MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 341 



acetic acid. Cobalt will immediately be precipitated. If lead 

 and nickel or copper are present the yellow or black or both triple 

 nitrites will eventually separate. If none appears, a little lead 

 acetate is added; tiny black squares and cubes indicate copper. 

 Powerful oxidizing agents must be absent. 



EXPERIMENTS. 



a. Test for Cu in CuS0 4 ; in Cu(NO 3 ) 2 . 



b. Try the reaction in acid solution; in ammoniacal solution. 



c. Try in like manner a mixture of Cu and Ni, Cu and Co. 



C. Other Useful Reactions for Copper, which may arise in 

 Testing for Other Elements. 



Cesium chloride forms two very characteristic double 

 chlorides with copper CsCl - CuCl 2 in golden yellow rectangular 

 plates, squares and short stout prisms and a less frequently met 

 with orange colored salt of unknown formula. These char- 

 acteristic double salts frequently appear when testing for tin, 

 antimony, or bismuth with cesium chloride or on rare occasions 

 when testing for aluminum. Ferric chloride also forms a yellow 

 double chloride with cesium chloride. The color and the appear- 

 ance of the cesium iron chloride is quite different from the copper 

 salt and the combination does not take place so readily. 



Potassium ferrocyanide in acetic acid solutions yields an 

 amorphous red-brown precipitate. Added to ammoniacal solu- 

 tions there appear after a time white dendrites of copper ferro- 

 cyanide ammonia 2 (NH 3 ) Cu 2 Fe(CN) 6 . 1 The addition of acetic 

 acid causes these dendrites to become red. 



ALUMINUM. 



Crystal Forms and Optical Properties of Common Salts of 

 Aluminum. 



A. ISOTROPIC. The alums (I). 



B. ANISOTROPIC. 



Hexagonal. Sulphate; chloride (6 H^O). 

 Tetragonal. 



1 Behrens, Anleitung, p. 75. 



