358 ELEMENTARY CHEMICAL MICROSCOPY 
It is unfortunate that this, which is one of the most character¬ 
istic as well as delicate of the microchemical tests for zinc, should 
be open to many difficulties. The chief of these lies in the fact 
that many elements are precipitated as carbonates, and that 
these often bulky precipitates interfere with or mask the zinc 
reaction. Among the interfering elements, those most frequently 
met with are doubtless calcium, strontium, barium, magnesium, 
cadmium, lead, iron, manganese, cobalt and nickel. Of this list, 
calcium, strontium, barium and lead will probably have been re¬ 
moved by previous treatment with sulphuric acid. Zinc may 
be separated from the remaining elements of this list by treating 
with ammonium hydroxide and hydrogen peroxide and finally 
extracting with a drop or two of moderately concentrated sodium 
hydroxide solution. To this clear extract primary sodium car¬ 
bonate is added. 
Schoorl has pointed out that the best results are to be obtained 
from acetic acid solutions of zinc to which normal sodium car¬ 
bonate is added. This method is unquestionably the best in the 
analysis of complex mixtures and when the per cent of zinc 
present is low. The Behrens method of direct addition of pri¬ 
mary carbonate is restricted to simple salts of zinc or to mix¬ 
tures known to contain no interfering elements. 
If only a very small amount of cadmium is present, it is pre¬ 
cipitated before the zinc, and by avoiding the addition of an 
excess of the reagent, decanting the clear liquid and adding to 
the decanted liquid a fresh portion of the reagent in sufficient 
quantity, the zinc can be precipitated as the double carbonate. 
When considerable cadmium is present this method is not feasible. 
In such an event recourse may be had to ammoniacal solutions, as 
suggested by Behrens. The test drop is made strongly ammonia¬ 
cal and to it primary sodium carbonate is added. Cadmium is 
immediately precipitated, while the zinc remains in solution. 
The clear solution is decanted at once. After a few seconds 
zinc separates from the decanted solution as the double car¬ 
bonate in the forms described above. Some little skill and 
experience is generally necessary in order to obtain good 
results. 
