and Laboratory Methods. 



1191 



Fig. 40. 



The crystals of magnesium ammonium phos- 

 phate belong to the orthorhombic system and 

 have a great tendency to assume hemihedral and 

 hemimorphic and skeleton forms. 



Only very little magnesium acetate should be 

 used since either a dense amorphous precipitate 

 of magnesium phosphate will result, or if the con- 

 ditions are favorable this salt will itself crystal- 

 lize in star-like prism aggregates. 



This method can be applied directly to the 

 solution of the substance without the necessity of 

 having recourse to the separation of the am- 

 monia. When applied directly it is advisable to 

 substitute sodium hydroxide for the carbonate. 

 The objection to this procedure is that many elements are precipitated as 

 phosphates in alkaline solution, and that magnesium hydroxide almost invariably 

 separates as a flocculent mass. 



Exercises for Practice. 



Expel the ammonia from an ammonium salt by the method above described, 

 and test by Method I. 



Repeat, and test the drop by Method II. First employing primary sodium 

 carbonate, then using sodium hydroxide. 



Make a mixture of various compounds introducing a salt of ammonium. 

 Test directly by II. Expel the ammonia and test by either I or II. 



LITHIUM. 



The element lithium can be considered as marking the transition between 

 the alkalies on the one hand and the alkaline earths on the other, and is there- 

 fore a link between Groups I and II. Because of this — -its peculiar behavior — 

 lithium is worthy of a brief consideration, although it is so seldom that the 

 chemist is required to test for its presence that it should properly not be con- 

 sidered in these articles. 



The solubility of its sulphate and oxalate excludes its appearance in testing 

 for calcium, strontium and barium ; while its precipitation with ammonium (or 

 potassium) carbonate and sodium phosphate brings it into close relationship with 

 these elements. 



With almost all the reagents used for Group I, as for example, chlorplatinic 

 acid, potassium antimonate, tartaric acid, ammonium silicomolybdate, bismuth 

 thiosulphate, etc., lithium resembles sodium in its behavior; yet on the other 

 hand the fact that in rare cases phosphomolybdic acid may cause a precipitate 

 and that hexagons are obtained with bismuth sulphate brings this element in 

 close analogy to potassium. 



Lastly, like magnesium, lithium forms a double ammonium phosphate of low- 

 solubility. Moreover, this salt is isomorphous with the magnesium ammonium 

 phosphate. In this respect lithium resembles the magnesium group. 



