92 Prof. E. F. Herroun on an Iodine Voltameter 



apparent with copper ; for, as its chemical equivalent is much 



smaller, the weight of copper yielded by a given number of 



31'75 

 coulombs is only nft of the weight of silver. In addition, 



its greater readiness to oxidize may cause the deposit of 

 copper to become superficially oxidized in the process of 

 washing and drying. This, however, may partly compensate 

 for the defect in weight due to dissolved oxygen round the 

 kathode. 



If a list of the electrochemical equivalents of elements be 

 examined, next to mercurows mercury, the one which has the 

 highest value will be found to be iodine ; and it is curious 

 that, although the liberation of iodine from potassic iodide 

 was very early used to detect electric currents, no attempt in 

 recent years seems to have been made to utilize it for quan- 

 titative measurements. 



As iodine is an anion, dissolved oxygen, which diminishes 

 the yield at the kathode in other voltameters, will have no 

 such action in a neutral solution of iodide ; and as its colour 

 reaction with starch is extremely sharp, and its titration with 

 standard sodium thiosulphate a process of greater accuracy 

 than the performance of the average balance, the exact deter- 

 mination of the amount of iodine liberated by a current is 

 both easy and rapid. 



Experimental Details. 



A voltameter consisting of platinum plates in a solution of 

 potassic iodide is not suited for quantitative measurement ; 

 as, upon electrolysis, it yields caustic potash and hydrogen 

 at the kathode, the former of which by diffusion would come 

 in contact with tli3 iodine set free at the anode, converting it 

 into iodide and iodate. If this were all it would be easy to 

 acidify after electrolysis, when the iodine would be again set 

 free ; but there is besides the additional disadvantage, that a 

 rather large electromotive force of polarization is set up 

 between the electrode surrounded by hydrogen and KOH 

 and the one surrounded by iodine. 



The solution actually employed was a 10 per cent, to 15 per 

 cent, solution of neutral zinc iodide (a solution that keeps 

 well if a small strip of zinc be suspended in it). The anode 

 was a plate or disk of platinum placed at the bottom of a tall 

 narrow beaker and joined by a platinum wire, sealed through 

 a glass tube extending beyond the top of the beaker, to the 

 outside circuit. In this way all the iodine is liberated at the 

 bottom of the column of liquid, and, owing to its high density. 



