CALCULATION OF RESULTS 331 



the operation may be considered finished. In the manipulation, 

 our chief variation from the official method consists in connect- 

 ing the combustion apparatus with the measuring tube before the 

 heat is applied to the front end of the combustion tube. Any 

 particles of the sample which may have stuck to the sides of the 

 tube on filling, will thus be subject to combustion and the gases 

 produced measured. Where it is certain that no such adhesion 

 has taken place, it is somewhat safer on account of the possible 

 presence of occluded gases to heat the front end of the tube before 

 connecting the combustion apparatus with the azotometer. 



291. Method of Johnson and Jenkins. In the method of 

 Johnson and Jenkins the principal variation from the process 

 described consists in introducing into the combustion tube a 

 source of oxygen whereby any difficultly combustible carbon may 

 be easily oxidized and all the nitrogen be more certainly set free. 84 

 The potassium chlorate used for this purpose is placed in the 

 posterior part of the tube, which is bent at a slight angle 

 to receive it. The sodium bicarbonate is placed in the anterior 

 end of the tube. The combustion goes on as already described, 

 and at its close the potassium chlorate is heated to evolve the 

 oxygen. The free oxygen is absorbed by the reduced copper 

 oxid, or consumed by the unburned carbon. Any excess of 

 oxygen is recognized at once by its action on the copper spiral. 

 As soon as this shows signs of oxidation the evolution of the gas 

 is stopped. Care must be taken not to allow the oxygen to come 

 off so rapidly as to escape entire absorption by the contents of 

 the combustion tube. In such a case the nitrogen in the meas- 

 uring tube would be contaminated. 



It is rarely necessary in fertilizer analysis to have need of more 

 oxygen than is contained in the copper oxid powder in contact 

 with the sample during the progress of combustion. 



292. Calculation of Results. The nitrogen originally present 

 in a definite weight of any substance having been obtained 

 in a gaseous form, its volume is read directly in the burette in 

 which it is collected. This instrument may be of many forms 

 but the essential feature of its construction is that it should be 



M American Chemical Journal, i88o-8r, 2 : 27. 



