112 Browning and Flint — Tellurium Dioxide. 



Art. XIII. — The Quantitative Precipitation of Tellurium 

 Dioxide and its Application to the Separation of Tellu- 

 rium from Selenium ; by Philip E. Browning and Wil- 

 liam R. Flint. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — cci.] 



All those processes for the estimation of tellurium in which 

 the tellurium is precipitated and weighed in elementary con- 

 dition are open to the objections that, first, there is more or 

 less difficulty in securing completeness of precipitation owing 

 to the rapid increase of free acid* in the solution ; and, second, 

 the product is extremely susceptible to oxidation. On the 

 other hand, those methods in which compounds decomposable 

 by heat are transformed to the dioxide by ignition are gener- 

 ally both tedious by reason of the length of time required (as 

 for example, the basic nitrate process as described by Norrisf) 

 and, what is more to the point, liable to errors caused not only 

 by lack of constancy of composition, but also by the volatiliza- 

 tion of the product to be weighed. 



Of all the forms in which tellurium has been weighed there 

 is no doubt that the dioxide is the best. It is unaffected by 

 the air, is anhydrous, is not hydroscopic, and can easily be 

 obtained in pure condition. Likewise it can be heated to any 

 temperature below low redness without any danger of volatili- 

 zation. It was in view of" these facts that some results obtained 

 from an extensive study, about to be published, of the 

 hydrolytic behavior of hydrochloric acid solutions of tellurium 

 tetrachloride suggested the process about to be described. 



When a tetrachloride solution containing the least possible 

 excess of hydrochloric acid is sufficiently diluted with hot 

 water, but a small portion, if any, of the tellurium is at first 

 precipitated. By the addition of as little ammonia in excess 

 as may be, and the restoration of the acidity by acetic acid in 

 the faintest possible excess and then allowing the liquid to 

 stand until cold, the tellurium is precipitated completely, as 

 Te0 2 , but in very finely crystalline condition. The precipi- 

 tate is insoluble in cold water and alcohol, in acetic acid and 

 ammonium acetate solutions of one per cent strength if cold, 

 and filters, washes, and dries with the greatest facility. 



In the first testing 'of the method, portions of pure dioxide 

 were weighed out, dissolved in two cubic centimeters of con- 

 centrated hydrochloric acid, diluted with two hundred cubic 

 centimeters of boiling water, and the ammonia, and subse- 



* Crane, Am. Chem. J., xxiii, 409. See also Lenher and Homburger, J. 

 Am. Chem. Soc., xxx, 387. 



f J. Am. Chem. Soc., xxviii, 1675. 



