47O AGRICULTURAL ANALYSIS 



Standard Sodium Nitrite. 0.275 gram pure silver nitrite is 

 dissolved in pure water, and a dilute solution of pure sodium 

 chlorid added until the precipitate ceases to form. It is then 

 diluted with pure water to 250 cubic centimeters and allowed to 

 stand until clear. For use, 10 cubic centimeters of this solution 

 are diluted to 100. It is to be kept in the dark. 



One cubic centimeter of the dilute solution is equivalent to 

 o.ooooi gram of nitrogen. 



The silver nitrite is prepared in the following manner : A hot 

 concentrated solution of silver nitrate is added to a concentrated 

 solution of the purest sodium or potassium nitrite available, fil- 

 tered while hot and allowed to cool. The silver nitrite will 

 separate in fine needle-like crystals, which are freed from the 

 mother-liquor by filtration with the aid of a filter pump. The crys- 

 tals are dissolved in the smallest possible quantity of hot water, 

 allowed to cool and crystallize, and again separated by means of 

 the pump. They are then thoroughly dried in the water bath, and 

 preserved in a tightly-stoppered bottle away from the light. Their 

 purity may be tested by heating a weighed quantity to redness in a 

 tared, porcelain crucible and noting the weight of the metallic 

 silver. One hundred and fifty-four parts of silver nitrite leave a 

 residue of 108 parts of silver. 



Analytical Process. Twenty-five cubic centimeters of the 

 water, soil or fertilizer solution are placed in a color-comparison 

 cylinder, the measuring vessel and cylinder having previously 

 been rinsed with the water to be tested. By means of a pipette 

 two cubic centimeters each of the test solutions are dropped in. 

 It is convenient to have three pipettes for this test, and to use 

 them for no other purpose. In any case the pipette must be 

 rinsed out thoroughly with nitrite-free water each time before 

 using, as nitrites, in quantity sufficient to give a distinct reaction, 

 may be taken up from the air. 



One cubic centimeter of the standard nitrite solution is placed 

 in another clean cylinder, made up with nitrite-free water to 25 

 cubic centimeters and treated with the reagents, as above. 



In the presence of nitrites a pink color is produced, which, in 

 dilute solutions, may require half an hour for complete develop- 



