Gooch and ^f organ — Determination of Tellurium^ etc. 271 



Art. XXXYIII. — Ttte Determination of Tellurium hy Precip- 

 itation as the Iodide ; by F. A. Gooch and W. C. Morgax. 



[Contributions from the Kent Chemical Laboratory of Yale University. — LYIII.] 



It was known to Berzelius that hydriodic acid and tellaroiis 

 acid interact with the formation of tellurium tetraiodide, which 

 is converted by water into an oxyiodide and by excess of an 

 alkaline iodide into a soluble double salt. Wheeler^ has shown 

 that the double salt which is formed when tellurous iodide is 

 boiled in a strong solution of potassium iodide in dilute hydri- 

 odic acid is definite and has the constitution represented by the 

 formula 2KI . Tel^ . ^H^O. We have observed, however, that 

 when potassium iodide is added to a cold solution of tellurous 

 acid containing at least one-fourth of its volume of strong 

 sulphuric acid, no tendency toward the formation of a double 

 salt becomes apparent until the potassium iodide amounts to 

 more than enough to convert all the tellurous acid present into 

 the tetraiodide according to the equation. 



H.^Te03 -^ -t ^fi^. ^ ^E:I = Tel^ - 4KHS0^ + 3H,0. 



The tellurium tetraiodide which is thus formed is extremely 

 insoluble in sulphuric acid of the strength mentioned, though 

 soluble in excess of potassium iodide, and acted upon by water 

 with the formation of tellurium oxyiodide and hydriodic acid. 

 It is produced at first in the condition of a finely divided dark 

 brown precipitate which upon agitation of the liquid contain- 

 ing it gathers in curdy masses and settles, leaving the super- 

 natant liquid clear. By taking advantage of this tendency to 

 curd it is possible to determine without great difficulty the 

 exact point during the gradual addition of potassium iodide 

 when the precipitation of the tellurium iodide is complete, and 

 we have been able to found upon this property a very simple 

 titrimetric method for the direct determination of small 

 amounts of tellurium. 



In our test experiments we used tellurium dioxide prepared 

 by oxidizing presumably pure tellurium with nitric acid and 

 igniting the residue at a low red heat. TTeighed amounts of 

 the oxide thus prepared were dissolved in Erlenmeyer beakers 

 in a very little of a strong solution of potassium hydroxide, 

 and dilute sulphuric acid was added carefully until the tellurous 

 acid which was precipitated upon the neutralization of the 

 alkaline hydroxide was just redissolved. To this solution sul- 

 phuric acid of half-strength was added in such amount that 

 the solution finally obtained, after adding the aqueous solution 

 of potassium iodide subsequently, should still contain at least 



* This Journal, xlv, 267. 



