METHOD OF HALLE EXPERIMENT STATION 91 



tion of the yellow salt takes place at too high a temperature. When 

 the temperature of precipitation in the method is above 50 the 

 results are likely to be too high, while a great excess of nitric 

 acid in the reagent may produce a contrary effect. In the lattei 

 case the filtrate from the yellow salt should be mixed with addi- 

 tional quantities of molybdate solution until no further precipitate 

 takes place. 



Many methods of conducting the citrate method have been pro 

 posed, but the best of them are based on the one elaborated at 

 the experiment station of Halle by Biihring, and which will be 

 given in the next paragraph, followed by some other methods in 

 use in other localities. 



90. Method of Halle Agricultural Experiment Station. 85 

 The citrate method elaborated by Biihring, as described by Mor- 

 gen, is the one employed. 66 The principle depends upon the direct 

 precipitation of the phosphoric acid by magnesia mixture. By 

 the addition of a solution of ammonium citrate the precipitation 

 of lime, iron, alumina and other bases is practically prevented. 

 The precipitate of ammonium magnesium phosphate is converted 

 by ignition into magnesium pyrophosphate and weighed as such. 

 By the use of this method a part of the phosphoric acid some- 

 times escapes precipitation and a portion of the other bases is 

 sometimes thrown down with the precipitate. Experience has 

 shown that by adhering to certain precautions the weight of im- 

 purities in the precipitate may be made to correspond very nearly 

 to the weight of the phosphoric acid which escapes precipitation. 



(i) Soluble Acid. The soluble phosphates are first brought 

 into solution in such a way that one liter of water contains the 

 soluble phosphoric acid from 20 grams of the substance. 

 Twenty grams are rubbed in a porcelain mortar with water and 

 through a wide-necked funnel washed into a bottle-shaped flask 

 in which a little water has been previously placed. The flasks em- 

 ployed are made of thick glass in order to withstand shaking. 

 After the substance is washed in, the flasks are filled to the mark 



65 Bieler und Schneidewind, Die agrikultur-chemische Versuchsstation, 

 Halle a/S, ihre Einrichtung undThatigkeit, 1892 : 56. 

 86 Die chenrische Industrie, 1890, 13 : 135, 139. 



