190 Wells and Willis — Caesium- Tellurium Fluoride. 



Art. XXI. — On a Ccesium- Tellurium Fluoride ; by H. L. 

 Wells and J. M. Willis. 



Several tellurium double fluorides have been described : 

 NaF*TeF 4 by Berzelius, KFTeF 4 , NH 4 F'TeF 4 and BaF 2 - 

 2TeF 4 *H 2 by Hogbom.* It is noticeable that all these fluo- 

 rides belong to a type which is different from that of the 

 double chlorides, bromides and iodides of tellurium, e. g., 

 2KCl-Te01 4 , 2KbBrTeBr 4 and 2CsI*TeI 4 , etc., which have been 

 thoroughly studied in this laboratory by Wheeler.f We have 

 undertaken, therefore, an investigation of the combination of 

 caesium fluoride with tellurium fluoride, with the expectation 

 that possibly several types of double fluorides might be obtained. 

 After a systematic examination of the matter, however, we 

 were able to prepare only one double fluoride, CsF*TeF 4 , 

 which corresponds in type to the previously known fluorides. 



A concentrated solution of TeF 4 was prepared by dissolving 

 about 10 g of pure Te0 2 in an excess of strong hot hydro- 

 fluoric acid, and to this caesium fluoride was added in small 

 portions, the liquid being concentrated by evaporation and 

 cooled after each addition. At the same time small portions of 

 tellurium fluoride were added to a concentrated solution of 

 about 50 g of caesium fluoride in hydrofluoric acid and this 

 solution was evaporated and cooled in the same manner. 

 Under the widest range of conditions, however, only a single 

 double salt was obtained. 



1:1 CcBsium-tellurium fluoride, CsF*TeF 4 . — This salt crys- 

 tallizes beautifully in large, transparent, colorless needles. The 

 presence of free hydrofluoric acid is necessary for its forma- 

 tion, for it is decomposed by water. Several crops, made 

 under widely varying conditions, were analyzed with the fol- 

 lowing results : 



Calculated for Found. 



CsTeF 5 . I. II. III. IV. 



Caesium 37-36 36'59 37*50 38*56 37-23 



Tellurium 35-96 35-51 36*45 35-82 35*60 



Fluorine 26*68 26*76 24*51 26-18 



Fluorine was determined volumetrically by converting it 

 into SiF 4 , collecting this in water, and titrating with a stand- 

 ard solution of potassium hydroxide. In another portion, 

 after evaporating with concentrated sulphuric acid and dissolv- 

 ing the residue in hydrochloric acid, tellurium was precipitated 

 with sulphur dioxide, collected on a Gooch crucible and 

 weighed as metal. From the filtrate from the tellurium 

 caesium was obtained, and weighed as normal sulphate. 



Sheffield Scientific School, May, 1901. 



* Bulletin, xxv, 60. f This Journal (3), xlv, 26V. 



