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 + K2S2O8 = I2 + 2K2SO4

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.