Identification of Organic Compounds. 23 



mixture is then heated to a higher temperature. Whilst 

 the test-tube is still hot, the lower end is plunged under 

 cold water and thus splintered ; the water then acts on 

 the excess of metal. The alkaline solution is then filtered 

 off from the excess of carbonaceous matter, and a few 

 drops of ferrous sulphate are added ; after warming for a 

 few moments, the liquid is acidified, and a drop of ferric 

 chloride is added. The formation of a bluish-green colour, 

 or of a Prussian blue precipitate, indicates the presence of 

 nitrogen. This test depends on the formation, in the first 

 instance, of sodium cyanide, which reacts with the ferrous 

 salt in alkaline solution to form a ferrocyanide, which 

 with ferric salts gives the well-known Prussian blue 

 reaction. 



The Halogens. These substances cannot be detected 

 directly in the majority of cases by silver nitrate. The 

 simplest test is to heat the substance with a little sodium, 

 as in the case of the nitrogen reaction given above, and 

 after getting rid of the excess of metal by water, to acidify 

 the liquid with nitric acid and to add silver nitrate. In 

 this reaction the halogens combine with sodium to form 

 the sodium halide salt. Cyanides also give the same 

 reaction. 



Sulphur. The substance is mixed with several times 

 its weight of a mixture of potassium nitrate and sodium 

 carbonate (2KN0 3 + Na 2 C0 3 ) in a crucible. The heating 

 should be very gentle at first, but the temperature is 

 afterwards raised until the mixture fuses. The sulphur 

 is thereby oxidized to sulphate, which can be detected by 

 extracting the fused mass with water, acidifying with 

 nitric acid, and adding barium chloride. 



Phosphorus. The substance is heated in a small flask 

 with a mixture of equal volumes of concentrated sulphuric 

 and nitric acids (in a fume cupboard). The heat is applied 



