1124 



Journal of Applied Microscopy 



Rubidium and cesium give an identical reaction, their primary tartrates being 

 even more insoluble than that of potassium, hence the salts of these two elements 

 will separate first. In fact, this last property can be utilized for detecting 

 rubidium or cesium in the presence of potassium, if a sufficiently dilute solution 

 be employed. 



Ammonium sometimes gives crystals not to be distinguished from those of 

 potassium ; at other times, when treated as above, the precipitate is distinctly 

 different. 



Many other elements yield relatively insoluble tartrates, which, while differing 

 from the potassium salt, yet resemble it sufificiently to lead to confusion (see 

 Calcium and Strontium). 



Double tartrates are also apt to be formed. This is far more liable to happen 

 when large drops are employed and the reagent added at once to the test drop, 

 than when evaporation to dryness is practiced, and the reagent then drawn 

 across. 



X. Perchloric acid added to solutions of salts of potassium precipitates Potassium 

 Perchlorate. 



K2S0^ + 2HC10^ = -iKClO^ + H.-,SO^. 



Method. — Next to the dilute solution of the substance to be tested place a 

 tiny drop of water, and to the latter add a drop of 

 perchloric acid or a little ammonium perchlorate. 

 Cause the drop of the reagent to flow into the drop 

 to be tested. In a few seconds colorless, highly 

 refractive, clear cut crystals of potassium perchlorate 

 separate (see Fig. 36). These crystals belong to the 

 orthorhombic system, but at first sight those first 

 formed seem to be isometric, while later, what would 

 be mistaken for monoclinic prisms appear. 



Pemarks. — The solutions must be dilute, other- 

 wise the potassium perchlorate is precipitated at 

 once. 



If the solution is too dilute, crystals may not 

 aopear for a considerable period. The addition of alcohol will, in such cases, 

 greatly hasten matters. 



Rubidium and cesium give a like reaction ; their perchlorates are more insol- 

 uble than that of potassium. Thallium forms a still more insoluble perchlorate. 



The perchlorates of the elements of the other groups which are generally met 

 with in ordinary work, are sufficiently soluble not to interfere. 



Behrens* has recently shown that in the presence of potassium permanganate, 

 the perchlorate of rubidium is colored pink. 



Advantage can be taken of a similar property of the potassium salt to obtain 

 an exceedingly beautiful test, for if the test drop contains sodium permanganate, 

 the potassium perchlorate separating therefrom will be colored. To obtain this 

 reaction, add to the test drop a little sodium manganate, so as to impart a dis- 



* van Breukeleveen, Rec. trav. chim. Pays-Bas. XVII, i, 94. 



Fig. 3fci. 



