1378 Journal of Applied Microscopy 



In A the reaction sometimes fails for lack of sufficient ammonium chloride, 

 magnesium hydroxide being precipitated. A slight excess of this salt will do no 

 harm. 



Both modifications fail if there is an insufficiency of ammonium hydroxide, 

 for it should be remembered that there must be not only enough ammonium 

 present to unite to form the proper compound, but that this salt will not separate 

 save in alkaline solution. 



The advantage of employing modification 7? 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 evapora- 

 tion is not completely soluble in the ammonium hydroxide solution, it is best, 

 though not essential, to draw off the clear liquid before adding to it the sodium 

 phosphate. 



Reactions A and B work equally well in the cold, but are then a trifle slower. 

 Generally, an amorphous precipitate is at first produced, which begins to crystal- 

 lize in a few seconds. The formation of merely an amorphous precipitate must 

 never be taken as evidence of the presence of magnesium. 



It must also be borne in mind that the use of too strong ammonium hydroxide 

 in excess so reduces the solubility of many salts as to cause their separation, 

 hence it is necessary to beware, in reactions of this character, of deciding too 

 hastily as to the result of a test. 



See remarks made under Ammonium, Method II (Journal, p. 1190), and 

 Calcium, Method V (Journal, p. 1247). 



In the presence of phosphates the detection of magnesium becomes quite 

 difficult, particularly if other elements are present which form phosphates insol- 

 uble in ammonium hydroxide. If arsenates are also present, a still further com 

 plication arises, for, as we have already seen, double ammonium arsenates of 

 calcium, zinc, etc., are formed, which are isomorphous with ammonium magnes- 

 ium phosphate. 



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

 hydroxide will cause the separation of characteristic crystals of ammonium mag- 

 nesium 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. Details will be given later. 



Exercises for Practice. 



Try method I A on z. solution of MgSO^, then try it on salts of Fe, Mn, Co, 

 Ni, Al, Zn, Cd. Repeat the experiments, this time adding the HNa2P04 before 

 the NH4OH. 



Try IB in like manner. 



Make mixtures, trying various combinations of the above with members of 

 Groups I and II. 



Consult notebook on the results obtained with the experiments tried under 

 Ammonium II and Calcium V. 



