and Laboratory Methods. 1127 



As in the case of the phosphomolybdates (see Potassium IV), the composi- 

 tion of the silicomolybdates is still in doubt. 



When the following method is employed, there is generally no difficulty in 

 distinguishing between rubidium and cesium. A drop of an exceedingly dilute 

 solution of the substance is spread out in a thin layer, and evaporated in the 

 usual manner. A drop of a dilute solution of ammonium silicomolybdate con- 

 taining a trace of free nitric acid is drawn across the dry film and the slide held 

 inclined for a second or two, placed on the stage of the 

 •* ' microscope, and a tiny drop of a saturated solution of the 



reagent added to the reagent drop on the slide. The 

 rubidium salt separates at once along the edges of the 

 streak of reagent in the form of lemon-yellow, highly 

 refractive cubes, octahedra, dodecahedra, and the usual 

 combinations of these forms, often rapidly passing into 

 spheroidal granules (Fig. 38). In size these crystals 

 approximate those of rubidium chlorplatinate. 

 When cesium is present, and the test is thus performed, the cesium salt is 

 instantly precipitated in the form of grains so minute that even a high power fails 

 to reveal any definite form other than what appear to be minute disks. The 

 solubility of the cesium salt is therefore so far below that of the corresponding 

 rubidium compound, that there is little difficulty in distinguishing between them 

 even when both are present in the same substance. So delicate is the reaction 

 that it is essential that there be only the smallest possible amount of these two 

 elements present. 



It is advisable to have no salts of ammonium present, since these compounds 

 seem to lower the solubility of the reagent sufficiently, at times, to cause the 

 appearance of octahedra of ammonium silicomolybdate. According to Laden- 

 berg* the octahedra of ammonium silicomolybdate act feebly on polarized light, 

 and hence are not to be referred to the isometric system. 



Potassium salts treated in the above manner give after a time, near the 

 edges, neat prisms, which under favorable conditions may attain a considerable 

 size. These crystals are far too soluble, and their form so different from the 

 rubidium compound that it is impossible for them to be mistaken for the latter. 

 Silver, thallous, and mercurous salts are also precipitated by this reagent. 

 Sodium and lithium yield no crystals ; the same is true of the magnesium and 

 the calcium groups. 



Better crystals of rubidium silicomolybdate can be obtained by the addition 

 of a dilute solution of the reagent to a dilute solution of the rubidium compound 

 than by the method suggested above, but this process does not permit of so 

 easily distinguishing between rubidium and cesium. 



Exercises for Practice. 



Try the above method on salts of Na, K, Rb, Cs, NH^, Li. 

 Test mixtures of K and Rb ; K and Cs ; NH^ and Rb ; NH^ and Cs ; Rb 

 and Cs; K, Rb, and Cs. 



*Handworterbuch, VII, 361. 



