and Laboratory Methods. 1495 



MICRO-CHEMICAL ANALYSIS. 

 XVII. 



MAGNESIUM GROUP— SEPARATIONS. 



When engaged upon the examination of a complex mixture of unknown 

 composition, the chemical behavior of all the elements and salts liable to be 

 present must ever be borne in mind. It is seldom indeed that the analyst is 

 called upon to make an analysis of a substance or mixture of absolutely unknown 

 composition. The chief constituents are almost always known, or at least sus- 

 pected ; and there are also always good reasons why certain other substances 

 cannot be present. He who would become a rapid worker must learn to reason 

 by exclusion, but the beginner must realize that there is a vast difference 

 between using one's judgment and common sense, and hazarding a mere guess. 

 Thus the choice of methods in micro-chemical work will depend as largely upon 

 what substances are absent as upon what are present. 



In all analytical work rapidity is to be striven for, but such rapidity must 

 never be gained at the expense of accuracy. 



In order to better understand the chemistry of the separation methods of 

 this group, it may be well to recall the most important of the chemical properties 

 of the elements composing it, upon which our procedures will depend. 



The hydroxides of all the members of the group are insoluble in pure water. 



Glucinum hydroxide is insoluble in excess of ammonium hydroxide, but is 

 soluble in ammonium carbonate, potassium hydroxide, and sodium hydroxide. 



Magnesium hydroxide is soluble in the presence of ammonium salts, 

 especially ammonium chloride, but is insoluble in excess of the fixed alkalies. 



Zinc hydroxide behaves like that of glucinum toward fixed alkalies, but 

 unlike glucinum, it is also soluble in ammonium hydroxide. 



Cadmium hydroxide is insoluble in excess of sodium or potassium hydroxides, 

 but is soluble in ammonium hydroxide. 



Magnesium is the only one of the group normally yielding a crystalline 

 precipitate with secondary sodium phosphate in ammoniacal solution. 



Zinc and cadmium are readily precipitated by oxalic acid, glucinum with 

 difficulty, and magnesium only when much acetic acid is present or the solution 

 is excessively concentrated. In the case of glucinum, the double potassium 

 oxalate is less soluble than the normal oxalate. 



All four elements are precipitated by alkaline carbonates. 



The chlorides and oxides of zinc and cadmium can be easily volatilized. 



Zinc and Cad^nium from Magnesium^ etc., by sublimation. 



A. Place a small portion of the material on a nickel or platinum spatula,* 

 moisten with nitric acid, evaporate gently, then ignite to convert into oxides, but 



* This Journal, III, p. 794, Fig. 6. 



