MILK AND MILK PRODUCTS 



29 



The filtering apparatus must be arranged prior to the pre- 

 cipitation, so that the cuprous oxid may be filtered without delay. 

 The precipitate is transferred as rapidly as possible to the filter, 

 well washed with hot water, alcohol, and ether successively, 

 dried, and the cuprous oxid reduced by gentle heating in a cur- 

 rent of hydrogen. When the reduction is complete, the heat is 

 withdrawn, but the flow of hydrogen is continued until the tube 

 is cold. It is then detached and weighed. 



Reduction of Copper by Electrolysis. — The filtration is per- 

 formed in a Gooch crucible with an asbestos-felt film and the 

 beaker in which the precipitation was made is 

 well washed with hot water, the washings 

 being passed through the filter, but it is not 

 necessary to transfer all the precipitate. When 

 the asbestos film is completely washed, it is 

 transferred with the adhering oxid to the 

 beaker; any oxid remaining in the crucible is 

 washed into the beaker by use of 2 c.c. nitric 

 acid (sp. gr. 1.42), added with a pipet. The 

 crucible is rinsed with a spray of water, the 

 rinsings being collected in the beaker. The 

 liquid is heated until all the copper is in solu- 

 tion, filtered, the filter washed until the filtrate 

 amounts to at least 100 c.c, and electrolyzed. 



Electrolytic apparatus has been constructed in a great variety 

 of forms. When the operation is carried out frequently, it is 

 best to have an electrolytic table. A platinum basin holding 

 not less than 100 c.c. is used. A cylindrical form with flat bot- 

 tom is convenient. It should rest on a bright copper plate, 

 which is connected with the negative pole of the electrical supply. 

 The positive pole should be also platinum, either. a spiral wire, 

 cylinder, or flat foil. Many operators use a funnel-shaped 

 perforated terminal for the negative pole ; in which case a glass 



\ 

 Fig. 6. 



