186 H. L. Wells — Quantitative Determination of Caesium. 



groups composed of combinations of the cube and octahedron. 

 The accompanying figure, by Mr. Pirsson, shows the prevailing 

 habit of these crystals. The groups are very small, usually not 

 over 0*015 mm in diameter. 



Sheffield Scientific School, March, 1893. 



Aet. XX VII. — On a Method for the Quantitative Determina- 

 tion of Caesium^ and the Preparation of Pure Caesium and 

 Rubidium Compounds / by H. L. Wells. 



Since no method has heretofore been devised for the accu- 

 rate quantitative determination of caesium in the presence of 

 both rubidium and potassium, some experiments have been 

 made in order to test the availability of the plumbic chloride, 

 described in the preceding article, for this purpose. The 

 results have not been as accurate as could be desired, but the 

 method will be useful until a better one is found. 



The solubility of Cs 2 PbCl 6 in a hydrochloric acid solution 

 (fuming acid diluted with water 1:1), containing twice the 

 theoretical amount of lead chloride and saturated with chlorine, 

 was determined by making a precipitation of about I s of 

 Cs 2 PbCl 6 under these conditions in 350 cc and determining the 

 caesium in the filtrate. The whole filtrate gave 0119 g of 

 Cs 2 S0 4 , which corresponds to a solubility of 0-000068^ of 

 Cs 9 PbCl 6 in l cc . A similar experiment in which concentrated 

 hydrochloric acid was used, and also a larger excess of lead 

 chloride, gave a solubility of 0'000049 s of Cs,PbCl e in l cc . It 

 has been shown in the preceding article that the solubility of 

 Kb 2 PbCl 6 is 0'003 g in l cc under similar conditions. 



Some actual determinations of caesium were made as follows : 

 Known quantities of Cs 2 PbCl 6 and about an equal weight of 

 PbCl 2 were dissolved in hot HC1 (1 : 1). Chlorine was passed 

 into the solutions until they became cold, and, after standing 

 about three hours, the precipitates were collected in porcelain 

 Gooch crucibles and washed with hydrochloric acid containing 

 chlorine. The precipitates were decomposed with hot water 

 and the caesium in the resulting solutions was determined as 

 sulphate. In one case a comparatively large amount of potas- 

 sium chloride was present. The details are as follows : 





Cs 2 PbCl 6 



KOI 



Volume 



Cs 2 S0 4 



Deficiency 





taken. 



taken. 



1 : 1 HC1. 



found. 



as Cs a S0 4 . 



A 



•16'74 g 





35 cc 



•0856 



•0026 



JB 



•1592 







35 



•080V 



•0031 



C 



•1280 



05 g 



35 



•0638 



•0035 



