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,^ it 
is found that the metallic elements are arranged thus: 
-h 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 shghtly 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. 
^ Zeit. phys. Chem., 36 (1901) 92. 
