ESTIMATION OF NITRITES 



475 



the hyposulfite, but the equivalent amount of free iodin repre- 

 senting it. In the case of a soil or fertilizer where the quantity 

 of nitrites is usually very small, it is well to use as much as one 

 kilogram for making the extract. The extraction should be made 

 rapidly, with water free of nitrites, in order to avoid any reducing 

 action on the nitrates which may be present. In the case of water, 



Figure 37. Apparatus of Chabrier. 



from five to 10 liters should be evaporated to a small volume. The 

 concentration should take place in a large flask, rather than in an 

 open dish, in order to avoid any possibility of the absorption of 

 nitrites produced by combustion in the flame of the burner. When 

 the volume has been reduced to about 100 cubic centimeters, 

 it is transferred to a small flask and the concentration con- 

 tinued until only 10 or 15 cubic centimeters are left. The residue 

 is filtered into a woulff bottle, shown in Fig. 37, of about 100 

 cubic centimeters capacity. 



