1126 Journal of Applied Microscopy 



/. Potassium Chiorplatifiate. 



Since the chlorplatinates of rubidium and cesium are so much more insoluble 

 than that of potassium, it is more convenient to employ a saturated solution of 

 the potassium salt than to make use of a solution of chlorplatinic acid. The 

 employment of this salt renders it possible to test for the two elements under 

 consideration even in the presence of salts of potassium and ammonium. 



Allow a drop of a saturated solution of potassium chlorplatinate to flow into 

 a drop of a dilute solution of the material to be tested. The test drop should be 

 neutral or only slightly acid with hydrochloric acid (see Potassium, Method I). 

 If cesium is present its chlorplatinate separates immediately as exceedingly 

 minute crystals of the same form as those of the potassium salt. The crystals 

 are always so small that a high power is required to enable 

 one to ascertain that the precipitate is not an amorphous 

 one. Rubidium chlorplatinate being a trifle more soluble, 

 separates later in crystals again of the same form, but at 

 least twice as large, though still much smaller than those 

 ^> ^iL> -^"^^ ^^ ^^^ corresponding potassium compound. 



If the solution to be tested is not exceedingly dilute, 

 skeleton crystals almost invariably result, resembling 

 ipiv=o.o>«„n, crosses or 5- and 6-pointed stars ; careful focusing will 



'*^' ^^' reveal with the latter the fact that the branches of the 



stars do not lie in one plane, but are arranged in the three dimensions of space 

 corresponding to the axes of an octahedron. In the case of rubidium, these skele- 

 ton forms often attain a considerable size. 



The usual forms assumed by rubidium chlorplatinate have been sketched in 

 Fig. 37. The crystal forms given by the cesium salt are the same, but much 

 smaller. 



Exercises for Practice. 



Try the reaction of chlorplatinic acid on dilute, and on concentrated solutions 

 of K, NH4, Rb, Cs. 



Repeat the experiments in the presence of considerable free sulphuric acid. 



Try, as directed under Rubidium and Cesium, the action of potassium chlor- 

 platinate on these two elements. Then try on solutions of K salts and of NH^ 

 salts. 



Make a mixture of Rb and Cs, and attempt to decide upon the presence of 

 both elements. Then try a mixture of NH4 and Rb, and one of NH^ and Cs. 



//. Avimoiiium Silicomolybdate. 



This reagent, which can be prepared according to the method given in a 

 previous article*, forms very insoluble compounds with rubidium and cesium. 

 The following reaction is supposed to take place : 



4RbCl+ [(NH^)4Si04. I2M0O38H2O] =: [Rb4Si04. 12Mo03- UHgO] + 

 4NH4CI. 



* Jour. App. Micro., Vol. Ill, 821. 



