360 
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 ZnC204 • 2 H2O; draw off the supernatant liquid, add 
NH4OH; warm gently and study the preparation. Prepare slides of different 
degrees of concentration. 
c. Recrystallize CdC204 • 3 H2O in the same manner as the Zn salt. 
d. Test mixtures of Zn and Cd. 
e. Recrystallize the mixed oxalates from NH4OH. 
/. Make mixtures of Zn and the interfering elements listed above. Treat the 
precipitated oxalates with NH4OH. Then try Cd in the same manner. 
g. Try precipitating Zn with HKC2O4; K2C2O4; (NH4)2C204. Then try Cd in 
like manner. 
