MICROCHEMICAL REACTIONS OF ZINC 357 



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. 



The thiocyanate reaction for zinc is not satisfactory in the 

 presence of colloids nor in the presence of organic acids. 



EXPERIMENTS. 



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

 different degrees of concentration. 



b. Try in turn pure salts of Cd, Cu, Co, Ni, Ag and Pb. 



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

 using more Cd. 



d. 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. 



e. Then try more complex mixtures, as, for example: Zn, Cd and 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 in the crystals which separate. 



B. By Means of Primary Sodium Carbonate. 



Apply a large drop of a saturated solution of the reagent 

 by Method /, page 299, to a neutral or very slightly acid drop of 

 the material to be tested. 



An amorphous precipitate of what is doubtless a basic car- 

 bonate of zinc is usually at first formed and may persist unless 

 the reagent is in large excess; in the latter case, after a few-min- 

 utes, a double carbonate of zinc and sodium separates at the 

 periphery of the drop. The crystals of this salt are constant and 

 peculiar to zinc. No other element yields compounds of like 

 appearance. The salt has the formula 3 Na 2 C0 3 -8 ZnCO 3 -8 H 2 O 

 (Deville). It takes the form of tiny colorless triangles and tet- 

 raheda or three-pointed or five-pointed agglomerates or rarely 

 short stout prisms with pointed ends. The characteristic form 

 upon which to base a decision are the triangles or tetrahedra. 

 The crystals cling tenaciously to the glass, rendering decantation 

 easy. After the removal of the mother liquor the double car- 

 bonate can be dissolved in acid and subjected to other tests. 



