J. T. Norton, Jr. — Titrations by Sodium Thiosulphate. 287 



Aet. XXX. — The Influence of Hydrochloric Acid in Titra- 

 tions by Sodium Thiosulphate, with special reference to the 

 Estimation of Selenious Acid; by John" T. Norton, Jr. 



[Contributions from the Kent Chemical Laboratory of Yale University — LXXX.] 



In the method of Morris and Fay* for the iodometric deter- 

 mination of selenious acid, advantage is taken of a direct and 

 unique action of sodium thiosulphate upon selenium dioxide 

 in the presence of hydrochloric acid. Most excellent results 

 are claimed for this method ; but the explicit statement of the 

 originators of the method, that the amount of hydrochloric 

 acid present does not influence the result, provided the titra- 

 tion is made at the temperature of melting ice, is so extraordi- 

 nary in view of generally accepted ideas in regard to the 

 interaction of hydrochloric acid and sodium thiosulphate, as to 

 suggest the necessity of careful investigation of this point. 



Pickering, f in his investigation of the reaction between iodine 

 and sodium thiosulphate, has shown that more iodine is required 

 to oxidize the thiosulphate as the proportion of hydrochloric 

 acid increases. He ascribed this effect to the formation of a 

 sulphate, apparently, by the increased activity of the iodine, 

 but the more rational explanation is that, although some sul- 

 phate is ultimately formed, the thiosulphate is first partially 

 decomposed into free sulphur and sulphur dioxide. FinkenerJ 

 and Mohr§ also mention the decomposing effect of free acid 

 upon sodium thiosulphate. 



The sodium thiosulphate used in the following experiments 

 was taken in nearly decinormal solution and w r as standardized 

 by running it into an approximately decinormal solution of 

 iodine, the value of which had been determined by comparison 

 with decinormal arsenious acid made from carefully resub- 

 limed arsenious oxide. In the experiments of Table I the 

 solutions were stirred continuously and kept at a temperature 

 of from 0° to 5° C, while the thiosulphate ran into the acidified 

 liquid. The volume of the solution, though fixed at the be- 

 ginning as given in the table, was considerably increased during 

 the operation by the melting of the ice. Titrations were con- 

 ducted as rapidly as possible to avoid the separation of sulphur, 

 which is likely to occur, especially when the acid and thiosul- 

 phate are present in large quantities. A perusal of the table 



* Am. Chem. Jour., vol. xviii. pp. *703. 

 f Jour. Chem. Soc , vol. xxxvii, pp. 135. 

 X Anal. Chem., 6 Aufl , pp. 620. 

 § Titrirmethode, 6 Aufl., pp. 279. 



