170 AGRICULTURAL ANALYSIS 



wash the filter with 100 cubic centimeters of water. Finally add 

 to the filtrate and wash-water 80 cubic centimeters of water 

 and 10 of strong sulfuric acid. Pass the mixture through the 

 jones reducing tube and follow it with 200 cubic centimeters of 

 water, taking care that no air enter the tube during the opera- 

 tions. The solution collected in the flask should be at once 

 titrated with potassium permanganate. 



In cases where the content of phosphorus is very high the solu- 

 tion of the yellow precipitate is made to a definite volume and 

 the reduction and titration performed on an aliquot thereof. 



Solutions Used: (i) Nitric Acid. One part of nitric acid of 

 1.42 specific gravity and two parts of water by volume. The 

 specific gravity of the mixture is about 1.18. 



(2) Permanganate Solution for Oxidizing. Dissolve 12.5 

 grams of potassium permanganate in one liter of water. 



(3) Ferrous Sulfate. Fresh crystals not effervesced and free 

 from phosphorus. 



(4) Ammonia. The strong ammonia used should have a 

 specific gravity of about 0.90 and the dilute of 0.96 at 15.5. 



(5) Molybdic Solution. Dissolve 100 grams of molybdic anhy- 

 drid in 400 cubic centimeters of ammonia of 0.96 specific gravity 

 and pour the solution slowly, with constant stirring, into one 

 liter of nitric acid of about 1.20 specific gravity. Heat the mix- 

 ture to 45 and add one cubic centimeter of a 10 per cent, solu- 

 tion of sodium phosphate, stir vigorously, and allow to stand in 

 a warm place for 18 hours. The object of adding the sodium 

 phosphate is to remove any substance which may contaminate the 

 yellow precipitate. Filter before using. 



(6) Acid Ammonium Sulfate. To half a liter of water add 

 27.5 cubic centimeters of 0.96 ammonia and 24 cubic centimeters 

 of strong sulfuric acid, and make the volume one liter with water. 



(7) Potassium Permanganate for Titration. Dissolve four 

 grams of potassium permanganate in two liters of water, heat 

 nearly to boiling for an hour, allow to stand for 18 hours, 

 and filter on asbestos felt. The. solution must not come in con- 

 tact with rubber or other organic matter. The solution may be 

 standardized with pure iron (piano wire), with thoroughly air- 



