462 AGRICULTURAL ANALYSIS 



Standard potassium nitrate: 0.722 gram of potassium nitrate, 

 previously heated to a temperature just sufficient to fuse it, is 

 dissolved in water, and the solution made up to 1000 cubic centi- 

 meters. One cubic centimeter of this solution contains o.oooi 

 gram of nitrogen. 



Analytical Process. A measured volume of the sample is evap- 

 orated just to dryness in a platinum or porcelain basin. One 

 cubic centimeter of the phenoldisulfonic acid is added and thor- 

 oughly mixed with the residue by means of a glass rod. One 

 cubic centimeter of water and three drops of strong sulfuric acid 

 are added, and the dish gently warmed. The liquid is then di- 

 luted with about 25 cubic centimeters of water, ammonium hy- 

 droxid added in excess, and the solution made up to 50 cubic 

 centimeters. 



The reactions are: 



Acid phenyl Trinitrophenol 



sulfate. (picric acid). 



HC 6 H 5 SO 4 +3HNO 3 =HC 6 H 2 (NO 2 ) 3 O+H 2 SO 4 +2H 2 O. 



Ammonium picrate. 



HC 6 H 2 (NO 2 ) 3 O+NH 4 HO=NH 4 C 6 H 2 (NO 2 ) 3 O+H 2 O. 



The ammonium picrate imparts to the solution a yellow color, 

 the intensity of which is proportional to the amount present. 



One cubic centimeter of the standard solution of potassium 

 nitrate is similarly evaporated in a platinum dish, treated as 

 above, and made up to 50 cubic centimeters. The color pro- 

 duced is compared to that given by the water, and one or the 

 other of the solutions diluted until the tints of the two agree. 

 The comparative volumes of the liquids furnish the necessary 

 data for determining the amount of nitrate present, as the follow- 

 ing example shows: 



One cubic centimeter of standard nitrate is treated as above 

 and made up to 100 cubic centimeters, representing o.oooi gram 

 nitrogen. 



Suppose loo cubic centimeters of the sample similarly treated 

 are found to require dilution to 150 cubic centimeters before the 

 tint will match that of the standard ; then 



loo : 150 :: o.ooi : 0.0015 



