Potassium Permanganate and Sulphuric Acid. 309 



instability of the already unstable molecules and so set up a 

 far-reaching decomposition. It seems to us that these consid- 

 erations throw some light upon the phenomena observed by 

 Brauner* in the oxidation of tellurous oxide in presence of 

 sulphuric acid ; and the fact that the liberation of free oxygen 

 in this special case is more noticeable than in the oxidation of 

 ferrous salts or oxalic acid, for example, is explicable in the 

 light of Brauner's observation that the attraction of tellurous 

 oxide for oxygen is greatly inferior to that of these substances 

 — not sufficient, in fact, to break up so unstable a substance as 

 manganic sulphate, which is at once reduced by ferrous salts 

 or oxalic acid. The practical lesson to be drawn is the desira- 

 bility of keeping the acid present in oxidations effected by the 

 agency of permanganate at the lowest limit consistent with 

 perfect oxidation. 



In Table Y are recorded the results of a comparison made 

 between Brauner's two excellent methods for the determina- 

 tion of tellurous oxide by titration with potassium permanga- 

 nate — the one in alkaline solution, the other in acid solution — 

 in which the precaution suggested as to the restriction of the 

 amount of acid which should be present was taken. The 

 tellurous oxide used in these experiments was prejDared for the 

 so-called pure crystallized element by oxidation with nitric 

 acid and the prepared and ignited oxide was dissolved in potas- 

 sium hydroxide. 



In series A the alkaline solution of the oxide was diluted to 

 100 cm 3 , potassium permanganate was added in excess, sul- 

 phuric acid [1 : 1] was introduced to an amount not exceeding 

 by more than 5 cm 3 that needed for neutralization, oxalic acid 

 was added in excess of the amount needed to destroy the" man- 

 ganic oxide and permanganate, the liquid was warmed to about 

 80° C, and the surplus of oxalic acid was titrated by perman- 

 ganate. 



In series B the alkaline solution of the oxide was treated 

 with sulphuric acid [1 : 1] until the precipitate first thrown 

 down was just redissolved, and 1 cm 3 of the same acid was 

 added in excess. Permanganate was added in excess and oxalic 

 acid in excess of the permanganate, the liquid was warmed, 

 and titration by permanganate to the final reaction was com- 

 pleted as usual. In the calculation of the results the atomic 

 weight of tellurium used by Brauner in his workf was adopted 

 and the figures obtained correspond fairly well (without the 

 application of any correction) with theory based upon this 

 assumption. Our purpose, however, was simply to test the 

 agreement between titrations made in alkaline solution and 

 those carried out in acid solution. 



* loc. cit. f loc. cit., p. 240. 



