The thermal decomposition of nitrogen iodide

Abstract

Bunsen (1852) was the first to suggest that the formula of nitrogen iodide was NH 3 . NI 3 , although this was disputed later on account of the fact that any attempts made to remove the ammonia resulted in complete decomposition. An alternative formula, NH 2 I.NHI 2 , which took account of this fact, was, however, disproved by Silberrad (1905), who prepared triethylamine by the action of zinc ethyl on nitrogen iodide. Eggert (1921) investigated the thermal decomposition of nitrogen iodide and showed that the following equation represented the changes occurring during detonation, thermal or photochemical decomposition: (1) 8NH 3 .NI 3 → 5N 2 + 6NH 4 I + 9I 2 . The primary step (also suggested by Chattaway and Orton (1900) for the photochemical decomposition) was supposed to be: (2) NH 3 .NI 3 = N 2 + 3HI, followed by either or both of the following reactions: (3) 5HI + NH 3 .NI 3 → 2NH 4 I + 3I 2 ; (4) 3HI + 7NH 3 .NI 3 = 4N 2 + 6NH 4 I + 9I 2 . These results were obtained when the products were allowed to accumulate up to atmospheric pressures. The present investigation has shown that the decomposition takes a different course if the products are removed in high vacua. The properties of the substance when subjected to mechanical shock are well known. Under the impact of a small falling weight it detonates even at liquid-air temperatures (Eggert 1921). Eggert states that the detonation of this substance occurs under the action of pressure alone (at 5000 atm.) even when the substance is wet. Garner and Latchem (1936) showed that the substance detonates in a hard vacuum immediately it is dry, an observation which was confirmed by Belajev and Chariton (1936). The decomposition proceeds quietly at - 10° C if the pressure of the gas above the crystals be not allowed to fall below 2 x 10 -3 cm., the reaction slowing up as the products accumulate and coming to a standstill before all of the iodide is decomposed. It was suggested by Garner and Latchem that one of the products acted as a retarding agent stabilizing the solid. It was thought that the retarding agent was iodine, but this has been shown by the present investigation to be incorrect. In spite of the large volume of experimental research on nitrogen iodide, the details of the kinetics of its decomposition are still very obscure. Neither the activation energy nor the heat liberated in its decomposition is known, and there has been no extended investigation into its sensitivity to heat or to shock. It has been the object of this investigation to obtain information along these lines and also to throw some light on the initiation of detonation generally.