//. L. Wells — Quantitative Determination of Caesium. 189 



free from potassium. To ensure the complete purification of 

 the caesium and rubidium, the precipitate is washed with 

 hydrochloric acid containing, chlorine and lead chloride, then 

 it is treated repeatedly with small quantities of boiling water 

 until completely decomposed, and the resulting solution is sub- 

 jected to a repetition of the foregoing process. The mixed 

 plumbic salts are decomposed with hot water and the resulting, 

 filtered solution is evaporated to dryness to remove hydrochloric 

 acid. The residue is dissolved in hot water,* the lead is precip- 

 itated by the addition of a slight excess of ammonium sulphide 

 and the -precipitate is removed by filtration. The solution is 

 evaporated to dryness and the residue consists of caesium and 

 rubidium chlorides and some ammonium chloride. 



The following directions for the separation and purification 

 of the caesium and rubidium do not involve any new methods, 

 but the course of procedure has been arrived at after a consid- 

 erable amount of experience, and it may be of use to others. 

 It is assumed that rubidium is more abundant than caesium in 

 the mixture. If caesium predominated it would be more ad- 

 vantageous to extract that metal first by an obvious modifica- 

 tion of the process. 



The mixed chlorides of rubidium and caesium are dissolved 

 in at least five parts of concentrated nitric acid and the solution 

 is evaporated to dryness and heated until the excess of nitric 

 acid is removed. The residue is dissolved in a small amount 

 of water, and as much oxalic acid as corresponds to twice the 

 weight of the original chlorides is added. The whole is evap- 

 orated to dryness and the residue is ignited in platinum until 

 the oxalates are completely converted into carbonates. f The 

 carbonates are dissolved in water, the solution is filtered and 

 exactly neutralized with a measured solution of tartaric acid, 

 as much more tartaric acid as has been used for the neutraliza- 

 tion is added, and the solution is evaporated until it becomes 

 saturated while hot. The solution on cooling deposits acid 

 rubidium tartrate, which is washed with a small quantity 

 of water and is recrystallized two or three times from a hot 

 saturated solution in the same way until it gives no caesium 

 spectrum. J The united mother-liquors from the acid rubidium 

 tartrate are evaporated to dryness and ignited in platinum. 

 The resulting carbonates are converted into chlorides, and, to 

 a solution of these in a small volume of 1 : 1 hydrochloric 

 acid, a solution of antimony trichloride in the same acid is 



* No part of this residue should be thrown away on the assumption that it is 

 lead chloride, for the salt CsPb 2 Cl 5 is difficultly soluble and resembles PbOL. 



f This method of converting alkaline chlorides into carbonates is due to J. L. 

 Smith, this Journal, II, xvi, 373. 



% This method is due to 0. D. Allen, this Journal, IT, xxxiv, 367. 



