Note 61 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE

In general, 2HI + O = I2 + H2O, if the oxygen proceed from a substance from which it is easily evolved. For this reason compounds corresponding with the higher stages of oxidation or chlorination frequently give a lower stage when treated with hydriodic acid. Ferric oxide, Fe2O3, is a higher oxide, and ferrous oxide, FeO, a lower oxide; the former corresponds with FeX3, and the latter with FeX2, and this passage from the higher to the lower takes place under the action of hydriodic acid. Thus hydrogen peroxide and ozone (Chapter IV.) are able to liberate iodine from hydriodic acid. Compounds of copper oxide, CuO or CuX2, give compounds of the suboxide Cu2O, or CuX. Even sulphuric acid, which corresponds to the higher stage SO3, is able to act thus, forming the lower oxide SO2. The liberation of iodine from hydriodic acid proceeds with still greater ease under the action of substances capable of disengaging oxygen. In practice, many methods are employed for liberating iodine from acid liquids containing, for example, sulphuric acid and hydriodic acid. The higher oxides of nitrogen are most commonly used; they then pass into nitric oxide. Iodine may even be disengaged from hydriodic acid by the action of iodic acid, &c. But there is a limit in these reactions of the oxidation of hydriodic acid because, under certain conditions, especially in dilute solutions, the iodine set free is itself able to act as an oxidising agent—that is, it exhibits the character of chlorine, and of the halogens in general, to which we shall again have occasion to refer. In Chili, where a large quantity of iodine is extracted in the manufacture of Chili nitre, which contains NaIO3, it is mixed with the acid and normal sulphites of sodium in solution; the iodine is then precipitated according to the equation 2NaIO3 + 3Na2SO3 + 2NaHSO3 = 5Na2SO4 +I2 + H2O. The iodine thus obtained is purified by sublimation.