290 J. T. Norton, Jr. — Influence of Hydrochloric Acid 



7% 



sulphate is greater than 20 cm3 of the — solution, care as to the 



restriction of the acid and dilution of the solution becomes a 

 necessity. Fortunately, in most analytical processes involving 

 the use of the thiosulphate it is possible to add that reagent 

 from the burette to the solution to be acted upon, so that it 

 is destroyed normally as fast as it is introduced and the danger 

 of interaction with the acid does not occur. In the process of 

 Norris and Fay, however, the method involves the addition 

 of an excess of the thiosulphate to the solution of selenious and 

 hydrochloric acids, and thus the conditions prevail which 

 demand care as to the relation of the acid, the thiosulphate 

 and the degree of dilution. I have experimented, therefore, 

 with this process under varying conditions. 



The process of Norris and Fay for the iodometric determi- 

 nation of selenious acid consists briefly in treating the solution 

 of that acid in ice water, in the presence of hydrochloric acid, 



with an excess of a — solution of sodium thiosulphate and 



titrating back the excess of the thiosulphate with iodine. 

 Four molecules of sodium thiosulphate act, apparently, upon 

 one molecule of selenious acid according to a reaction which 

 the authors propose to study. 



The selenium dioxide used was made by dissolving pre- 

 sumably pure selenium in nitric acid and evaporating to dryness. 

 The residue was then treated with water, and a little barium 

 hydroxide was added to remove any sulphate which might be 

 present. The solution was then filtered and the filtrate 

 evaporated to dryness. The residue was mixed with four or five 

 times its volume of dried pulverized pyrolusite, and the whole 

 was put into a porcelain crucible and heated. The sublimate of 

 selenium dioxide was carefully collected on a dry watch-glass 

 and put into a drying bottle as quickly as possible. The pyro- 

 lusite prevents any reduction of the selenium dioxide to 

 selenium and the product consisted of beautiful long white 

 needles. This method of preparing the selenium dioxide, 

 which has been used for some time in this laboratory, avoids 

 contamination of the selenium dioxide by nitric acid or water, 

 resulting from the decomposition of the latter, which would 

 be possible in case this reagent were employed in the final 

 sublimation, as is recommended by Norris and Fay. The 

 hydrochloric acid used was of a sp. gr. 112, as recommended 

 by the originators of the process. For the experiments of Table 

 IV the dilution at the beginning was fixed at 400 cm3 , and this 

 was increased in every case by the melting of the ice used to 

 cool the liquid. A glance at the preceding part of this paper 



