Ashley — Estimation of Sulphites by Iodine. 13 



Art. III. — The Estimation of Sulphites hy Iodine • by 

 R. Harman Ashley. 



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



Yolhard's method for the determination of sulphur dioxide 

 and sulphites is accurate and reliable, but involves the incon- 

 venience of making up every solution to be examined accurately 

 to a standard volume of which portions are to be drawn from 

 a burette and made to react with definite amounts of a stand- 

 ardized solution of iodine. The method consists in running the 

 unknown sulphite or sulphurous acid solution into a known 

 amount of a standardized solution of iodine, acidified with 

 hydrochloric acid, to the disappearance of the iodine reaction 

 with starch. This procedure rests upon the facts that the 

 oxidation of sulphite is brought about in the acidified solution 

 and that, as Bunsen showed, no more than a small proportion 

 of hydriodic acid should be present at the point at which the 

 bodies are made to react. The reaction for the oxidation of 

 sulphur dioxide proceeds normally in dilute solutions according 

 to the equation 



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



In solutions too concentrated, however, the secondary reaction 



S0 9 + 4HI = 2l a + 2H,0 + S 



takes place as Yolhard has shown*, and vitiates the indications. 

 To avoid the inconvenience of the Yolhard method it has 

 been proposed by Ruppf to bring about the oxidation of sul- 

 phites- by treatment with an excess of standardized iodine in a 

 solution made alkaline by acid sodium carbonate, and then, 

 after fifteen minutes, to titrate the excess of iodine by sodium 

 thiosulphate. This procedure, however, is, as has been shown 

 by Ruff and Jaroch^: and by the present writer,§ faulty in 

 principle and practice, and gives correct results only by a chance 

 balancing of opposing errors. Theoretically it might be possi- 

 ble to overcome the difficulties by treating with acid the alka- 

 line mixture of iodine and sulphite and acid sodium carbonate 



* Ann. Chem. 242, 93. 

 f Ber. Dteh. Chem. Ges. xxxv, 3694. 

 JBer. Dtch. Chem. Ges. xxxviii, 409. 

 § This Journal, vol. xiv, p. 237. 



