376 Gooch and Rowland — lodometrie 



iodine is slow in coming, though finally definite ; but if the 

 solution is hot the thiosulphate precipitates tellurium in flocky 

 condition. 



The excessive action of the iodide suggested the substitu- 

 tion of potassium bromide — an agent which proved of value 

 in reducing arsenic acid as described in a recent paper from 

 this laboratory — and, the preliminary experimentation proving 

 to be encouraging we set about the preparation of telluric 

 acid of definitely known strength. We weighed out accur- 

 ately tellurous oxide, prepared by oxidizing presumably pure 

 crystallized tellurium with nitric acid and igniting the product 

 at a low red heat, dissolved the weighed oxide in a few cubic 

 centimeters of a strong solution of potassium hydroxide, pre- 

 cipitated the tellurous acid by the careful addition of dilute 

 sulphuric acid, and dissolved the precipitate in 10 cm 3 of sul- 

 phuric acid of half-strength. The solution thus obtained was 

 treated with potassium permanganate in excess, the manganic 

 oxide and the excess of permanganate were bleached by oxalic 

 acid added to the hot solution, and the excess of oxalic acid 

 was destroyed by the addition of a dilute solution of the per- 

 manganate until the faintest possible blush of color appeared. 

 For the experiments involving the treatment of the larger 

 amounts of tellurium every preparation was made individ- 

 ually in the manner described ; for the experiments with 

 smaller amounts a single preparation was made and the solu- 

 tion thus obtained was diluted to a half liter and portions of 

 this standard solution were drawn from a burette as occasion 

 required. 



The equivalent of the so-called element tellurium is, as 

 Brauner has shown* so variable with the mode of determining 

 that constant and the preparation of material that we em- 

 ployed a preparation of tellurous oxide which had been tested as 

 to its equivalent weight by Brauner's excellent method of oxida- 

 tion by potassium permanganate. The results of these tests 

 are recorded in the accompanying table. In series I the alka- 

 line solution of the oxide was diluted to 100 cm 3 and oxida- 

 tion was effected in it by standard permanganate added in 

 excess. Sulphuric acid of half-strength was added to neutral- 

 ization and then 5 cm 3 more, the liquid was heated, a solution 

 of oxalic acid of known value was introduced to an amount 

 slightly in excess of that needed to destroy the manganic 

 oxide and excess of permanganate, and the surplus of oxalic 

 acid was removed by standard permanganate. In series II the 

 alkaline solution of the oxide was neutralized with dilute 

 sulphuric acid and 1 cm 3 of sulphuric acid of half-strength was 



* Jour. Chem. Soc, 1SS9, 382. 



