As with chlorine, so with iodine, a periodic acid, HIO4, is formed. This acid is produced in the form of its salts, by the action of chlorine on alkaline solutions of iodates, and also by the action of iodine on chloric acid.94 It crystallises from solutions as a hydrate containing 2H2O (corresponding with HClO4,2H2O), but as it forms salts containing up to 5 atoms of metals, this water must be counted as water of constitution. Therefore IO(OH)5 = HIO4,2H2O corresponds with the highest form of halogen compounds, IX7.95 In decomposing (at 200°) or acting as an oxidiser, periodic acid first gives iodic acid, but it may also be ultimately decomposed.
Compounds formed between chlorine and iodine must be classed among the most interesting halogen bodies.96 These elements combine together directly with evolution of heat, and form iodine monochloride, ICl, or iodine trichloride, ICl3.97 As water reacts on these substances, forming iodic acid and iodine, they have to be prepared from dry iodine and chlorine.98 Both substances are formed in a number of reactions; for example, by the action of aqua regia on iodine, of chlorine on hydriodic acid, of hydrochloric acid on periodic acid, of iodine on potassium chlorate (with the aid of heat, &c.) Trapp obtained iodine monochloride, in beautiful red crystals, by passing a rapid current of chlorine into molten iodine. The monochloride then distils over and solidifies, melting at 27°. By passing chlorine over the crystals of the monochloride, it is easy to obtain iodine trichloride in orange crystals, which melt at 34° and volatilise at 47°, but in so doing decompose (into Cl2 and ClI). The chemical properties of these chlorides entirely resemble those of chlorine and iodine, as would be expected, because, in this instance, a combination of similar substances has taken place as in the formation of solutions or alloys. Thus, for instance, the unsaturated hydrocarbons (for example, C2H4), which are capable of directly combining with chlorine and iodine, also directly combine with iodine monochloride.
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