392 ELEMENTARY CHEMICAL MICROSCOPY 



plumbo-fluorides the salts have the composition M 2 RF 6 , 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. 



Precautions. 



Employ only neutral solutions. 



Always have an excess of ammonium fluoride, for if not a 

 compound of different formula results appearing as very tiny 

 rods, worthless as an identity test for aluminum. 



Salts of lithium, sodium and iron must be absent. 



The presence of silicon and analogous elements will generally 

 seriously complicate matters, and may ruin the test, owing to 

 the formation of silico-fluorides, etc. (See ammonium silico- 

 fluoride tests, under sodium and barium.) Aluminum silico- 

 fluoride is gelatinous, and does not crystallize. 



Testing for aluminum with ammonium fluoride generally yields 

 results a trifle quicker than Method A, but the delicacy of the 

 reaction is very little greater. Moreover, Method B is subject 

 to many complications and interferences, and there is always 

 danger, in spite of great care, of damaging objectives by the 

 corrosive vapors arising from the test drop, since objectives of 

 moderate power and therefore short working distance must be 

 employed. For these reasons, testing with ammonium fluoride 

 cannot be considered as being as satisfactory as the cesium 

 sulphate method. One of the chief reasons for inserting the test 

 in this series is the fact that crystals of ammonium alumino- 

 fluoride may occasionally appear when ammonium fluoride is 

 being employed for other purposes, and the presence of alu- 

 minum is not suspected. 



The method of testing for aluminum by heating with ammo- 

 nium fluoride in a platinum cup has been described under 

 Method XV, page 318 (q.v.). The results thus obtained are in 

 most cases somewhat more reliable than those given above but 

 require more time, patience and care. 



