343 AGRICULTURAL ANALYSIS 



and the materials under the tube confined to the proper posi- 

 tion by asbestos plugs. The gases used act in a merely mechan- 

 ical manner and their use affords so few advantages over the 

 method of aspirating air at the end of the combustion as to ren- 

 der it unadvisable. 



302. Coloration of the Product. It often happens, especially 

 in the combustion of animal products, such as tankage and fish 

 scrap, that the acid receiving the ammonia is deeply colored by 

 the condensation of some of the other products of combustion. 

 This coloration interferes, in a very serious way, with the delicacy 

 of the indicator used to determine the end of the reaction. In 

 this case the liquid may be mixed with an alkali and distilled, and 

 the ammonia secured in a fresh portion of the standard acid as 

 in the moist combustion process to be hereafter described. 



303. General Considerations. (i) Preparation of the Sample. 

 In the soda-lime method it is of great importance that the 

 organic substances be in a fine state of subdivision so as to ad- 

 mit of intimate mixture with the alkali. In cases where frag- 

 ments of hoof, horn, hair, or similar substances are to be pre- 

 pared for combustion, it is advisable to first decompose them by 

 heating with a small quantity of sulfuric acid. The excess of 

 acid may be neutralized with marble dust and the resulting mix- 

 ture dried, rubbed to a fine powder, and mixed with the soda- 

 lime in the usual way. Care must be taken not to lose any of 

 the ammonia from the sulfate, which may possibly be formed in 

 mixing with the soda-lime in filling the tube. 



(2) Purity of Soda-Lime. The soda-lime employed must be 

 entirely free of nitrogenous compounds, and blank combustions 

 should be made to establish its purity or to determine the mag- 

 nitude of the corrections to be made. 



(3) Temperature. The temperature of the combustion should 

 not be allowed to exceed low redness. At very high tempera- 

 tures there would be danger of decomposing the ammonia. 



(4) Aspiration of Air. Before aspiring a current of air 

 through the tube to remove the last traces of ammonia, the gas 

 should be put out under the furnace and the tube be allowed to 



