1 86 AGRICULTURAL ANALYSIS 



centimeters of ammonium nitrate solution, warmed for 20 minutes 

 at 80, filtered after cooling, and the yellow precipitate washed 

 with cold water. The water will gradually dissolve all the silicic 

 acid from the yellow precipitate and carry it into the filtrate. The 

 yellow precipitate is then dissolved in 2.5 per cent, liquid ammonia 

 and precipitated with magnesia mixture and the precipitate 

 washed, ignited and weighed in the way described. 



159. Determination of Phosphoric Acid in Superphosphates. 

 (i) Citrate-Soluble Phosphoric Acid. Five grams of the su- 

 perphosphate are rubbed with 100 cubic centimeters of Wagner's 

 acid citrate solution in a mortar and washed into a flask of 500 

 cubic centimeters content and .diluted to 500 cubic centimeters 

 with water. With frequent shaking, the flask is allowed to 

 stand for 12 hours, after which its contents are filtered. Fifty 

 cubic centimeters of the filtrate are treated with 10 cubic centi- 

 meters of the Joulie solution and 15 cubic centimeters of the mag- 

 nesia mixture and, if necessary, made distinctly alkaline with am- 

 monia, vigorously stirred, and, after two hours, filtered. The 

 precipitate is washed, ignited, and weighed as described, or titrat- 

 ed, after solution in nitric acid and the addition of sodium ace- 

 tate, with uranium solution. Example: The weighed precipi- 

 tate has 0.1272 gram Mg 2 P 2 O 7 , then the phosphate contains 

 12.72X2X0.64=16.28 per cent, of citrate-soluble P 2 O 5 . 



(2) Water-Solitble Phosphoric Acid. Twenty grams of super- 

 phosphate are rubbed in a mortar and washed into a flask of one 

 liter content and made up to the mark with water. After two 

 hours digestion with frequent shaking, the contents of the flask 

 are filtered through a folded filter. Twenty-five cubic centime- 

 ters of the filtrate equivalent to 0.5 gram of the substance are 

 precipitated with magnesia mixture, the precipitate filtered, 

 washed, ignited, and weighed, or the moist filtrate may be dissolved 

 upon the filter with a little nitric acid, treated with sodium acetate 

 and titrated, as described, with uranium solution. 



Example: 14.5 cubic centimeters of the uranium solution are re- 

 quired for the precipitate from 25 cubic centimeters of the orig- 

 inal solution=o.5 gram superphosphate; it contains then 14.5 X 



