Ashley — Oxidation of Sulphites by Iodine. 237 



Art. XXI. — The Oxidation of Sulphites by Iodine in 

 Alkaline Solution; by R. Harman Ashley. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — cxxxiv.] 



According to Bunsen, Dupasquer's method of oxidizing 

 sulphurous acid by iodine in an acid solution, proceeds to com- 

 pletion according to the equation 



2S0 2 + 2I 2 +4H 2 0=4HI + 2H 2 S0 4 

 only when the concentration of the sulphur dioxide does not 

 exceed 0*5 per cent of the solution. When, on the other hand, 

 the proportion of sulphur dioxide exceeds this value, there is a 

 secondary reaction, which according to Volhard, involves the 

 reduction of the sulphur dioxide by the hydriodic acid pro- 

 duced. This difficulty may be obviated, however, as has been 

 shown by Volhard,* if the solution of the sulphurous acid or a 

 sulphite is run with stirring into a solution of iodine in potas- 

 sium iodide, acidified with hydrochloric acid, to the bleaching 

 of color, using starch as an indicator, — a procedure which is 

 obviously less convenient than direct titration by a standard 

 solution of iodine. 



It has been recently further proposed by E. Ruppf to accom- 

 plish the oxidation in an alkaline solution, by treating the solu- 

 tion of sulphur dioxide or a sulphite, with an excess of standard 

 iodine in presence of acid sodium carbonate, 1 gr., during an 

 interval of fifteen minutes, and determining the excess of iodine 

 with sodium thiosulphate. Rupp's analytical examples, three 

 in number, involving amounts of sulphur dioxide approximating 

 0-0343 gr., show small errors of excess, and from them the 

 conclusion is drawn that sulphites like arsenites, may be esti- 

 mated in a solution made alkaline with sodium bicarbonate, by 

 the process indicated. 



This very unusual use of sodium thiosulphate for the deter- 

 mination of free iodine in the presence of an alkali bicarbonate, 

 suggests the question as to whether the sulphite was in reality 

 completely oxidized by the treatment, or whether the appar- 

 ently good results were in fact due to a chance balancing of 

 errors of incomplete oxidization of the sulphite, on the one 

 hand, and on the other hand, of the excessive use. of iodine in 

 the action of the thiosulphate in an alkaline solution, it being 

 generally supposed that in alkaline solutions not only is the 

 expected tetrathionate formed, but that some of the thiosul- 

 phate is oxidized to the extent of forming sulphate rather than 

 tetrathionate exclusively. 



In the following table of experiments are data of experi- 

 ments upon the action of iodine and sodium thiosulphate in 

 *Ann. Chem., ccxlii, 93. fBer. Dteh. Chem. Ges., xxxy, 3694. 



Am. Jour. Sci.— Fourth Series, Vol. XIX, No. Ill— March, 1905. 

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