MICROCHEMICAL REACTIONS OF THE COMMON ELEMENTS 313 



Precautions. 



Salts of ammonium must be absent or present only in small 

 amounts. 



The separation of typical crystals is always slow and cannot 

 safely be hastened. 



It is essential that an excess of the reagent be employed. 

 Failure not infrequently results from a neglect of this precaution. 

 This is particularly true if the test drop is acid. Because of the 

 necessity of adding large amounts of primary sodium carbonate, 

 the test drop must be of greater volume than is usual in micro- 

 chemical testing. 



EXPERIMENTS. 



a. Try precipitating Zn in acid, neutral and ammoniacal solutions. 



b. Test mixtures of Zn and Cd, first in neutral, and then in ammoniacal solu- 

 tions. 



c. Experiment with Zn in the presence of the interfering elements noted above. 



C. By Means of Oxalic Acid. 



The reagent is applied by Method 7, page 251; see Cal- 

 cium, Method B, page 291, Strontium, Method B, Barium, 

 Method B, pages 295 and 298. 



Zinc yields ZnC 2 O 4 2 H 2 O as small double spherulites, as 

 pseudo-octahedra singly or united in twos, and as thin rhombs. 

 The great majority of the crystals separating usually have their 

 angles rounded. It is rare that a preparation is obtained giving 

 clear-cut crystals. 



These crystals, when examined with a low power, often bear a 

 striking resemblance to the oxalates of calcium and strontium; 

 therefore to avoid error the alkaline earths should first be re- 

 moved. 



Cadmium gives clear colorless monoclinic prisms and tabular 

 crystals of the formula CdC 2 O 4 3 H 2 O. The prisms are usually 

 very long and show a marked tendency to form large X's, and 

 radiating aggregates. From concentrated solutions octahedral 

 crystals are also obtained. The typical prisms of cadmium oxa- 



