ANALYTICAL PROCESS 91 



loosen the residue and bring the acid well in contact with 

 every portion of it. The time of action on the nitrate 

 should be about ten minutes. At the end of this time the 

 acid is diluted with about 15 cc. of water and made al- 

 kaline with ammonium hydroxide, a yellow color being 

 developed when the solution becomes alkaline. This is 

 then diluted to 50 cc. or 100 cc. and compared with the 

 standard colorimetric solution. If the color is too in- 

 tense for direct comparison with this standard, an aliquot 

 portion may be taken and diluted to definite volume and 

 the strength of this determined." 



To clear the soil extracts, Greaves and Hirst (6) found 

 the following methods to give good results: The addition 

 of 2 grams of lime, ferric sulphate, ferric alum, sodium 

 alum, or potassium alum to the soil-water mixture; filter- 

 ing through Pasteur-Chamberland filter, or centrifuging. 



To eliminate possible error due to the presence of chlo- 

 rides or other inorganic materials, certain reduction 

 methods have given better results than the above method. 

 The iron reduction method, as described below, was found 

 by Greaves and Hirst (6) to give more satisfactory results 

 in the presence of inorganic salts and in the presence of 

 organic matter than did other methods. The soil is first 

 agitated for five minutes with five times its weight of water 

 and clarified by one of the methods described above, prefer- 

 ably with alum. 



"An aliquot part (100 cc.) of the supernatant liquid is 

 pipetted off, and, together with 2 cc. of a saturated solu- 

 tion of sodium hydroxide, evaporated to about one-fourth 

 of its original volume to free from ammonia. If urea is 

 present, it is necessary to evaporate to dryness. To this 

 is added 50 cc. of ammonia-free water, 5 grams of 'iron- 

 by-hydrogen,' and 30 cc. of sulphuric acid (sp. gr. 1.35). 



