2 I). A. Kr eider— Iodine Titration Voltameter. 



into the general literature. The work of both men, on voltam- 

 eters, was incidental to another research arid the titles of their 

 articles fail to afford any clue to this particular contents. My 

 own preliminary search of possible reactions had been com- 

 pleted and I had settled upon the form of the iodide voltameter 

 before I learned of the work just mentioned. 



Herroun rather erroneously points to the high electro- 

 chemical equivalent of iodine as its chief advantage ; this is 

 obviously immaterial in a volumetric method. As a basis of a 

 titration method, however, iodine has peculiar advantages in 

 the rapid action of sodium thiosulphate upon it and in the 

 extreme sharpness of the end reactions, intensified when desir- 

 able by the addition of starch solution. Potassium iodide is, 

 moreover, much to be preferred to the zinc salt. It is obtain- 

 able commercially quite free from iodates and its solutions 

 may, therefore, be acidified without the liberation of iodine, 

 except for the very slow action of atmospheric, or dissolved, 

 oxygen. 



It is, of course, impracticable to electrolyze the potassium 

 iodide directly, when the liberated iodine is to afford a measure 

 of the current ; this is because of the recombination of the 

 electrode products by diffusion. Acidifying prevents this 

 recombination, or even after it has taken place, in an acid 

 solution the original amount of iodine is liberated, providing 

 the iodine has not been allowed to diffuse to the cathode 

 during the electrolysis, where it would tend to combine with 

 hydrogen and would be irrecoverable. 



A most satisfactory action is obtained when the anode is 

 submerged in a strong aqueous solution of the potassium 

 iodide under dilute hydrochloric acid in which the cathode is 

 suspended. With the electrodes thus placed vertically, the 

 anode below, the liberated iodine, because of its great density 

 and solubility in potassium iodide, diffuses very slowly. After 

 a run of hours, and even on subsequent standing for hours 

 more, the solution about the cathode remains perfectly color- 

 less and free of iodine. 



The Potassium Iodide Cell. 



As a cell, I have employed a side-necked test tube, fig. 1, 

 drawn out at the bottom to a long capillary. Into the junction 

 of this capillary a glass rod was ground. The test tube was 

 closed by a rubber stopper with three perforations, through 

 the middle one of which the ground glass rod passes air tight, 

 though not so tightly as to prevent easy motion when moist. 

 The other perforations receive the glass tubes through which 

 the wires connect with the platinum electrodes; anode at the 



