Kinetics of the reaction OH + HNO2 to H2O + NO2 at high temperatures behind shock waves

The thermal decomposition of nitrous acid at temperatures ranging from 1000 to 1400 K and pressures from 3.5 to 11 atm, using a shock tube chemical relaxation technique. Mixtures of NO, NO2, and H2O in argon, containing nitrous acid due to the reversible equilibrium reaction NO + NO2 + H2O to 2HNO2, were heated by incident shock waves; the rate of HNO2 decomposition was followed by monitoring spectroscopically the increase in NO2 concentration as the equilibrium shifts to the left. Time-dependent NO2 absorption traces at 4500 A are diagrammed. There is a sudden decrease in transmittance, resulting from compression of the gases at the shock front. This is followed by a slower decrease due to NO2 formation as the HNO2 decomposes. The NO2 absorption eventually approaches a steady-state level due to completion of reaction. Good correlation between the steady-state absorption and its calculated value indicates that no appreciable quantities of N2, NO3, etc. are formed and is consistent with the reversible reaction HNO2(+M) to NO + OH(+M) (slow) and the reaction OH + HNO2 to H2O + NO2 (fast) being the only important reactions in the system.