70 The Estimation of Nitrogen [104-106 



Add distilled water to the 100-c.c. mark. Shake well. Mea- 

 sure out 25 c.c. of this solution with a pipette into an 8-oz. 

 flask. Add 5 grams of reduced iron and 20 c.c. of dilute sul- 

 phuric acid (one volume of acid to three of water). Place the 

 flask in an inclined position on the stand (fig. 28) and allow 

 the reaction to continue without heating until effervescence 

 ceases. Now heat to boiling for six minutes, then allow to cool. 

 The nitrogen is now in the state of sulphate of ammonia, and 

 may be estimated as in paragraph 100, operation I. Instead of 

 magnesia, however, add 20 c.c. of caustic soda solution and two 

 or three lumps of clean granulated zinc. The calculation in this 

 case is similar to the one in paragraph 89 ; but it must be re- 

 membered that, whereas about 2 grams of the potassium nitrate 

 were weighed out and dissolved, only one quarter of this solu- 

 tion was used. 



104. Schloesing's Method (modified).— This is a very 

 rapid method, and is generally used for the estimation of the 

 nitrates in soils. It may, however, be advantageously used in 

 the estimation of nitrogen in alkaline nitrates and manures. 



Method. — The nitrate is decomposed by sulphuric acid 

 and ferrous sulphate according to the equation 



2NaN0 3 + 6FeS0 4 + 4H 2 S0 4 - 2NO + 3Fe 2 (S0 4 ) 3 + Na 2 S0 4 + 4H.O. 



The nitric oxide is measured. 



105. Apparatus. — Many forms of apparatus are used for 

 this operation. Of the two described here, the first is in use 

 at the laboratory of the Royal Agricultural Society of England, 

 and has the great advantage that no other gas excepting the 

 nitric oxide and a little water vapour is present in any part of 

 the apparatus. The second one is more readily prepared but 

 less easily worked. 



106. The first apparatus is shown in fig. 31. a is a round- 

 bottomed flask, in which the reaction is to take place, b is a 



