Gooch and Howland — Iodometrio Method, etc. 375 



Art. LII. — An Iodometrio Method for the Estimation of 

 Telluric Acid; by F. A. Gooch and J. Howland. 



[Contributions from the Kent Chemical Laboratory of Yale College— xxxvii.] 



In his valuable and extended study of possible volumetric 

 methods for the estimation of tellurium, Brauner* has investi- 

 gated the action of iodine upon an alkaline tellurite and finds 

 that the oxidation is slow and incomplete at ordinary tempera- 

 tures, while at 100° C. and in presence of a sufficient excess of 

 iodine complete oxidation takes place, though the difficulty of 

 estimating the excess of iodine not directly utilized in the 

 reaction is so great as to render the method practically inap- 

 plicable to the determination of tellurium. The difficulty in 

 effecting oxidation in alkaline solution naturally suggests the 

 reversal of the reaction in an attempt to reduce telluric acid 

 by the action of hydriodic acid in acid solution. Upon put- 

 ting the matter to the test we find the reduction of telluric 

 acid does take place, but we have been unable to prevent its 

 going too far. Thus, on boiling a solution made by adding 

 10 cm 3 of sulphuric acid of half-strength and 3 grm. of potas- 

 sium iodide in 90 cm 3 of water containing a little more than 

 O'l grm. of telluric acid, the liquid darkened, deposited dark 

 gray crystalline scales and evolved iodine which, when col- 

 lected and titrated with sodium thiosulphate proved to be 

 twenty per cent in excess of the theoretical yield assuming 

 that the reduction should result in the production of tellurous 

 acid. A similar experiment made in like manner excepting 

 that the liquid was heated for fifteen minutes in a closed bottle 

 on the water-bath instead of being subjected to boiling, 

 with the idea that the presence of the free iodine might tend 

 to prevent excessive reduction of the tellurium, indicated a 

 yield of free iodine about six per cent, in excess of what it 

 should be if tellurous acid is the sole product of reduction. 

 In this case the titration of the free iodine was made by 

 sodium thiosulphate in the presence of the reduced tellurous 

 acid, but we found by independent experiment that the deter- 

 mination of free iodine associated with tellurous acid may be 

 accomplished with a fair degree of accuracy provided the 

 solution be cold and dilute, and the final reaction secured by 

 adding the thiosulphate in slight excess and restoring the color 

 permanently by standard iodine. Even under the most favor- 

 able conditions the tellurous acid is somewhat acted upon by 

 sodium thiosulphate even before the free iodine is entirely 

 bleached and the reaction in the reverse titration by standard 



* Jour. Chem. Soc, 1891, pp. 58, 238. 



