On Chemical reaction in the electric discharge II—The synthesis of nitric oxide

The electrical synthesis of nitric oxide has long been regarded as highly anomalous from the standpoint of reaction kinetics. It is now generally considered to be primarily an electrical process, which may, in the absence of special experimental dispositions, easily be masked by a secondary thermal reaction which conceals its true nature. An admirable discussion and bibliography of the earlier work is given by Knox (1924). Attempts to establish the detailed mechanism of the reaction have been made by Wansbrough-Jones (1930) and by Brewer and Westhaver (1930). The former used controlled electrons in a triode system and gas at a pressure of the order of 0.1 mm. Since the reaction commenced with electrons at 16 V, and became more rapid at 24 V, he concluded that synthesis proceeds via N+2 or N+'2 excited or ionized oxygen playing no part since no discontinuities in the curves were observed at any of the critical potentials of this element. The latter employed D. C. with a discharge tube immersed in liquid air. They found that under the optimum conditions approximately 1 mol. of NO2 was formed for every electron flowing in the outer circuit, and that the rate of reaction is favoured by nitrogen and retarded by oxygen in excess. With further assumptions as to the relation between electron and positive-ion currents, Brewer and Westhaver arrive at N+2 + 2O2 -> N2O+4 -> N2O4 as the mechanism of reaction, with an M/N value of 2. The experiments to be described in the present communication derive from an observation, made incidentally to researches upon impulse discharges (Willey 1935), that whereas the yield of active nitrogen in pure nitrogen is proportional, over a wide range, to the applied wattage, the yield of nitrogen oxides, after a short initial stage, is independent of the power. This suggested that some destructive agent is formed simultaneously with the main product. Verification of this hypothesis leads to rejection of some existing reaction theories and their replacement by a very simple alternative. Experimental The nitrogen and oxygen were obtained from cylinders and thoroughly dried by phosphorous pentoxide ; earlier experiments showed that no advantage is gained by further purification. In all cases the flow speed of the gas was maintained at 6.5 I./hr. at N. T. P., the pressure in the apparatus being 7 mm.