MICROCHEMICAL REACTIONS OF ALUMINUM 391 



The difficulties often experienced with this test by the beginner 

 are generally due to too much sulphuric acid in dissolving the 

 aluminum hydroxide and to too much acid in preparing the 

 cesium sulphate. 



EXPERIMENTS. 



a. To a test drop consisting of a solution of A1 2 (S0 4 ) 3 add a fragment of the 

 reagent. 



b. Precipitate another drop with NH 4 OH, decant, wash the precipitate, dis- 

 solve in the least possible amount of H 2 S0 4 and test. 



c. Try Rb 2 S0 4 as reagent; then K 2 S0 4 ; Na 2 S0 4 , (NH 4 ) 2 S0 4 . Try CsCl. 



d. Test for Al in the presence of free HC1; free HNO3. 



e. Test preparations containing Al and Fe; Al and Cr; Al and Mn; Al, Fe and 

 Cr; Al and Mg; Al in the presence of phosphates. 



/. Prepare slides of chrome alum, iron alum, etc., then mixtures of these various 

 alums; note isomorphism. 



B. By Means of Ammonium Fluoride. 



See Method XV, page 316. Apply the fluoride in solid 

 form (Method III). 



Use a celluloid object slide. 



From neutral solutions or those containing at the most only a 

 trace of free mineral acid a double fluoride separates having the 

 formula 3 NH 4 F • A1F 3 or considering this to be an alumino- 

 nuoride its formula may be written (NH 4 ) 3 A1F 6 . It crystallizes 

 in very tiny clear-cut colorless octahedra belonging to the iso- 

 metric system. 



Alumino-fluorides of the same formula of potassium, rubidium, 

 cesium and sodium are known; they are even less soluble than 

 that of ammonium and therefore can be obtained only in such 

 minute crystals as to be useless as a test. Lithium alumino- 

 fluoride is also very insoluble. 



The ammonium, potassium, rubidium and cesium salts are 

 isometric and form isomorphous mixtures; but the sodium salt 

 is monoclinic. 



In these alkali fluorine compounds the aluminum can be re- 

 placed by titanium, chromium, iron and vanadium. But in the 

 case of zircono-fluorides, silico-fluorides (see page 325) and 



