312 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. 



