352 ELEMENTARY CHEMICAL MICROSCOPY 



iron, manganese, cobalt and nickel form, with ammonium and 

 phosphoric acid, salts of similar composition to, and isomorphous 

 with, the magnesium salt. 



The ammonium glucinum phosphate, ammonium zinc phos- 

 phate and ammonium cadmium phosphate are not precipitated 

 in crystal form. 



The advantage of employing modification II lies in the fact 

 that owing to the presence of ammonium citrate, there is little 

 danger of the interference of the elements listed above. If in 

 following this method, the residue after evaporation is not com- 

 pletely soluble in the ammonium hydroxide solution, it is best, 

 though not essential, to decant the clear liquid before adding 

 to it the sodium phosphate. 



Reactions I and II work equally well in the cold, but are then 

 a trifle slower. Generally, an amorphous precipitate is at first 

 produced which begins to crystallize in a few seconds. The 

 formation of merely an amorphous precipitate must never be 

 taken as evidence of the presence of magnesium. 



In the presence of phosphates the detection of magnesium 

 becomes quite difficult, particularly if other elements are present 

 which form phosphates insoluble in ammonium hydroxide. If 

 arsenates are also present, a still further complication arises, for, 

 as we have already seen, double ammonium arsenates of calcium, 

 zinc, etc., are formed, which are isomorphous with ammonium 

 magnesium phosphate. 



Of course it may happen that in some cases the mere addition 

 of ammonium hydroxide will cause the separation of character- 

 istic crystals of ammonium magnesium phosphate. Generally, 

 however, it is first necessary to remove the phosphoric acid. 

 This can be accomplished by tin and nitric acid, or by means of 

 ammonium tungstate and nitric acid (see Phosphates, page 426). 



Precautions. 



In I, the reaction sometimes fails for lack of sufficient ammo- 

 nium chloride, magnesium hydroxide being precipitated. A 

 slight excess of this salt will do no harm. 



Both modifications fail if there is an insufficiency of ammo- 



