324 AGRICULTURAL ANALYSIS 



in cases where comparison with other data is concerned. Future 

 experience will doubtless improve the processes both in respect 

 of accuracy and simplicity, but it must be granted that, as at 

 present practiced, they give essentially accurate results. 



284. Volumetric Estimation by Combustion with Copper Oxid. 

 This classical method of analysis is based on the principle, that 

 by the combustion of a substance containing nitrogen in copper 

 oxid and conducting the products of the oxidation over red-hot 

 copper oxid and metallic copper, all of the nitrogen present in 

 whatever form will be obtained in a free state and can subse- 

 quently be measured as a gas. The air originally present in all 

 parts of the apparatus must first be removed either by a mer- 

 cury pump or by carbon dioxid, or by both together, the resid- 

 ual carbon dioxid being absorbed by a solution of caustic alkali. 

 Great delicacy of manipulation is necessary to secure a perfect 

 vacuum, and, as a rule, a small quantity of gas may be measured 

 other than nitrogen, so that the results of the analyses are often 

 a trifle too high. The presence of another element associated 

 with mtrogen, or the possible allotropic existence of that ele- 

 ment, may also prove to be a disturbing factor in this long prac- 

 ticed, analytical process. For instance, if nitrogen be contaminated 

 with another element, e. g., argon, of a greater density, the com- 

 monly accepted weight of a liter of nitrogen is too great and 

 tables of calculation based on that weight would give results 

 too high. 



First will be given the official method for this process, fol- 

 lowed by a few simple variations thereof, as practiced in the 

 laboratory of the Bureau of Chemistry. 



285. The Official Volumetric Method. This process may be 

 used for nitrogen in any form of combination. Practically, it is 

 no longer used in fertilizer analysis, but the method is inserted 

 here because of its historic and scientific value. 



The apparatus and reagents needed are as follows : 

 Combustion tube of best hard Bohemian glass, about 66 centi- 

 meters long and 12.7 millimeters internal diameter. 



Asotometer of at least 100 cubic centimeters capacity, accu- 

 rately calibrated. 



