INTRODUCTORY REMARKS. 445 



except oxygen, to be absent, the quantity of oxygen is, then, equal 

 to 49.384 per cent, and the composition of the substance is as 

 follows : 



Carbon . 44.444 per cent. 



Hydrogen 6.213 " 



Oxygen 49.343 



100.000 



Determination of nitrogen. Nitrogen is generally determined 

 by the Kjeldahl method, which consists in boiling in a suitable flask a 

 weighed quantity of the organic compound with 30 to 40 times its 

 weight of sulphuric acid and a little potassium permanganate or mer- 

 curic oxide. By this treatment all nitrogen present is converted into 

 ammonium sulphate, from which by the addition of an excess of 

 sodium hydroxide ammonia is liberated. This ammonia is distilled 

 over into a known volume of normal acid. By titration with normal 

 alkali the unsaturated portion of acid is determined and from the 

 result the percentage of nitrogen is calculated. 



Nitrogen may also be determined by the Will- Varr entrap method, which is 

 based on the formation of ammonia whenever nitrogenous matter is heated 

 with soda-lime (a mixture of sodium hydroxide and calcium oxide). The 

 method is not applicable to all compounds, because the nitrogen of some is not 

 all converted into ammonia by the process. 



A third method, known as the Dumas or absolute method, consists in oxidiz- 

 ing, at a red heat, the nitrogenous substance by means of cupric oxide and 

 then decomposing, by means of highly-heated metallic copper, any oxide of 

 nitrogen which may have been formed. By this operation all nitrogen is 

 obtained in the elementary state ; it is collected, measured, and from the volume 

 the weight is calculated. 



For the details of manipulation in the above method, which are simply out- 

 lined, large works on quantitative analysis must be consulted. 



Determination of sulphur and phosphorus. These elements are 

 determined by mixing the organic substance with sodium carbonate 

 and nitrate, and heating the mixture in a crucible. The oxidizing 

 action of the nitrate converts all carbon into carbon dioxide, hydrogen 

 into water, sulphur into sulphuric acid, phosphorus into phosphoric 

 acid. The latter two acids combine with the sodium of the sodium 

 carbonate, forming sulphate and phosphate of sodium. The fused 

 mass is dissolved in water, and sulphuric acid precipitated by barium 

 chloride in the acidified solution, phosphoric acid by magnesium 

 sulphate and ammonium hydroxide and chloride. From the weight 

 of barium sulphate and magnesium pyrophosphate (obtained by heat- 

 ing the magnesium ammonium phosphate) the weight of sulphur and 

 phosphorus is calculated. 



