Ashley — Estimation of Sulphites by Iodine. 



15 



(B) 



Iodine 





Iodine value 



Error 





Excess Vol at 



value of 



Iodine 



of Na 2 S 2 3 



In terms 



In terms 



of HC1 



titra- 



S0 2 taken. 



taken. 



used. 



of Iodine. 



of S0 2 . 



1 :4 



tion. 



grm. 



grm. 



grm. 



grm. 



grm. 



cm 3 . 



cm 3 . 



0-1576 



0-3187 



0-1622 



—o-ooii 



— 0-0003 



7-5 



125 



0-1560 



0-3195 



0-1660 



— 00025 



-0-0006 



a 



a 



0-1992 



0-4460 



0-2482 



— 0-0014 



— 0-0003 



a 



a 



0-1915 



03825 



0-1919 



+ 0-0009 



— 0002 



a 



a 



0-2056 



0-3771 



0-1701 



4-0-0014 



+ 0-0003 



a 



a 



a 



a 



0-1697 



+ 0-0018 



+ 0-0004 



a 



a 



a 



a 



0-1707 



+ 0-0008 



+ 0-0002 



a 



a 



a 



• it 



0-1709 



+ 0-0006 



+ 0-0002 



a 



a 



[0-2131 



0*4470 



0-2412 



— 0-0073 



— 0-0018] 



a 



a 



0-2354 



0-3825 



0-1490 



— 0-0019 



— 0-0005 



a 



a 



0-2597 



0-4463 



0-1869 



— 0-0003 



—o-oooi 



a 



it 



0-2038 



0-4463 



0-1847 



— 0-0022 



— 0-0005 



a 



a 



0-2908 



0-6375 



0-3505 



— 0-0038 



— 0-0009 



a 



tt 



0-3187 



0-4463 



0-1326 



+ 0-0050 



+ 00012 



a 



a 



0-3395 



0-6275 



0-2842 



+ 0-0038 



+ O-O0O9 



a 



a 



a 



a 



0-2852 



+ 0-0028 



+ 0-0007 



a 



tt 



a 



a 



0-2844 



+ 0-0036 



+ 0-0009 



u 



a 



a 



it 



0-2855 



+ 0-0025 



+ 0-0006 



a 



a 



Ruff and Jaroch* take the ground that in the favorable 

 results occasionally obtained by Rupp's process, an error due 

 to the over-oxidation of the tetrathionate normally formed in 

 the action of sodium thiosulphate upon the residual iodine is 

 apparently balanced by some oxidation of sulphur dioxide by 

 dissolved air, the iodine in solution acting catalytically as well 

 as directly. The theory, however, is quite at variance with the 

 evidence supplied in the table : for, if it were true, under no 

 conditions could iodine in the presence of air act as a correct 

 measure of sulphur dioxide, as it apparently does when used 

 in a sufficiently large excess ; nor does the theory of the cataly- 

 tic action of iodine explain the fact that when a greater mass 

 of iodine is used, under conditions otherwise similar, we get a 

 larger oxidation of sulphur dioxide. 



The most obvious explanation is that at a low concentration 

 of iodine an intermediate oxidation product may be formed and 

 that the formation of this product may be prevented by suffi- 

 cient concentration of the iodine. It is not unreasonable to 

 suppose that the formation of a small amount of dithionate 

 instead of sulphate is the occasion of the deficient expenditure 

 of iodine noted when the concentration of this element is low, 

 and that the dithionate is not formed appreciably when the 



*Loc. cit. 



