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 Al2(S04)3 add a fragment of the 
reagent. 
h. Precipitate another drop with NH4OH, decant, wash the precipitate, dis¬ 
solve in the least possible amount of H2SO4 and test. 
c. Try Rb2S04 as reagent; then K2SO4; Na2S04, (NH4)2S04. Try CsCl. 
d. Test for Al in the presence of free HCl; 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 NH4F • AIF3 or considering this to be an alumino- 
fluoride its formula may be written (NH4)3A1F6. 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 
