364 ELEMENTARY CHEMICAL MICROSCOPY 



No other element yields a similar appearing 



compound. 



The reaction is an exceptionally sensitive one; exceedingly 

 small amounts of nickel may be thus detected save in the pres- 

 ence of large amounts of cobalt or copper. Neither cobalt 1 nor 

 copper alone yield a precipitate, but both these metals mask or 

 prevent the formation of the typical nickel compound; a yellow 

 amorphous precipitate results in which can be found only a few 

 masses of the pink needles. 



Copper can be easily removed by deposition upon a piece of 

 zinc foil prior to the addition of the ammonium hydroxide. 

 This is accomplished by placing the weakly acid drop upon a 

 clean bright piece of zinc. As soon as a black spot is formed 

 the drop is decanted to a new position, and as soon as it is ob- 

 served that the zinc is not at once stained the drop is decanted 

 upon an object slide, ammonium hydroxide added and the test 

 for nickel applied. 



Cobalt may be removed by adding to the almost neutral drop, 

 a fragment or two of potassium nitrite, warming to hasten solu- 

 tion, and then adding a drop of acetic acid. Potassium cobalt 

 nitrite is precipitated. After a few seconds the liquid is de- 

 canted from the precipitate which clings tenaciously to the glass 

 and ammonium hydroxide is added, ignoring any few tiny par- 

 ticles of the nitrite which may have been carried over. The 

 glyoxime test can now be applied with assurance of detecting 

 nickel if present. 



An excess of neither silver nor zinc appears to influence the 

 reaction for nickel. 



Dimethyl glyoxime gives with iron salts a red color. In 

 testing for nickel, therefore, we often obtain an indication of 

 the presence of iron in spite of the fact that ammonium hydroxide 

 has been added; for in the presence of ammonium salts the 

 addition of ammonium hydroxide to ferrous solutions will not 

 precipitate all the iron, owing to the formation of soluble double 



1 All the samples of cobalt salts sold as C.P. tested by the author have given a 

 slight precipitate with the reagent, probably due to traces of nickel present in the 

 material. 



