COMPOSITION OF ORGANIC COMPOUNDS 39 



3. Halogens. 



After completely oxidising all the organic matter to carbon dioxide 

 and water, the halogens are present as inorganic compounds and are 

 estimated in the usual way. 



The commonest method is to oxidise a known weight of the sub- 

 stance in a sealed tube with fuming nitric acid at 200 for several hours 

 a few crystals of silver nitrate being at the same time placed in the 

 tube : silver halide is formed and this is washed out of the tube and 

 weighed (Carius). 



In practice, about -2 gm. of the substance is weighed out into a narrow 

 test tube about 2 in. long. Some silver nitrate crystals are placed in a thick- 

 walled combustion tube sealed at one end and covered with 10-20 c.c. of 

 concentrated nitric acid, care being taken not to wet the sides of the tube with 

 acid. This is done by introducing the acid through a tube with a long capillary. 

 The test tube containing the substance is put in avoiding contact of the sub- 

 stance with the acid. The open end of the tube is now sealed in the following 

 way : a glass rod for a handle is fastened by heating to the side of the tube at 

 the open end. The tube is heated near this end in a blow-pipe flame in such 

 a way that the walls collapse together ; when it is nearly closed the end is 

 drawn out so as to form a capillary tube with even and thick walls. The 

 capillary is sealed by pulling off the end with the handle attached. The tube 

 is wrapped in asbestos paper and carefully placed, capillary outwards, in an 

 iron tube which can be closed with a screw cap. The iron jacket and tube 

 are placed in a specially constructed furnace in such a position that the capillary 

 point faces a wall. Should the tube burst, the contents are then not blown into 

 the room. The tube is heated to about 200-220 for 4 or 5 hours. The 

 furnace and contents are allowed to cool. The cap of the iron tube is re- 

 moved and the capillary point allowed to project a little. The point is heated 

 in a flame. When the glass is soft the pressure inside the tube forces an 

 opening. Owing to the high pressure inside the tube it is unsafe to open the 

 tube in any other way. The capillary is cut off and the contents washed out 

 into a beaker. The silver halide is filtered off and weighed by the usual 

 method employed in inorganic chemistry. 



Less frequently, halogens are estimated by heating the substance 

 in a combustion tube with quicklime. 



A thin layer of quicklime is placed at the closed end of a combustion 

 tube. Next to this is put a mixture of the substance (about -2 gm.) with 

 quicklime and then another layer of quicklime. The tube is heated in a 

 furnace, as in the estimation of carbon and hydrogen, the layers of quicklime 

 being heated to redness before the mixture of substance and lime. 



The contents of the tube, after the oxidation, are dissolved in nitric acid 

 and the halogen precipitated with silver nitrate. The silver halide is filtered 

 off, washed, dried and weighed. 



A convenient method of estimating chlorine is that described by Neumann. 

 The substance is oxidised with a mixture of nitric acid and sulphuric acid. 

 Hydrochloric acid is evolved, and this is collected in a solution of silver 

 nitrate. After boiling the solution for about half an hour to remove hydrogen 

 cyanide, which is also formed if nitrogen be present in the substance, the 

 silver chloride is filtered off, washed, dried and weighed. 1 

 1 See J. Physiol., vol. 31, p. 65. 



