OFFICIAL VOLUMETRIC METHOD 325 



Sprengel mercury air-pump. 



Small paper scoop, easily made from stiff writing paper. 



Coarse cupric oxid, to be ignited and cooled before using. 



Fine cupric oxid, prepared by grinding ordinary cupric oxid 



Metallic copper, granulated copper, or fine copper gauze, 

 heated and cooled in a current of hydrogen. 



Sodium bicarbonate, free from organic matter. 



Caustic potash solution, a supersaturated solution of caustic 

 potash in hot water. When absorption of carbon dioxid during 

 the combustion ceases to be prompt, the solution must be re- 

 placed with a fresh portion. 



Filling the tube. Use from one to two grams of ordinary 

 commercial fertilizers. In the case of highly nitrogenized sub- 

 stances, the amount to be used must be regulated by the amount 

 of nitrogen estimated to be present. Fill the tube as follows: (i) 

 About five centimeters of coarse cupric oxid. (2) Place on the 

 small paper scoop enough of the fine cupric oxid to fill, after 

 having been mixed with the substance to be analyzed, about 10 

 centimeters of the tube ; pour on this the substance, rinsing the 

 watch-glass with a little of the fine oxid, and mix thoroughly 

 with a spatula; pour into the tube, rinsing the scoop with a little 

 fine oxid. (3) About 30 centimeters of coarse cupric oxid. (4) 

 About seven centimeters of metallic copper. (5) About six 

 centimeters of coarse cupric oxid (anterior layer). (6) A 

 small plug of asbestos. (7) From eight-tenths to one gram of 

 sodium bicarbonate. (8) A large, loose plug of asbestos. Place 

 the tube in the furnace, leaving about two and five-tenths centi- 

 meters of it projecting; connect with the pump by a rubber stop- 

 per smeared with glycerol, taking care to make the connection 

 perfectly tight. 



Operation. Exhaust the air from the tube by means of the 

 pump. When a vacuum has been obtained, allow the flow of 

 mercury to continue; light the gas under that part of the tube 

 containing the metallic copper, the anterior layer of cupric oxid 

 (see (5) above), and the sodium bicarbonate. As soon as the 

 vacuum is destroyed and the apparatus filled with carbon dioxid, 

 shut off the flow of mercury and at once introduce the delivery 



