Browning and Flint— Tellurium Dioxide. 117 



wasbeci with water until free from soluble matter, and dried 

 to constant weight, yielding O'OOtfcl: grin. The amount of 

 TeO„ by difference was consequently 2010 grm. But the 

 residue, when dissolved in hydrochloric acid and tested with 

 stannous chloride, showed the presence of a trace of tellurium. 



In experiment 2, two cubic centimeters of the potassium 

 hydroxide solution were nsecl, and the hot, diluted solution 

 acidified with acetic acid. In 3 and 1, after the solution in 

 two cubic centimeters of potassium hydroxide, hydrochloric 

 acid was added in faint excess, and the hot, diluted solution 

 treated with ammonia and then acetic acid ; the difference 

 between these two determinations is apparently explained by 

 the fact that in 3 the solution was allowed to cool a little before 

 addition of ammouia, and thus the floccy precipitate was 

 attacked by the acetic acid. In experiments 5, 6, and 7, potas- 

 sium hydroxide was used to dissolve the oxides, hydrochloric 

 acid was added to faint acidity, and the dilution made with cold 

 water, which was then heated to boiling. It was evident that 

 the floccy precipitation caused by the cold water included some 

 selenium, which was not released by the change to crystalline 

 form, since not only are the errors positive, but also the pre- 

 cipitate, when tested for selenium with potassium iodide by 

 the delicate method of Norris, Fay, and Edgerly,* showed the 

 presence of a trace of that element. In order to be certain 

 that this is the true explanation of the fact, two more experi- 

 ments, 8 and 9, were performed, in the first of which special 

 care was taken to dilute with water- actively boiling, aud to 

 carry out the subsequent operations as quickly as possible in 

 order that the change of condition of the precipitate might 

 occur before the acetic acid was introduced. Ko selenium 

 could be detected in the precipitate of experiment 8. In the 

 ease of 9, the hot solution was allowed to cool somewhat before 

 the precipitation, in consequence of which the abundant, floccy 

 precipitate included a minute trace of selenium, as afterwards 

 proved by the above mentioned test. 



In order, therefore, to estimate tellurium as the dioxide by 

 this method, it is evident that fairly accurate and concordant 

 results can be obtained by dissolving the material in ten per 

 cent potassium hydroxide solution, about two cubic centime- 

 ters for 0*2 grm. of dioxide, acidifying this solution slightly 

 with hydrochloric acid, diluting to 200 cubic centimeters with 

 boiling hot water, and precipitating the tellurium in a finely 

 crystalline form of dioxide from the still hot solution by the 

 careful addition of dilute ammonia in faint excess and the 

 restoration of the acidity by the faintest possible excess of 

 * Am. Chem. J., xxiii, 105. 



