o 



98 Gooch and Mar — Determination of Ghlorine. 



occurs a time in the course of action when for a given degree 

 of dilution an equilibrium is established between the tendency 

 of the oxidizer to oxidize and that of the reduced residue to 

 reverse the action. The obvious modes of securing completed 

 action are the concentrating of the liquid and the reinforcing 

 of the oxidizer. Application of the former is precluded by 

 the danger of volatilizing hydrochloric acid ; the simplest 

 mode of realizing the latter — and as the result proved, a feasi- 

 ble one — is the re-oxidation of the reduced ferrous sulphate by 

 means of nitric acid. 



When a sufficient amount of nitric acid is added to restore 

 the iron to the ferric state, the boiling of the solution resulted 

 in the complete liberation of the iodine. In dilute solutions 

 the amount of nitric acid necessary to oxidize a fixed quantity 

 of ferrous salt is greater than in concentrated solutions. Thus, 

 while 0*1 cm 3 of strong nitric acid should be more than enough 

 to reoxidize the iron reduced by 1 grm. of potassium iodide 

 when the full oxidizing action is brought out, we found it 

 necessary to add to the dilute solutions with which we worked 

 about 2 cm 3 of the acid to complete the action satisfactorily. 

 Incidentally, we found that the nitric acid by itself — that is, 

 without the presence of the iron salt — is not effective in liber- 

 ating the iodine; for, the successive addition of portions of 

 1 err? of nitric acid to a solution containing 10 cm 3 of sulphuric 

 acid [I :1], and 1 grm. of potassium iodide in a total volume 

 of 400 cm 3 until the amount of nitric acid reached 5 cm 3 , the 

 liquid boiling all the time, liberated but little iodine, while 

 the addition at this point of 2*5 grm. of ferric alum deter 

 mined the evolution of iodine in dense fumes. 



The addition of 2 cm 3 , or at the most 3 cm 3 , of strong nitric 

 acid to solutions constituted as has been described proved to 

 be sufficient to liberate the entire amount of iodine present. 

 The question as to whether the hydrochloric acid is affected by 

 the addition of so much nitric acid was settled in the experi- 

 ments of the following series. In these determinations a little 

 sulphurous acid was added to each distillate to insure the com- 

 plete precipitation of the chlorine by silver nitrate, and suffi- 

 cient nitric acid was added to re-dissolve the silver sulphite 

 thrown down also at first. 



It appears in these results that the ferric salt has np per- 

 ceptible influence upon the hydrochloric acid, and that the 

 presence of nitric acid within reasonable limits does not sen- 

 sibly increase the loss of hydrochloric acid which takes place 

 under conditions otherwise similar, strong nitric acid to the 

 amount of 5 cm 3 in a total volume of 300 cm 3 producing no 

 apparent increment of loss from half a gram of hydro- 

 chloric acid. The nitric acid itself passes easily into the 



