Using the absorption of light by nitrogen dioxide as a measure of its concentration, the rate of the gas phase reaction NO+NO2+H2O=2HNO2 was measured over a fivefold range of water vapor and nitrogen dioxide concentrations, with nitric oxide greatly in excess. Changes in light intensity were detected by means of an electron‐multiplier photo‐tube and recorded by photographing the screen of a cathode‐ray oscilloscope. Half‐times as short as 0.014 sec were observed. The reaction rate was found to depend more strongly upon the concentration of water vapor than upon that of nitrogen dioxide and to be kinetically consistent with a mechanism involving termolecular collisions.
The equilibrium constant of the reaction has been calculated and its order of magnitude experimentally confirmed, and a lower limit has been fixed for the rate of dissociation of nitrogen sesquioxide.