14 



Ashley — Estimation of Sulphites by Iodine. 



"before attempting to titrate by sodium thiosulphate the excess 

 of iodine. 



In the experiments recorded in the table the following proce- 

 dure was followed : the sulphite was treated with 1 grm. of 

 acid sodium carbonate and an excess of standardized iodine 

 solution. The solution was then acidulated with a safe amount 

 of hydrochloric acid, it having been found by experiment that 

 the presence of 10 cm3 of 1 : 4 hydrochloric acid in 125 cm3 of water 

 was without effect upon the determination of iodine by sodium 

 thiosulphate. The excess of iodine after acidification was 

 titrated by standardized sodium thiosulphate. It will be 

 noticed that in the .experiments recorded under A of the table, 

 the excess of iodine used was small and in these experiments 

 large negative errors are obtained ; while in the experiments 

 recorded under B, in which a large excess of iodine was employed, 

 the results are better. They are best when at least twice as 

 much iodine is added as is theoretically required to oxidize the 

 sulphur dioxide. The length of time during which the iodine 

 may act does not affect the results to any very marked degree- 



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 . 



0-2197 



0-3143 



0-0978 



— 0-0032 



— 0-0008 



7-5 



125 



a 



it 



0-0965 



— 0-0019 



— 0-0005 



a 



tt 



it 



ti 



0-0970 



— 0-0024 



— 0-0006 



a 



a 



0-1535 



0-1913 



00464 



— 0-0086 



— 0-0022 



a 



a 



k 



tt 



0-0467 



— 0-0089 



— 0-0022 



u 



a 



a 



it 



0-0472 



— 0-0094 



— 0-0024 



a 



u 



a 



a 



0-0465 



— 0-0087 



— 0-0022 



a 



(( 



0-2366 



0-3194 



0-0903 



— 0-0075 



—0-0019 



u 



u 



0-2906 



0-3825 



0-1132 



— 0-0213 



— 0-0054 



u 



a 



0-3825 



0-4463 



0-0750 



— 00112 



— 0.0028 



u 



It 



0-1143 



0-3143 



0-1990 



+ 0-0010 



+ 0-0003 



it 



it 



0-1982 



+ 0-0018 



+ 0-0004 



a 



a 



0-1992 



+ 00008 



+ 0-0002 



a 



a 



0-1986 



+ 0-0014 



+ 0-0003 



0-1482 



0-3187 



0-1708 



— 0-0003 



—o-oooi 



0-1576 



0-3187 



0-1586 



+ 0-0025 



+ 0-0006 



u 



u 



0-1643 



— 0-0032 



— 0-0008 



a 



it 



0-1598 



+ 0-0013 



+ 0-0003 



a 



a 



0-1606 



+ 0-0005 



+ 0-0001 



(( 



it 



0-1602 



+ 0-0009 



+ 0-0002 



5-0 125 



7-5 



u a 



