Note 78 bis — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE

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.