44 



DETERMINATION OF THE PHOSPHORIC ACID. 



The following solutions are used in .the determination of phosphoric 

 acid; 1, aniuioniuni molybdate: 2, acid ammonium nitrate, and 3 mag- 

 nesia mixture. 



PREPARATION OF THE AMMONIUM MOLYBDATE SOLUTION. 



Dissolve 75 grams of ammonium molybdate in 500 c. c. of distilled 

 water, adding the water in small quantities at a time, and filter into 

 500 c. c. of nitric acid of a specific gravity 1.20. One c. c. of this 

 solution is equivalent to 0.001 grams of phosphoric acid. 



PREPARATION OF THE ACID AMMONIUM NITRATE. 



Add to 325 c. c. of nitric acid, specific gravity 1.2, 200 c. c. of a mixt- 

 ure of equal parts of ammonia, specific gravity 0.96, and water. Allow 

 to cool, and keep in a glass-stoppered bottle. 



PREPARATION OF THE MAGNESIA MIXTURE. 



Dissolve 125 grams of crystallized magnesium sulphate and 125 grams 

 of ammonium chloride in 1 liter of water ; when all is dissolved add 500 

 c. c. of ammonia, specific gravity 0.96. 



The determination of phosphoric acid is made in duplicate. 



Add to 200 c. c. of the solution, equal to 4 grams of the air-dried soil, 

 50 c. c. of ammonium molybdate and 10 c. c. of acid ammonium 

 nitrate, and heat on the water-bath at 80 C., with frequent stirring for 

 four hours. Allow the precipitate to settle, and decant the clear fluid 

 on a small filter, wash the precipitate with about 25 c. c. of acid ammo- 

 nium nitrate, decant the clear fluid, and then transfer the precipitate to 

 the filter, washing it with the same. 



Set the filtrate aside for twelve hours in a warm place, after adding 

 10 c. c. of ammonium molybdate to insure the precipitation of all the 

 phosphoric acid. If a precipitate should occur, which rarely happens, 

 filter it off and add to the main precipitate. 



Dissolve the precipitate through the filter, into the same beaker, 

 with warmed dilute ammonia, 1 of ammonia to 3 of water, and as there 

 often remains upon the filter a small quantity of iron, arising from the 

 phosphate of that metal which the dilute nitric acid has disolved, re- 

 pass the ammoniacal liquid through the filter several times. To the 

 solution is added enough hydrochloric acid to make it decidedly acid, 

 and then enough ammonia to render it decidedly alkaline and to redis- 

 solve the precipitate formed, and finally 5 c. c. of magnesia mixture. 

 The latter is not to be added, however, until the solution becomes cold. 

 After adding the magnesia mixture set the solution aside in a cool 

 place for twelve hours, to allow the crystalline precipitate of ammonium 

 magnesium phosphate to thoroughly settle. Filter by decantation, 



