460 AGRICULTURAL ANALYSIS 



the length of the column of liquid. A comparison camera such 

 as is described in Volume I, page 591, may be used to advantage. 



Influence of Nitrites. If the quantity of nitrous acid in the 

 water is known a correction can be applied for nitrates by deduct- 

 ing one-fifth of the number found for nitrites when estimated as 

 nitrates. 



Influence of Iron. Although ferrous salts give no reaction 

 with carbazol, nitrates are apt to be overestimated in their pres- 

 ence. On the other hand, ferric compounds, like other oxidiz- 

 ing agents, may give a characteristic green color with carbazol. 

 In all cases when iron is present in any considerable quantity it 

 is best to remove it by rendering the sample slightly alkaline, 

 evaporating to dryness, and redissolving the soluble residue 

 until the solution reaches the original volume. 



Influence of Chlorids. The presence of chlorids furnishes by 

 far the most serious source of error in the process by intensifying 

 the action of the nitric acid. If, however, nitrates be absent, 

 chlorids give no reaction with carbazol. The chlorids are re- 

 moved by the standard silver sulfate solution, the quantity of 

 chlorids present having been first determined by a standard sil- 

 ver nitrate solution. For this purpose an ordinary sugar flask 

 can be employed, marked at 100 and no cubic centimeters. 

 This flask is filled to the 100 cubic centimeter mark with the 

 sample to be examined; the necessary quantity of silver sulfate 

 is added and then two cubic centimeters of the solution of alumi- 

 num sulfate, previously described, and the contents of the 

 flask brought up to no cubic centimeters by the addition of 

 pure distilled water. The whole is shaken up and filtered, the 

 first portion of the filtrate being rejected. The aluminum sul- 

 fate, by reacting with the carbonates usually present in the water 

 and producing the precipitation of alumina, facilitates the re- 

 moval of the precipitated silver chlorid. 



The above described method, on account of its delicacy, is not 

 well suited to aqueous solutions of soils and fertilizers except 

 where the quantity of nitric nitrogen present is extremely minute. 



Spiegel also first suggested the use of diphenylamin for de- 



