THE AMMONIO-MANGANOUS METHOD 151 



It is to be noted that the determinations with uranium require 

 always two successive titrations. It would therefore be an ad- 

 vantage in all operations to precipitate a weight of ammonium 

 magnesium phosphate sufficient for allowing this precipitate to be 

 dissolved and made up to 100 cubic centimeters, on which amount 

 it would be possible to execute two, three or four determinations, 

 and thus to obtain a result of great accuracy. 



138. Conclusions. It has been seen from the above data that 

 the French chemists have worked out the uranium volumetric 

 method with great patience and attention to detail. Where many 

 determinations are to be made it is undoubtedly possible for an 

 analyst to reach a high degree of accuracy as well as to attain a 

 desirable rapidity by using this method. For a few determina- 

 tions, however, the labor of preparing and setting the standard 

 solutions required would be far greater than the actual determina- 

 tions either by the molybdate or citrate gravimetric methods. For 

 control work in factories and for routine work connected with fer- 

 tilizer inspection, the method has sufficient merit to justify a com- 

 parison with the processes already in use by the official chemists 

 of this country. 



The use of an alkaline ammoniacal citrate solution, however, 

 for the determination of reverted acid renders any comparison of 

 the French method with our own impossible. On the other hand, 

 the French method for water-soluble acid is based on the same 

 principle as our own ; viz., washing at first with successive small 

 portions of water, and thus avoiding the decomposition of the 

 soluble phosphates, which is likely to occur when too great a vol- 

 ume of water is added at once. 



In the matter of the temperature and time as affecting the sol- 

 ubility of reverted acid, the French method is also distinctly in- 

 ferior to our own. The digestion is allowed to continue from 

 12 to 24 hours, at the pleasure of the analyst, and meanwhile it 

 is subjected to room temperature. It is not difficult to see that 

 this treatment in the same sample would easily yield disagreeing 

 results between 12 hours at a winter temperature and 24 hours at 

 summer heat. 



139. The Ammonio-Manganous Method. The principle of this 



