Browning and Flint — Tellurium Dioxide. 115 



acidified with acetic acid, the precipitate being afterwards 

 made crystalline* by further heating. It was noted that the 

 precipitate formed by this variation of the method is not so 

 quickly transformed to the crystalline condition as when the 

 procedure of the experiments described in Table I is followed. 

 It is, besides, still more finely divided and does not settle quite 

 so well. There seems to be a distinct advantage in the use of 

 ammonia, when added to the solution acidified with hydro- 

 chloric acid, since, if the diluted solution is sufficiently hot, 

 the precipitate formed by the ammonia begins to become 

 crystalline, apparently, at about the time when the point of 

 neutrality is reached. Under these conditions, a few drops of 

 dilute ammonia in excess have an inappreciable solvent effect 

 upon the Te0 2 and consequently there is also no opportunity 

 for the slight excess of acetic acid subsequently introduced to 

 dissolve and thus hold up a trace of the tellurium. On the 

 other hand, it seems probable that, when the acetic acid is 

 introduced, in faint excess, into the hot, diluted solution, 

 alkaline with potassium hydroxide, since the tellurium is 

 precipitated in floccy form which does not become entirely 

 crystalline until again heated, the excess of acid must dissolve 

 up a more or less minute portion of the precipitate and retain 

 it in solution in such a form as not to be again thrown down 

 upon cooling. Two facts may be adduced in support of this 

 theory, namely : first, that the errors in Table III show much 

 greater irregularity than those of Table I ; and second, that 

 whereas the filtrates of I were shown by testing with stannous 

 chloride to be free from tellurium, several of those in II, 

 notably experiments 4, 5, and 7, were proved to contain it in 

 traces. 



And finally, the last three experiments of Table III show 

 that it is perfectly possible to use quite as successfully one half 

 gram of the basic nitrate, equivalent to four tenths gram of 

 dioxide, in a single determination, employing a bulk of solution 

 no greater than 200 to 250 cubic centimeters. 



Attempts to separate tellurium from copper and bismuth 

 by treatment with small amounts of potassium hydroxide solu- 

 tion, and to estimate the tellurium in the filtrate by this 

 ammonia-acetic acid process, met with only moderate success. 

 Under the conditions, the copper and bismuth apparently tend 

 to form insoluble tellurites unclecomposable by the allowable 

 excess of alkali, f and consequently there was always a loss of 

 tellurium. And further, if the bismuth or copper is precipi- 

 tated together with the tellurium, it is practically impossible to 

 dissolve out from the mixed precipitate all the tellurium by a hot 



* Berzelius, Ann. de Chim. et de Phys., 2 serie, lviii, 134 sq. 

 flbid., lviii, 114. 



