98 AGRICULTURAL ANALYSIS 



nition oven, Fig. 5. When the muffle of the furnace shows a 

 white heat or a white-red heat, it is at the proper temperature for 

 the estimation. At higher temperatures, the filtering property 

 of the asbestos felt is easily injured. Generally, an ignition of five 

 minutes is sufficient, but with double superphosphates, 10 min- 

 utes are required. 



91. The Swedish Citrate Method. 08 This method of determina- 

 tion is a modification of the citrate method already described and 

 is founded on the observation that phosphoric acid in the pres- 

 ence of calcium salts, without the necessity of previously con- 

 verting into phosphomolybdate, is precipitated directly by mag- 

 nesia mixture from a solution to which ammonium citrate has 

 been added, provided first, that the solution contain a sufficient 

 quantity of sulfuric acid to convert all the calcium compounds 

 into sulfates, and second, that only as much citrate be added as 

 is required to keep the calcium salts in alkaline solution. In other 

 words, it is the method of separation devised by C. Glaser and 

 others. 69 



Reagents, (i) Citric Acid Solution. Prepared by dissolving 

 500 grams of citric acid in water and completing to a volume of 

 one liter. 



(2) Ammonia of 0.959 specific gravity. 



(3) Magnesia Mixture, composed of 140 grams of magnesium 

 sulfate, 150 grams ammonium sulfate, and 30 grams of chlorid of 

 ammonium dissolved in 350 cubic centimeters of 16 per cent, am- 

 monia and 1650 cubic centimeters of water. The ammonium 

 chlorid is added to prevent the precipitation of basic magnesium 

 sulfate. 



The various processes are conducted as follows : 

 (a) Water-Soluble Phosphoric Acid. Add 20 cubic centi- 

 meters of citric acid solution to 50 cubic centimeters of the 

 water-soluble solution obtained according to the Swedish molyb- 

 date method, and then add 33 cubic centimeters of ammonia. 

 68 Official Swedish Methods Translated for the Author by F. W. Woll. 

 89 Zeitschrift fur analytische Chemie, 1885, 24 : 178; 1886, 25 : 416. 

 Chemiker-Zeitung, 1888, 12 =85,492. 

 Zeitschrift fur angewandte Chemie, 1888, 1 : 354. 

 Die landwirtschaftlichen Versuchs-Stationen, 1888, 35 : 439. 



