MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 345 



EXPERIMENTS. 



a. To a test drop consisting of a solution of AkCSO^s 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 H2SO4 and test. 



c. Try Rb2SO 4 as reagent; then K 2 SO 4 ; Na 2 SO 4 , (NHOaSOi. Try CsCl. 



d. Test for Al in the presence of free HC1; free HNO 3 . 



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 268. Apply the fluoride in solid 

 form (Method///). 



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 NEUF - A1F 3 or considering this to be an alumino- 

 fluoride its formula may be written (NH^sAlFe. 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 279) and 

 plumbo-fluorides the salts have the composition M 2 RFe, where 

 M is an alkali metal and R may be Zr, Si or Pb. 



Crystalline double fluorides of aluminum with copper, nickel 

 and zinc have been described, but these are too soluble to appear 

 under the conditions which usually obtain in an analysis. 



