Hydrobromic acid forms two hydrates, HBr,2H2O and HBr,H2O, which have been studied by Roozeboom with as much completeness as the hydrate of hydrochloric acid (Chapter X. Note [37]).
With metallic silver, solutions of hydriodic acid give hydrogen with great ease, forming silver iodide. Mercury, lead, and other metals act in a similar manner.
[78 bis] Iodide of nitrogen, NHI2 is obtained as a brown pulverulent precipitate on adding a solution of iodine (in alcohol, for instance) to a solution of ammonia. If it be collected on a filter-paper, it does not decompose so long as the precipitate is moist; but when dry it explodes violently, so that it can only be experimented upon in small quantities. Usually the filter-paper is torn into bits while moist, and the pieces laid upon a brick; on drying an explosion proceeds not only from friction or a blow, but even spontaneously. The more dilute the solution of ammonia, the greater is the amount of iodine required for the formation of the precipitate of NHI2. A low temperature facilitates its formation. NHI2 dissolves in ammonia water, and when heated the solution forms HIO3 and iodine. With KI, iodide of nitrogen gives iodine, NH3 and KHO. These reactions (Selivanoff) are explained by the formation of HIO from NHI2 + 2H2O = NH3 + 2HIO—and then KI + HIO = I2 + KHO. Selivanoff (see Note [29]) usually observed a temporary formation of hypoiodous acid, HIO, in the reaction of ammonia upon iodine, so that here the formation of NHI2 is preceded by that of HIO—i.e. first I2 + H2O = HIO + HI, and then not only the HI combines with NH3, but also 2HIO + NH3 = NHI2 + 2H2O. With dilute sulphuric acid iodide of nitrogen (like NCl3) forms hypoiodous acid, but it immediately passes into iodic acid, as is expressed by the equation 5HIO = 2I2 + HIO3 + 2H2O (first 3HIO = HIO3 + 2HI, and then HI + HIO = I2 + H2O). Moreover, Selivanoff found that iodide of nitrogen, NHI2, dissolves in an excess of ammonia water, and that with potassium iodide the solution gives the reaction for hypoiodous acid (the evolution of iodine in an alkaline solution). This shows that HIO participates in the formation and decomposition of NHI2, and therefore the condition of the iodine (its metaleptic position) in them is analogous, and differs from the condition of the halogens in the haloid-anhydrides (for instance, NO2Cl). The latter are tolerably stable, while (the haloid being designated by X) NHX2, NX3, XOH, RXO (see Chapter XIII. Note [43]), &c., are unstable, easily decomposed with the evolution of heat, and, under the action of water, the haloid is easily replaced by hydrogen (Selivanoff), as would be expected in true products of metalepsis.
[79] Hypoiodous acid, HIO, is not known, but organic compounds, RIO, of this type are known. To illustrate the peculiarities of their properties we will mention one of these compounds, namely, iodosobenzol, C6H5IO. This substance was obtained by Willgerodt (1892), and also by V. Meyer, Wachter, and Askenasy, by the action of caustic alkalis upon phenoldiiodochloride, C6H5ICl2 (according to the equation, C6H5ICl2 + 2MOH = C6H5IO + 2MCl + H2O). Iodosobenzol is an amorphous yellow substance, whose melting point could not be determined because it explodes at 210°, decomposing with the evolution of iodine vapour. This substance dissolves in hot water and alcohol, but is not soluble in the majority of other neutral organic solvents. If acids do not oxidise C6H5IO, they give saline compounds in which iodosobenzol appears as a basic oxide of a diatomic metal, C6H5I. Thus, for instance, when an acetic acid solution of iodosobenzol is treated with a solution of nitric acid, it gives large monoclinic crystals of a nitric acid salt having the composition C6H5I(NO3)2 (like Ca(NO3)2). In appearing as the analogue of basic oxides, iodosobenzol displaces iodine from potassium iodide (in a solution acidulated with acetic or hydrochloric acid)—i.e. it acts with its oxygen like HClO. The action of peroxide of hydrogen, chromic acid, and other similar oxidising agents gives iodoxybenzol, C6H5IO2, which is a neutral substance—i.e. incapable of giving salts with acids (compare Chapter XIII. Note [43]).
