MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 317 



Tetragonal. 



Orthorhombic . Bromide. 



Monoclinic. Acetate; chloride; sulphate. 



Tridinic. 



DETECTION. 



A. By Means of Ammonium Mercuric Sulphocyanate. 

 Read Method A, Zinc, page 307. 



The prismatic crystals of Cd (CNS) 2 Hg(CNS) 2 are, in a 

 similar manner to the zinc salt, colored a faint chocolate brown 

 by traces of copper. This brown color intensifies with an in- 

 crease in the amount of copper. When considerable copper is 

 present, the copper double salt first separates, since it is slightly 

 less soluble than the cadmium compound; then mixed crystals 

 form, in which the copper apparently predominates over the 

 cadmium. These mixed crystals are of a deep bluish green 

 color. By this time most of the copper and but little of the 

 cadmium have been precipitated, and the concentration has also 

 reached such a point that the cadmium double salt begins to 

 separate in the crystal forms described on page 309. These 

 are, however, still mixed crystals, for they are colored brown 

 by the small amount of copper still in solution. 



As in the case of the zinc reaction, iron may sometimes color 

 the cadmium salt a reddish brown. 



Cobalt colors the cadmium salt blue. Much cobalt gives an 

 intense blue color and alters the crystal form. 



Magnesium and aluminum have even less effect than in the 

 case of zinc. 



Before testing for cadmium with the sulphocyanate reagent, 

 it is best to first remove any lead or silver which may be present. 



If a small amount of zinc is also present, mixed crystals con- 

 taining zinc and cadmium first separate, whose crystal form can 

 be described as non-feathery skeletons; soon after this the 

 cadmium double salt separates in its typical form. In order 

 that this sequence shall be brought about, it is best to employ a 

 solution somewhat more dilute than when zinc is known to be 

 absent. Much zinc usually prevents the formation of any of the 



