Equilibrium constant of the reaction .OH+HNO3←→H2O+NO3. in aqueous solution

The equilibrium reaction ·OH + HNO₃ ⇆ H₂O + NO₃. was investigated by pulse radiolysis in aqueous solutions. An estimate of the equilibrium constant was derived from the dependence of the optical absorbance due to NO₃. radicals upon the concentration of HNO₃. In addition, estimates of the forward and reverse rate constants were obtained from the dependence of the formation rate constant on the ratio of the activities of nitric acid and water. Studies were carried out at different values of the dose per pulse and at pH values of 1 and 0. The observations were modeled, taking into account all relevant reactions for the formation and decay of the nitrate radical. From these modeling studies the estimated rate and equilibrium constants were refined. The rate constant of the forward reaction is found to be (8.6 ± 1.3) × 10⁷ L mol^-1 s^-1, the rate constant of the reverse reaction is found to be (3 ± 1) × 10² L mol^-1 s^-1, and the equilibrium constant K_eq = (2.8 ± 0.4) × 10⁵, all at zero ionic strength. From the latter value of K_eq and taking E⁰(H⁺, .OH/H₂O) = 2.72 V versus NHE, we calculate the reduction potentials E⁰(H⁺, NO₃./HNO₃) = (2.40 ± 0.01) V and E⁰(NO₃./NO₃^-) = (2.48 ± 0.01) V. This value of the reduction potential for this couple leads to a Henry's law coefficient of K_H = 0.018 mol L^-1 bar^-1 at 298 K for NO₃.. This value suggests that the impact of NO₃. on atmospheric droplets will be due solely to reactive uptake.