuric acid. 



By Reduction in Alcohol. — This method was originally proposed by 

 Votocek and Lexa^^. As carried out by Wedderbum^^ the cuprous oxide 

 is collected in an alundum crucible. Some alcohol is made to boil in a beaker 

 and the heated crucible \nth its contents placed therein on a stand. The 

 crucible should not be heated sufficiently to ignite the alcohol. After 

 placing the crucible in the beaker, the latter is covered with a clock face. 

 Reduction to copper is rapid and complete. 



By Electrolytic Deposition. — In the United States Agricultural Depart- 

 ment'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^". 



" \\liere a direct current is used in lighting the sugar-house, it is the 

 most convenient soiurce of electricity for depositing the copper. The cmrent 

 must be passed through a resistance or regulator in addition to the lamp. 

 A convenient and durable regulator is showTi in Fig. 346 ; c is a glass tube 

 partly filled with water shghtly acidulated %vith sulphuric acid ; the wire a 