[79 bis] The oxidation of iodine by strong nitric acid was discovered by Connell; Millon showed that it is effected, although more slowly, by the action of the hydrates of nitric acid up to HNO3,H2O, but that the solution HNO3,2H2O, and weaker solutions, do not oxidise, but simply dissolve, iodine. The participation of water in reactions is seen in this instance. It is also seen, for example, in the fact that dry ammonia combines directly with iodine—for instance, at 0° forming the compound I2,4NH3—whilst iodide of nitrogen is only formed in presence of water.
[80] Bromine also displaces chlorine—for instance, from chloric acid, directly forming bromic acid. If a solution of potassium chlorate be taken (75 parts per 400 parts of water), and iodine be added to it (80 parts), and then a small quantity of nitric acid, chlorine is disengaged on boiling, and potassium iodate is formed in the solution. In this instance the nitric acid first evolves a certain portion of the chloric acid, and the latter, with the iodine, evolves chlorine. The iodic acid thus formed acts on a further quantity of the potassium chlorate, sets a portion of the chloric acid free, and in this manner the action is kept up. Potilitzin (1887) remarked, however, that not only do bromine and iodine displace the chlorine from chloric acid and potassium chlorate, but also chlorine displaces bromine from sodium bromate, and, furthermore, the reaction does not proceed as a direct substitution of the halogens, but is accompanied by the formation of free acids; for example, 5NaClO3 + 3Br2 + 3H2O = 5NaBr + 5HClO3 + HBrO3.
[81] If iodine be stirred up in water, and chlorine passed through the mixture, the iodine is dissolved; the liquid becomes colourless, and contains, according to the relative amounts of water and chlorine, either IHCl2, or ICl3, or HIO3. If there be a small amount of water, then the iodic acid may separate out directly as crystals, but a complete conversion (Bornemann) only occurs when not less than ten parts of water are taken to one part of iodine—ICl + 3H2O + 2Cl2 = IHO3 + 5HCl.
[82] Schönbein and Ogier proved this. Ogier found that at 45° ozone immediately oxidises iodine vapour, forming first of all the oxide I2O3, which is decomposed by water or on heating into iodic anhydride and iodine. Iodic acid is formed at the positive pole when a solution of hydriodic acid is decomposed by a galvanic current (Riche). It is also formed in the combustion of hydrogen mixed with a small quantity of hydriodic acid (Salet).
[83] Kämmerer showed that a solution of sp. gr. 2·127 at 14°, containing 2HIO3,9H2O, solidified completely in the cold. On comparing solutions HI + mH2O with HIO3 + mH2O, we find that the specific gravity increases but the volume decreases, whilst in the passage of solutions HCl + mH2O to HClO3 + mH2O both the specific gravity and the volume increase, which is also observed in certain other cases (for example, H3PO3 and H3PO4).
[83 bis] Ditte (1890) obtained many iodates of great variety. A neutral salt, 2(LiIO3)H2O, is obtained by saturating a solution of lithia with iodic acid. There is an analogous ammonium salt, 2(NH4IO3)H2O. He also obtained hydrates of a more complex composition, such as 6(NH4IO3)H2O and 6(NH4IO3)2H2O. Salts of the alkaline earths, Ba(IO3)2H2O and Sr(IO3)2H2O, may be obtained by a reaction of double decomposition from the normal salts of the type 2(MeIO3)H2O. When evaporated at 70° to 80° with nitric acid these salts lose water. A mixture of solutions of nitrate of zinc and an alkaline iodate precipitates Zn(IO3)22H2O. An anhydrous salt is thrown out if nitric acid be added to the solutions. Analogous salts of cadmium, silver, and copper give compounds of the type 2Me′IO34NH3 and Me″(IO3)24NH3, with gaseous ammonia (Me′ and Me″ being elements of the first (Ag) and second (Cd, Zn, Cu) groups). With an aqueous solution of ammonia the above salts give substances of a different composition, such as Zn(IO3)2(NH4)2O, Cd(IO3)2(NH4)2O. Copper gives Cu(IO3)24(NH4)2O and Cu(IO3)2(NH4)2O. These salts may be regarded as compounds of I2O5, and MeO and (NH4)2O; for example, Zn(IO3)2(NH4)2O may be regarded as ZnO(NH4)2OI2O5, or, as derived from the hydrate, I2O52H2O = 2(HIO3)H2O.