and Laboratory Methods. 1453 



to the formation of mixed crystals, and there generally result crystallites of the 

 shape of an arrowhead. Small amounts of zinc in the presence of much cad- 

 mium will usually escape detection. 



The presence of both copper and cobalt in a solution containing zinc gives 

 rise to the formation of mixed crystals of very peculiar color and form. These 

 peculiarities are accentuated when cadmium is also present. The experienced 

 worker thus will have little difficulty in detecting a number of elements in one 

 single operation. 



Indium forms with the reagent a double sulphocyanate, crystallizing in forms 

 resembling those of the corresponding cadmium double salt. The reaction is 

 quite slow in the case of indium. 



When iron is present in sufficient amount to give a blood-red color to the 

 preparation on the addition of the reagent, the crystals of the double sulphocy- 

 anate of zinc and mercury, separating from such solutions, are colored a deep 

 reddish brown, appear jet black by transmitted light, and have at first the usual 

 form of the zinc double salt. The appearance of these crystals usually changes 

 rapidly, and in a few seconds bunches and masses of curving, branching, fili- 

 form crystals are seen. The change is a very remarkable one and takes place 

 rapidly. 



Lead, unless present in large amount, seems to have little or no effect on the 

 zinc reaction. Under some conditions it seems to interfere, however, and it is, 

 therefore, always best to first remove the lead by means of dilute sulphuric acid. 

 Add the acid, draw off or filter; evaporate the clear solution to dryness; fume 

 off the free sulphuric acid ; dissolve in water ; add ammonium acetate, and test 

 as above. 



Silver gives with the reagent a white amphorous precipitate, soon crystalliz- 

 ing in the form of small, thin, slender prisms with square or oblique ends, 

 somewhat resembling those of the cadmium-mercury salt, but very much smaller 

 than the latter. In the presence of silver the test for zinc is sometimes masked. 

 In such an event, first remove the silver with hydrochloric acid and test, after 

 evaporation, in the usual manner. 



Exercises for Practice. 



Apply the reagent, in the manner indicated, to solutions of a pure Zn salt of 

 different degrees of concentration. 



To a Zn solution add a very little Cd and test. Repeat the experiment, 

 using more Cd. 



In like manner try mixtures of Zn and Cu ; Zn and Co ; Zn and Ni ; Zn 

 and Fe ; Zn and Mg ; Zn and Al ; Zn and Pb ; Zn and Ag. 



Then try more complex mixtures, as for example : Zn, Cd ana Cu ; Zn, Cd 

 and Co ; Zn, Cu and Co ; etc. 



In each case prepare several slides under different conditions and note well 

 the changes in the appearance of the crystals which separate. 



See also remarks and suggestion of experiments given under Cadmium, 

 Copper and Cobalt. 



