300 ELEMENTARY CHEMICAL MICROSCOPY 



EXPERIMENTS. 



a. Addition of Chloroplatinic Acid (platinum chloride) to a solution of a 

 potassium salt (KC1). Application : testing for K, NH 4 , Rb, Cs, Na, many organic 

 bases, etc. 



Repeat the experiment, using a fragment of CsCl in a drop of the same size 

 as that of the potassium salt just employed. Note the instantaneous forma- 

 tion of a precipitate and that crystals are very much smaller. Repeat again, using 

 a very dilute solution of CsCl. Next try a solution drop of KC1 containing very 

 little CsCl. Allow to evaporate spontaneously after the addition of the reagent, 

 Cs separates first, then K. 



b. Addition of Potassium Mercuric Thiocyanate to a dilute solution of a copper 

 salt. 



II. The substance to be tested is added to a drop of the 

 reagent. 



This method of applying tests is the one least often employed. 

 It will prove successful in such reactions as require for the sepa- 

 ration and characteristic development of the crystalline phase a 

 constant addition of one component, in this case that to be 

 tested for in small but almost uniform amount. 



The fragment of material is added to the center of a shallow 

 broad drop. Warming gently will accelerate the separation of 

 crystals. 



EXPERIMENTS. 



a. To a drop of a solution of Bi 2 (SO4)a containing a trace of free HNOs, add 

 a fragment of K 2 SO4. 



Applications Testing for K, for Na, for Bi, etc. 



777. A tiny fragment of the solid reagent is added to a drop of the 

 solution of the substance to be tested. 



This case is substantially similar to Method 77, and is governed 

 by the same general conditions. It will be found to be the safest 

 procedure in nearly all reactions where the solid phase at first 

 formed is soluble in excess of the reagent, for there will always be 

 during an appreciable time (owing to the rather slow solution of 

 the reagent) a zone in which the equilibrium is such that the solid 

 phase can exist. Thus the fragment of reagent will be surrounded 

 by a clear space or ring, at the outer edge of which the solid 



