GENERAL, PRINCIPLES 89 



have been present in the original sample. The phosphorus is thus 

 obtained in composition with molybdenum and ammonium in a 

 form easily soluble in ammonia, from which it can be accurately 

 separated by means of a soluble salt of magnesia. 



The citrate method has for its object the suppression of this in- 

 termediate step and the determination of the phosphoric acid by 

 direct precipitation in presence of iron, lime, and alumina. The 

 principle in which it is based rests on the well known power of 

 an alkaline ammonium citrate to hold in solution the salts of iron, 

 alumina, and lime, while at the same time it permits of the separa- 

 tion of phosphoric acid, as ammonium magnesium phosphate. In 

 no case can the citrate method be regarded as a rigidly exact an- 

 alytical process, but large experience has shown that the errors 

 of the method are compensatory and that it affords a good and 

 ready method for fertilizer control. 



When phosphoric acid solutions which contain no iron, lime, 

 alumina, or manganese, are precipitated in presence of ammonium 

 citrate the results obtained vary markedly with the quantity of 

 magnesia mixture employed. Grupe and Tdlens were the first 

 to point out that a portion of the phosphoric acid might remain 

 in solution, but that the precipitate might contain a sufficient 

 excess of magnesia to compensate for the loss. 61 If lime, iron and 

 alumina, moreover, are present, the precipitate obtained is not 

 wholly free from these bodies. It is true that the quantities of 

 these bodies found in the precipitate are quite small, but they may 

 at times influence the accuracy of the results. The presence of 

 lime and magnesia in the precipitate, as already mentioned, is 

 indicated by the yellow color produced by moistening the white 

 ignited precipitate with silver nitrate. 62 It has been further shown 

 by Glaser,'as well as by Neubauer, that a portion of the phos- 

 phoric acid may be lost by volatilization in the citrate method. 68 

 When the ignition is carried on in a crucible where the cover is 

 coated with magnesia to intercept the volatilized acid, a considera- 

 ble quantity of it can be recovered by the molybdate method. 



61 Journal fur Landwirtschaft, 1882, 30 : 23. 



6i Tollens, Journal fur Landwirtschaft, 1882, 30 : 48. 



63 Zeitschrift fiir angewandte Chemie, 1894, 7 : 544- 



