MICROCHEMICAL REACTIONS OF CADMIUM 363 



Tetragonal. 



Orthorhombic. Bromide. 



Monoclinic. Acetate; chloride; sulphate. 



Triclinic. 



DETECTION. 



A . By Means of Potassium Mercuric Thiocyanate. 

 Read Method A, Zinc, page 353. 



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

 similar manner to the zinc salt, colored a faint lavender or brown 

 by traces of copper. This brown color intensifies with an 

 increase 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 355. These 

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

 or 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 thiocyanate 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 solu- 

 tion somewhat more dilute than when zinc is known to be absent. 

 Much zinc usually prevents the formation of any of the prismatic 

 crystals of the cadmium salt, only mixed crystals resulting. 



