PHOSPHORIC ACID DETERMINATION 83 



nesium solution of which from 15 to 20 cubic centi- 

 meters are required for each 0.2 gram of phosphoric anhydrid 

 present. After the addition of the magnesium solution and dur- 

 ing the cooling it is desirable to frequently shake the vessel con- 

 taining the precipitate, especially if the precipitate has changed 

 into the dense, crystalline form, which is apt to be the case, for 

 the addition of the magnesium mixture has been slow and there 

 is not a sufficient excess of ammonia. The ordinary stirring ap- 

 paratus which is used can give valuable service at this point. 

 The nitration of the precipitated phosphoric acid should not take 

 place until after four hours' standing. A longer time than four 

 hours does not have any influence upon the results. After the 

 collection of the material in the filter it is washed with 2.5 per 

 cent, of ammonia. It is very convenient to have the bottom of 

 the crystal covered with precipitated platinum, since in this case 

 the heating over a blast-lamp is unnecessary, the ordinary heating 

 over a common burner being sufficient. For conversion the factor 

 0.63757 is used. (Log. 0.80453^-1). 



86. Influence of Aluminium, Magnesium and Calcium upon the 

 Phosphoric Acid Determination. In solutions containing alumin- 

 ium, iron, magnesium and calcium in which phosphoric acid is 

 to be determined, the influence of these bases upon the deter- 

 mination must not be neglected. Neubauer has called attention 

 to this point, especially in connection with the determination of 

 phosphoric acid in the hydrochloric acid solutions of soils. 57 



The well known fact that the chlorid of lime and aluminium 

 at moderate heating in the air and in water produce insoluble 

 oxids with which the phosphoric acid as an insoluble sulfate is 

 entangled is well known. The chlorids of alkalies, however, 

 are not changed by this heating, and this difference in deport- 

 ment is the principle upon which Neubauer bases his observations. 

 Where only potassium and phosphoric acid are to be determined 

 in a hydrochloric acid solution, for instance, of a soil, Neubauer 

 uses the following process: 



A volume of the solution corresponding to 25 grams of the 

 original substance (soil) is evaporated to dryness in a platinum 



" Die landwirtschaftlichen Versuchs-Stationen, 1905-6, 63 : 141. 



