APPENDIX G 

 DETERMINATION OF NITRATE ^ 



The nitrates, if present in small quantities, are best determined 

 by means of the color produced by the action of phenoldisulphonic 

 acid and making alkaline with ammonia. 



Chlorides, when present in considerable quantities, interfere quite 

 markedly with the determination of nitrates and must be previously 

 removed. This is best accomplished by means of silver sulphate free 

 from nitrates. This can be added in the solid form, thus avoiding 

 dilution of the original solution. 



REAGENTS REQUIRED 



1. Phenoldisulphonic acid reagent. This is prepared by mixing 30 g. 

 of pure crystallized phenol with 37 g. (20.1 cc.) of concentrated sul- 

 phuric acid (sp. gr. 1.84) and heating for six hours at 100° C. by 

 setting the lightly stoppered flask in boiling water. 



The acid thus prepared may crystallize out on standing, especially 

 during the cold season. It may be brought into solution by heat, but 

 the addition of water to affect solution is to be avoided. 



2. Ammonium hydroxide. Dilute strong ammonium hydroxide so- 

 lution (sp. gr. 0.9) with an equal volume of water. 



3. Standard nitrate solution. Dissolve 0.1631 g. of pure, dry potas- 

 simn nitrate in water and make up to 1 liter. Of this stronger solu- 

 tion 100 cc. are diluted to 1 liter. This constitutes the standard 

 nitrate solution and contains 0.01 of a milligram of NO^ in each 

 cubic centimeter. 



4. Standard colorimetric solution. Evaporate 10 cc. of the standard 

 nitrate solution (3) to dryness in a porcelain dish on a water or steam 

 bath and treat as described under Analytical Process below, finally 

 diluting the solution to 100 cc. This standard colorimetric solution 

 has the strength of one part of NO, per million. 



1 Adapted from Schreiner and Failyer, Bulletin No. 31, Bureau of Soils, U.S. 

 Dept. Agr., 1906. 



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