and Laboratory Methods. 



1457 



Traces of copper color the cadmium crystals a faint chocolate brown ; this 

 brown color intensifies with an increase in the amount of copper. When con- 

 siderable 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 has been precipitated, and the concentration has also 

 reached such a point that the cadmium double salt begins to separate in the 

 crystal forms shown in Fig. 71. These are, however, still mixed crystals, for 

 they are colored brown by the small amount of copper yet in solution. 



It is improbable that these brown copper-cadmium-mercury sulphocyanates 

 are isomorphous mixtures. 



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. 



See also remarks under Zinc, Method I. 



Exercises for Practice. 



Experiment with salts of cadmium in the manner suggested under " Zinc," 

 trying all the exercises mentioned, but having cadmium as the element in excess 

 instead of zinc. 



//. Oxalic Acid added to solutions of salts of Cadmium pi-ecipitates Cadmium 

 Oxalate. 



CdSO^ + H2C20^ = C^CO^ ' 3H2O + H2SO4. 



Method. — To the test drop add a solution of the reagent by the flowing in 

 method. Clear, colorless monoclinic prisms and tabular crystals separate, either 

 singly, in Xs, or in clusters. (Fig. 72.) 

 The tabular crystals have the appearance 

 of rhombs and rectangles. 



Frequently very concentrated solutions 

 yield crystals having an octahedral aspect. 



Retnarks. — The solution to be tested 

 should be neutral or only slightly acid, 

 and rather concentrated with respect to 

 cadmium. 



Dilute solutions fail to give good re- 

 sults. 



The typical crystals of cadmium oxa- 

 late are seen only when working with al- 

 most pure salts of this element. Fig. 72. 



