434 AGRICULTURAL ANALYSIS 



nitrates are present. For the minute traces of nitrates found in 

 rain and drainage waters, and in some soil extracts, the reduction 

 method may also be applied, but in these cases the ammonia which 

 is formed must be determined colorimetrically (nesslerizing) and 

 not by distillation. The processes about to be described are 

 especially applicable to the examination of fertilizers containing 

 only small quantities of nitrates and of soils and waters rich in 

 nitrates. 



REDUCTION IN ALKALINE SOLUTIONS 



372. Extraction of the Nitrates. Place one kilogram of the dry 

 soil or fertilizers poor in nitrates, calculated to water-free sub- 

 stance, on a percolator of glass or tin. Moisten the soil thor- 

 oughly with pure distilled water, and allow to stand for half an 

 hour. Add fresh portions of pure distilled water until the filtrate 

 secured amounts to one liter. If the first filtrate be cloudy before 

 use it may be refiltered. 



373. Qualitative Test for Nitrates. Evaporate five cubic centi- 

 meters of the extract as obtained above in a porcelain crucible, 

 having first dissolved a small quantity of pure brucin sulfate 

 therein. When dry, add to the residue a drop of concentrated sul- 

 furic acid free of nitrates. If the nitrate calculated as potassium 

 nitrate does not exceed the two-thousandth part of a milligram, 

 only a pink color will be developed; with the three-thousandth 

 part of a milligram, a pink color with reddish lines; with the 

 four-thousandth part of a milligram, a reddish color ; with the 

 five-thousandth part of a milligram, a distinct red color. 



374. Sodium-Mercury Amalgam Method. The reduction is 

 effected by means of hydrogen evolved by the action of a prep- 

 aration of sodium amalgam. Place 100 cubic centimeters of 

 mercury in a flask of half a liter capacity; warm until paraffin 

 will remain melted over the surface; drop successively in the 

 paraffin-covered mercury pieces of metallic sodium of the size of 

 a pea until 6.75 grams have united with the mercury. The amal- 

 gam thus prepared contains 0.5 per cent, of metallic sodium and 

 may be preserved indefinitely under the covering of paraffin. 



Estimation of the Nitrates. Evaporate 100 cubic centimeters 



