90 AGRICULTURAL ANALYSIS 



Where too little magnesia mixture is employed, therefore, two 

 sources of loss are to be guarded against; viz., a part of the 

 phosphoric acid may remain in solution and another part be vol- 

 atilized on ignition. The explanation of the volatilization is as 

 follows : In the presence of ammonium citrate, magnesium chlorid 

 may be partly converted into magnesium citrate and ammonium 

 chlorid. There may be a time, therefore, in the precipitation 

 with not too great excess of magnesia mixture, when propor- 

 tionally there is little magnesium chlorid and much ammonium 

 chlorid present. The formation of a salt represented by the 

 formula 2Mg(NH 4 ) 4 (PO 4 ) 2 may take place which, upon ignition, 

 breaks up into 2Mg(PO 3 ), and finally passes into Mg 2 P 2 O 7 with 

 less of P 2 O 5 . This theoretical condition has but little weight, 

 however, practically in the analysis of fertilizers, since in these 

 cases a large quantity of lime is always present. But even in 

 these cases traces of volatile P 2 O 5 may be discovered. 



Wells has shown that the citrate method gives good results 

 in certain conditions, but that this accuracy is reached by a for- 

 tunate compensation of errors. 6 * The ammonium magnesium salt 

 does not precipitate all the phosphoric acid in this process, but 

 contains enough impurities to make up for this loss. 



Johnson in conjunction with Osborne has shown that the re- 

 sults by the citrate method practiced in accordance with the details 

 laid down by Vogel, are too low, but that this difficulty could be 

 overcome by using more and stronger magnesia mixture and a 

 larger quantity of strong ammonia solution. 04 The citrate method 

 was found to give unsatisfactory results when iron and alumina 

 were present in any considerable quantity. In the examination of 

 the final ignited precipitate, which should be pure magnesium 

 pyrophosphate, it was found to consist of only from 94.98 to 97.83 

 per cent, of that salt. The chief impurity found was calcium oxid, 

 the percentage of which varied from 2.05 to 3.95 in six cases. 

 There was also a considerable percentage of loss due, probably, 

 to magnesia and pyrophosphoric acid. 



The presence of large quantities of iron and alumina also im- 

 pairs the accuracy of the molybdate method when the precipita- 

 M Journal of the American Chemical Society, 1894, 16 : 462. 



