314 ELEMENTARY CHEMICAL MICROSCOPY 



late are seen only when working with comparatively pure salts. 

 In the presence of cadmium the oxalic acid test for zinc is un- 

 reliable. 



Magnesium salts must be absent, for under certain conditions 

 a double magnesium-zinc oxalate in hexagons and more or less 

 irregular plates will separate. 



From a number of other precipitated oxalates, zinc oxalate 

 may be separated by dissolving it in ammonium hydroxide and 

 decanting from the insoluble precipitate. Upon evaporation 

 the ammoniacal solution will deposit zinc oxalate, but no longer 

 in the typical form described above, but as masses of radiating 

 curving needles. Unfortunately this method is not applicable 

 in the presence of magnesium and cadmium. 



Precautions. 



The solution to be tested should be neutral or only slightly 

 acid, and rather concentrated with respect to zinc. 



Lead, silver, copper, cobalt, nickel, iron, aluminum, manganese 

 and chromium interfere with the detection of zinc by means of 

 oxalic acid. They should first be removed if reliable results are 

 to be obtained. 



As stated above, zinc oxalate may be confused with the oxa- 

 lates of calcium and strontium, while magnesium and barium 

 seriously modify its characteristic appearance. 



EXPERIMENTS. 



a. Test a pure salt of Zn in dilute and in concentrated solution. Repeat the 

 experiments, substituting Cd for the Zn. 



b. Make a preparation of ZnC 2 O 4 2 H 2 O; draw off the supernatant liquid, add 

 NH4OH; warm gently and study the preparation. Prepare slides of different 

 degrees of concentration. 



c. Recrystallize CdC 2 O 4 3 H 2 O in the same manner as the Zn salt. 



d. Test mixtures of Zn and Cd. 



e. Recrystallize the mixed oxalates from NH 4 OH. 



/. Make mixtures of Zn and the interfering elements listed above. Treat the 

 precipitated oxalates with NH 4 OH. Then try Cd in the same manner. 



g. Try precipitating Zn with HKC 2 O 4 ; K 2 C 2 O 4 ; (NH 4 ) 2 C 2 O 4 . Then try Cd in 

 like manner. 



