16 Ashley — Estimation of Sulphites hy Iodine. 



iodine concentration is high. The dithionate once formed is 

 bnt slowly attacked by iodine, and that is apparently the rea- 

 son why long standing of the mixtures containing a small pro- 

 portion of iodine does not result in complete oxidation of the 

 sulphite to sulphate. From these considerations it will be seen 

 that the secondary error of Rupp's process may very probably 

 be due to the formation of some dithionate from the sulphite 

 where the concentration of the iodine is low. 



The practical estimation of sulphurous acid or a soluble 

 sulphite may, then, be accomplished with a reasonable degree 

 of accuracy by adding to the solution of the substance, not 

 exceeding 100 cm3 in volume and containing a gram of acid 

 sodium carbonate, at least twice as much iodine as is theoreti- 

 cally necessary to effect oxidation, acidifying cautiously with 

 hydrochloric acid, and determining with standard sodium 

 thiosulphate the excess of iodine remaining in the acidified 

 solution. 



The author takes this occasion to thank Prof. F. A. Gooch 

 for much kind assistance. 



