268 H. I. Cole — Estimation of Iodine and Bromine iti 



The preliminary tests of Table I, A, show that the iodine is 

 completely eliminated from 0'5 grm. of potassium iodide dur- 

 ing the process of concentrating to 75 cm3 the solution contain- 

 ing 1 grm. of telluric acid and 15 cm3 of the dilute sulphuric 

 acid, sp. gr. 1015 ; and that no bromine is evolved from 0\5 

 grm. of potassium bromide similarly treated until the concen- 

 tration of 50 cm3 is reached. From the results given in B and C 

 it appears that; while the presence of a chloride does not affect 

 the determination of the iodine of the iodide, the presence of 

 bromine tends to introduce slight errors of excess. On the 

 other hand, in those determinations in which an excess of 

 iodine is indicated the results show a slight deficiency in the 

 bromine. This complementary relation of the errors of the 

 iodine and the bromine points to an easier evolution of bromine 

 if free iodine is present during concentration to a limit in the 

 neighborhood of that at which the bromide by itself begins to 

 evolve bromine. A similar phenomenon was noted in previous 

 work* when the presence of free bromine had the effect of 

 raising the limit at which chlorine began to be evolved from a 

 chloride. 



In these tests the iodine collected in the receiver after the 

 concentration of the solution to a volume of 65 cm3 to 70 cm3 was 

 estimated by titration with ~N/10 thiosulphate, the receiver was 

 recharged with 200 cm3 of a 1*5 per cent solution of potassium 

 iodide and connected with the distilling flask, 10 cm3 of sulphuric 

 acid [1 : 1] was then added through the separating funnel to 

 the reaction mixture, and the solution concentrated as before 

 to a volume between i^cms^gcms^ ijijj e l^roinine was estimated 

 by titrating with JST/10 thiosulphate the iodine set free in the 

 receiver. 



The effect of the presence of free iodine upon the elimina- 

 tion of bromine from the bromide, as of free bromine upon the 

 elimination of chlorine from the chloride, is clearly manifest in 

 the results, although a part of the irregularity in the bromine 

 determination was no doubt due to the difficulty in measuring 

 with exactness the final volumes in so large a flask. So long- 

 as hydriodic acid remains undecomposed the formation of either 

 iodic acid or a compound of iodine with chlorine or bromine 

 is not possible, and so long as hydrobromic acid is present the 

 formation of a compound in which the bromine will drag along- 

 chlorine is also an impossibility. The conditions of concentra- 

 tion at which the hydriodic acid and the hydrobromic acid, 

 respectively, have been completely broken up while the free 

 halogen remains in solution are points at which there is danger 

 that the halogen less easily evolved will enter into combination 

 with the volatile, free halogen. It is desirable, therefore, that 

 the free halogen should be eliminated as it is evolved and from 



* Loc. cit. 



