132 THALLIUM. 



tion may also be effected by chloride of platinum, 

 which precipitates only the protoxide salt. 



The method of precipitation with iodide of potas- 

 sium serves also to separate thallium from most other 

 metals, the solution being first mixed with an alkaline 

 sulphite to insure the reduction of any peroxide salt 

 that may be present to the state of protoxide salt. If 

 copper is present the iodide of potassium will throw 

 down copper as well as thallium iodide ; but by treat- 

 ing the washed precipitate with ammonia, in contact 

 with the air, copper will be dissolved out, and 

 the thallium will remain as iodide of thallium. The 

 separation of copper from thallium may also be effect- 

 ed, though not so exactly, by precipitating the copper 

 with sulphuretted hydrogen in an acid solution. The 

 same method serves also to separate thallium fromlead 

 and silver. The precipitated sulphides are apt, how- 

 ever, to curry down small quantities of sulphide of 

 thallium. 



Small quantities of thallium often occur in bismuth 

 minerals, and preparations are made from them, espe- 

 cially the carbonate. To detect the thallium, the dilute 

 solution of the substance is mixed with a slight excess 

 of carbonate of soda and a small quantity of cyanide of 

 potassium, then gently warmed and filtered. If the 

 bismuth compound contained only 1 pt. of thallium in 

 100,000, the addition of a few drops of ammonium- 

 sulphide will produce a dark-brown precipitate of 

 sulphide of thallium, which gradually collects together 

 and may be further examined by the spectroscopic 

 method. From carbonate of bismuth, thallium may 

 be easily dissolved out by digestion with cyanide of 

 potassium, less completely with carbonate of soda. 



Volumetric Estimation. 



Thallium may be estimated volumetrically with per- 

 manganate of potassa in the same manner as iron. 



