136 AGRICULTURAL ANALYSIS 



beaker, and there are added, first, 10 cubic centimeters of mag- 

 nesium citrate solution, and second, a large excess of ammonia. 

 If the quantity of the magnesium citrate solution be sufficient, the 

 mixture should at first remain perfectly limpid and only become 

 turbid at the end of some moments and especially after the mix- 

 ture is stirred. 



If there should be an immediate turbidity, it is proof that the 

 quantity of magnesium citrate solution employed has been in- 

 sufficient to hold the iron and aluminum phosphates in solution 

 until the new compounds are formed, and it is necessary to be- 

 gin again by doubling its amount. Good results can not be ob- 

 tained by adding a second portion of the magnesium citrate solu- 

 tion to the original, since the iron and aluminum phosphates which 

 are once formed are redissolved with difficulty. Many chemists 

 at the present time abstain from using the magnesium citrate solu- 

 tion and replace it by a solution of citric acid and one of magne- 

 sium sulfate, which they pour successively into the sample under 

 examination. This is a cause of grave errors which it is neces- 

 sary to point out. Joulie has indeed recognized the fact that 

 the precipitation of the phosphoric acid is not completed in pres- 

 ence of ammonium citrate unless it is employed in conjunction 

 with a sufficient excess of magnesia. But the foreign matters 

 which accompany the phosphoric acid require different quanti- 

 ties of ammonium citrate in order to keep them in solution, and 

 it is important to increase the magnesium solution at the time of 

 increasing the citric acid in order to maintain them always in 

 the same proportion. This is easily accomplished by measuring 

 the two solutions, but it is much more easily done by uniting them 

 and adding them together. 



121. The Magnesium Citrate Solution. The formula originally 

 proposed by Joulie, modified by Millot, and adopted by the French 

 Association of Chemists, is as follows : Citric acid, 400 grams ; 

 pure magnesium carbonate, 40 grams ; caustic magnesia, 20 grams ; 

 distilled water, half a liter. After solution, add enough of am- 

 monia to render strongly alkaline, requiring about 600 cubic 

 centimeters. Make the volume up with distilled water to one 

 and a half liters. Tf the solution be turbid, it is proof that the 



