388 Proceedings of Royal Society of Edinburgh. [sess. 
The Action of Persulphates on Iodine. 
By Hugh Marshall, D.Sc. 
(Read December 19, 1898.) 
When solutions of potassium persulphate and potassium iodide 
are mixed, iodine is liberated and potassium sulphate formed. 
The reaction takes place somewhat slowly in the cold, much more 
rapidly on warming. It has generally been supposed that the 
equation 
2KI + K 2 S 2 O s il 2 + 2K 2 S0 4 
completely represents the action which takes place, and although 
I have had occasion to try the reaction very many times since I 
first noted it as a reaction for persulphates, it was only about a 
year ago that I observed a further change. 
A solution of potassium iodide was being boiled with ammonium 
persulphate (which is much more soluble than the potassium salt) 
in order to decompose it and drive off the iodine by sublimation. 
It was observed that the free iodine in the liquid seemed to 
disappear very rapidly, more so than corresponded to the amount 
volatilised, and that the final disappearance of the brown colour 
took place with unexpected suddenness. This pointed to the 
possible conversion of the iodine into some compound, probably 
by the oxidising action of the persulphate. This proved to be the 
case, because, on the cautious addition of sulphurous acid to the 
colourless solution, iodine again appeared ; on standing, the liquid 
again became decolourised, and was again turned brown by 
sulphurous acid. These operations could be repeated so long as 
persulphate remained in the solution. The action takes place 
slowly in the cold, and at high temperatures there is considerable 
volatilisation of iodine ; when a solution of potassium iodide was 
corked up with ammonium persulphate in a test tube at the 
ordinary temperature, the iodine which separated in the solid 
form gradually dissolved on further standing, and had entirely 
disappeared in the course of several days. The oxidation product 
