In a gaseous form hydrobromic and hydriodic acids are closely analogous to hydrochloric acid; they are liquefied by pressure and cold, they fume in the air, form solutions and hydrates, of constant boiling point, and react on metals, oxides and salts, &c.85 Only the relatively easy decomposability of hydrobromic acid and especially of hydriodic acid, clearly distinguish these acids from hydrochloric acid.
For this reason, hydriodic acid acts in a number of cases as a deoxidiser or reducer, and frequently even serves as a means for the transference of hydrogen. Thus Berthelot, Baeyer, Wreden, and others, by heating unsaturated hydrocarbons in a solution of hydriodic acid, obtained their compounds with hydrogen nearer to the limit CnH2n+2 or even the saturated compounds. For example, benzene, C6H6, when heated in a closed tube with a strong solution of hydriodic acid, gives hexylene, C6H12.
The easy decomposability of hydriodic acid accounts for the fact that iodine does not act by metalepsis on hydrocarbons, for the hydrogen iodide liberated with the product of metalepsis, RI, formed, gives iodine and the hydrogen compound, RH, back again. And therefore, to obtain the products of iodine substitution, either iodic acid, HIO3 (Kekulé), or mercury oxide, HgO (Weselsky), is added, as they immediately react on the hydrogen iodide, thus: HIO3 + 5HI = 3H2O + 3I2, or, HgO + 2HI = HgI2 + H2O.
From these considerations it will be readily understood that iodine acts like chlorine (or bromine) on ammonia and sodium hydroxide, for in these cases the hydriodic acid produced forms NH4I and NaI. With tincture of iodine or even the solid element, a solution of ammonia immediately forms a highly-explosive solid black product of metalepsis, NHI2, generally known as iodide of nitrogen, although it still contains hydrogen (this was proved beyond doubt by Szuhay 1893), which may be replaced by silver (with the formation of NAgI2): 3NH3 + 2I2 = 2NH4I + NHI2. However, the composition of the last product is variable, and with an excess of water NI3 seems to be formed. Iodide of nitrogen is just as explosive as nitrogen chloride.86
In the action of iodine on sodium hydroxide no bleaching compound is formed (whilst bromine gives one), but a direct reaction is always accomplished with the formation of an iodate, 6NaHO + 3I2 = 5NaI + 3H2O + NaIO3 (Gay-Lussac). Solutions of other alkalis, and even a mixture of water and oxide of mercury, act in the same manner.87
This direct formation of iodic acid, HIO3 = IO2(OH), shows the propensity of iodine to give compounds of the type IX5. Indeed, this capacity of iodine to form compounds of a high type emphasises itself in many ways. But it is most important to turn attention to the fact that iodic acid is easily and directly formed by the action of oxidising substances on iodine. Thus, for instance, strong nitric acid directly converts iodine into iodic acid, whilst it has no oxidising action on chlorine.88 This shows a greater affinity in iodine for oxygen than in chlorine, and this conclusion is confirmed by the fact that iodine displaces chlorine from its oxygen acids,89 and that in the presence of water chlorine oxidises iodine.90 Even ozone or a silent discharge passed through a mixture of oxygen and iodine vapour is able to directly oxidise iodine91 into iodic acid.
It is disengaged from solutions as a hydrate, HIO3, which loses water at 170°, and gives an anhydride, I2O5. Both these substances are crystalline (sp. gr. I2O5 5·037, HIO3 4·869 at 0°), colourless and soluble in water;92 both decompose at a red heat into iodine and oxygen, are in many cases powerfully oxidising—for instance, they oxidise sulphurous anhydride, hydrogen sulphide, carbonic oxide, &c.—form chloride of iodine and water with hydrochloric acid, and with bases form salts, not only normal MIO3, but also acid; for example, KIO3HIO3, KIO32HIO3.93 With hydriodic acid iodic acid immediately reacts, disengaging iodine, HIO3 + 5HI = 3H2O + 3I2.