THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 301 



crystalline phase will easily be distinguished under the micro- 

 scope. If the fragment of reagent added is too large, the clear 

 ring rapidly increases in diameter as the reagent dissolves, and 

 the solid phase is correspondingly rapidly forced toward the cir- 

 cumference of the test drop and eventually disappears completely. 

 The test drop should be somewhat deeper than usual and should 

 cover a relatively small area. 



Reactions involving no re-solution of the crystals first sepa- 

 rating require no such careful attention to equilibrium conditions, 

 nor do they necessitate such constant observation under the 

 microscope in order that the progress of the reaction may be 

 followed. In this class fall the precipitations of one metal by 

 another metal which is more electropositive. If, for example, we 

 make use of the electrochemical series of Wilsmore-Ostwald, 1 it 

 is found that the metallic elements are arranged thus: 



+ *- Mg, Al, Mn, Zn, Cd, Fe, Tl, Co, Ni, Sn, Pb, 

 (H), Cu, As, Bi, Sb, Hg, Ag, Pd, Pt, Au,^ -. 



Theoretically each element in this series is able to replace the 

 elements below it in the series which are less electropositive. 

 Since in many instances the metal displaced will separate in 

 characteristic crystalline form, the addition of a tiny piece of Mg 

 or of Al to a very slightly acidified drop may be made to yield a 

 beautiful test for metals farther along in the series. This type 

 of reaction is also of great value in effecting separations prior to 

 the application of identity tests, or in the separation of elements 

 which may interfere with future testing. 



A knowledge of the electrochemical series is absolutely essential 

 in all analyses of alloys where tiny fragments are not completely 

 dissolved since there will be solution of one or more components 

 and the precipitation of others upon the surface of the undissolved 

 material. Furthermore, a study of the above series will reveal 

 at once the fact that the addition to a test drop of a reagent with 

 reducing properties will in all likelihood be followed by the par- 

 tial precipitation of any metals present which fall in the electro- 

 negative end of the series. 



1 Zeit. phys. Chem., 36 (1901) 92. 



