386 ELEMENTARY CHEMICAL MICROSCOPY 



thiocyanate have been discussed at length under Zinc on page 

 355, to which the student is referred. 



To obtain the truly characteristic moss-like and radiating 

 crystallites the drop being tested must contain but little copper. 

 The double thiocyanate is sufficiently soluble to require several 

 minutes for its appearance in very dilute solution. 



Sin^e the zinc salt is much less soluble and possesses the prop- 

 erty of adsorbing any copper present with a change of color from 

 white through brown and black, a little zinc acetate or sulphate 

 added to the drop to be tested before the reagent is applied will 

 greatly increase the delicacy of the reaction. Infinitesimal per- 

 centages of copper may be thus detected. 



The thiocyanate test is the most satisfactory and generally 

 useful identity test for copper we possess. 



EXPERIMENTS. 



These have already been performed under Zinc. 



B. By Means of the Triple Nitrite Reaction. 



When copper alone is to be tested for, proceed as follows: 

 To the moderately concentrated drop add a fragment or two of 

 sodium acetate if free mineral acid is present, if not add a tiny 

 drop of dilute acetic acid, next add a fragment of lead acetate 

 and stir until dissolved. Finally add a fragment of potassium 

 nitrite. The black triple nitrite of potassium, copper and lead 

 K 2 CuPb(N0 2 )6 which is formed has been described under Lead, 

 page 373 (q.v.). By adding rubidium, cesium or thallous salts 

 the delicacy of the reaction may be greatly increased. 



If nickel is present it will separate as a triple nitrite of similar 

 composition K 2 NiPb(N0 2 ) 6 , light yellow or yellow-brown, in 

 squares and cubes of larger size. They differ from the copper 

 compound in never being black. 



Cobalt is immediately precipitated as insoluble potassium 

 cobalt nitrite. 



In testing alloys or mixtures likely to contain lead, copper, 

 nickel and cobalt, it is best to modify the above procedure. 

 Sodium acetate is first added, then potassium nitrite followed by 



