THALLIUM. 131 



conveniently estimated as protiodide, Til, in which 

 state it is obtained by precipitation with iodide of po- 

 tassium. The precipitate is quite permanent in the 

 air, and at the temperature of which it is weighed. It 

 is but very slightly soluble in water, insoluble or 

 nearly so in saline solutions, alcohol of 92 per cent., 

 and aqueous ammonia ; but perceptibly soluble in 

 water containing free acids or fixed alkalies. On mix- 

 ing the hot ammoniacal solution of a protoxide salt with 

 iodide of potassium, the thallium iodide separates im- 

 mediately as a curdy precipitate, which, after standing 

 for several hours, may be collected on a weighed filter, 

 and washed with alcohol ; or, if this is inadmissible 

 with ammonia, it is then dried at 115, and weighed 

 it contains 49.40 per cent, thallium. 



Thallium may also be estimated in the form of pro- 

 tosulphate, but not quite so exactly as by the method 

 just described. The sulphate bears a dull red heat 

 without perceptible volatilization, but is volatilized at 

 a bright red heat. Thallium is very completely pre- 

 cipitated from solutions of thallium salts by chloride of 

 platinum ; but the precipitated chloroplatinate is trou- 

 blesome to manage, as it is very finely divided, and is 

 apt to run through the filter when washed with water 

 or alcohol. 



In solutions of peroxide salts the thallium may be 

 estimated by reducing the peroxide to protoxide salts 

 with an alkaline sulphite, and then precipitating with 

 iodide of potassium ; or by precipitating the thallium 

 with ammonia as sesquioxide, and collecting the pre- 

 cipitate on a weighed filter. The separation of protox- 

 ide from peroxide salts may be effected, at least in the 

 case of the chlorides or sulphates, by first precipitating 

 the sesquioxide with ammonia, and then throwing 

 down the remaining portion of thallium from the hot 

 dilute filtrate with iodide of potassium. The separa- 



