MICROCHEMICAL REACTIONS OF ZINC 361 



D. By Means of Sodium Nitroprusside. 1 



Apply the reagent by Method /, page 299, to a neutral 

 or slightly acid solution. 



Zinc yields a nitroprusside of low solubility in the form of 

 spherical grains, botryoidal masses or tiny circular disks of a very 

 faint brownish color. Upon standing, a large number of distinct 

 faces develop upon the spheres (combination of cube and dodeca- 

 hedron ?). These crystals are isotropic. The formula of the 

 compound has not yet been established; that of the reagent can 

 be written Na 2 NO - Fe (CN) 5 2 H 2 O. If the zinc merely re- 

 places the sodium, we should obtain Zn NO Fe (CN) 5 #H 2 O, or, 

 on the other hand, we may be dealing with a sodium-zinc nitro- 

 prusside. In the presence of free mineral acids there is a ten- 

 dency for zinc nitroprusside to separate in tiny squares and stout 

 prisms or in fusiform rods. 



A moderate amount of free mineral acid does not appear to 

 prevent the reaction but retards the appearance of the crystals. 

 Much acetic acid (or acetates) retards the separation even more. 

 Heat hastens the reaction, but warming does not appear to be of 

 value in obtaining a better development of the crystal form. 



Cadmium yields tiny rough globulites, octahedra with rough, 

 corrugated or even bristling faces, and drusy masses. Cadmium 

 nitroprusside polarizes strongly and the largest of the crystals 

 exhibit brilliant polarization colors. 



Mixtures of zinc and cadmium yield rough globulites, most of 

 them anisotropic. 



Manganous salts give globulites similar in all respects to those 

 obtained with zinc; they appear later and rarely develop to as 

 large a size or exhibit the many faces. Like the zinc salt they 

 are isotropic. In ordinary routine analysis it is practically im- 

 possible to distinguish between zinc and manganese. 



Copper yields an immediate amorphous pale blue precipitate. 

 Often this shows a tendency toward the formation of star-like 

 skeleton crystals. Mixtures of copper and zinc yield, in addition 

 to an amorphous precipitate, the spherical grains of the zinc salt, 

 but in this case there is a tendency toward spherulites, tiny 

 1 Bradley, Am. J. Sci., 22 (1906), 326. 



