Determination <>t' Iodine in Haloid Salts. L89 



animation of the distillate by treatment of the residne is, how- 

 ever, bo great as to constrain ag to Beard) for Borne substitute 

 for the \wv\r salt, which, by virtue of easy reducibility may 

 act as a liberator of iodine from hydriodic acid, and, at the same 

 time, by reason of stability after reduction, shall register accu- 

 rately the quantity o^ iodine set free in the reducing process. 

 The results of our experience are contained in the following 

 account. 



Strong sulphuric acid, as is well known, acts upon an iodide 

 in a way to liberate iodine at the cost of its own loss of oxygen, 

 though by simple dilution of the mixture thus formed the 

 action is reversed, the iodine going hack into the form of hy- 

 driodic acid, and the products of the reduction of sulphuric 

 acid again taking back their oxygen and re-forming the acid. 

 In the presence of any substance easily reducible by the deoxi- 

 dation products of sulphuric acid the liberation of iodine, by 

 the action of that acid upon iodides, should take place without 

 interference, and even more easily and completely than in the 

 absence of such a substance, while the sulphuric acid should 

 remain at the end of the process in its original form. If the 

 products of reduction which appear in such a case in the place 

 of those of the sulphuric acid should be neither readily oxidiz- 

 able nor easily volatilizable, it ought to be possible to remove by 

 heat the iodine set free in the action without disturbing the 

 record kept of its amount by the reduced substance remaining 

 in the residue. 



The qualities of arsenic acid suggest it as a substance likely 

 to possess just these qualities ; for, though arsenious acid is 

 converted into arsenic acid by the action of iodine in alkaline 

 solution, in acid solution the reverse is true to at least a limited 

 extent, and arsenic acid liberates iodine according to the equa- 

 tion, 



H 3 As0 4 -f2lII=H 3 As0 3 +n Q + I-L 



In company with sulphuric acid of such strength as to liberate 

 the iodine from hydriodic acid, the reduction should fall in the 

 end upon the arsenic, and the arsenious oxide produced should, 

 under proper conditions, preserve the record of the iodine lib- 

 erated and removed by volatilization. We therefore under- 

 took experimentation upon this line, and the accompanying 

 table shows the results of a preliminary investigation of the 

 mode of action of a mixture of sulphuric and arsenic acids upon 

 an alkaline iodide. In making these tests a standard solution 

 of potassium iodide was put into a test-tube, a solution of potas- 

 sium arseniate was added, sulphuric acid mixed with its own 

 volume of water was introduced, the volume of the liquid was 

 adjusted, a film of kerosene 3 mm. thick was placed upon the 

 surface of the liquid, and the whole was heated gently and agi- 



