Solubility and decomposition kinetics of nitrous acid in aqueous solution

Abstract

The Henry's law solubility and the decomposition reaction kinetics of nitrous acid (HNO2) in aqueous solutions have been determined by measuring the material distribution between the gas and liquid phase of the pertinent species with the use of a bubbler-type gas-liquid reactor in conjunction with a high-sensitivity chemiluminescence NOx detector. The pH-dependent solubility of N(III) (≡HNO2 + NO2-) was measured for the pH range 2.13-3.33. The Henry's law coefficient and the acid dissociation constant of HNO2 corresponding to 25°C are determined to be 49 ± 3 M atm-1 and (5.3 ± 0.4) × 10-4 M, respectively. The temperature dependence of the solubility over the range 0-30°C yielded AΔH°sol = -9.7 ± 0.3 kcal mol-1 and ΔS°sol = -24.8 ± 0.7 cal mol-1 K-1. The rate constants of the aqueous-phase reactions 2HNO2 ⇄ NO + NO2 + H2O (5) and 2NO2 + H2O → H+ + NO3- + HNO2 (6) determined at 22.0 ± 0.1°C are ks = 13.4 ± 1.0, k-5 = (1.6 ± 0.1) X× 108, and k6 = (8.4 ± 1.5) × 107, all in units of M-1 s-1. The values of ΔG°sol and ΔS°sol for the dissolution of HNO2 determined in this work are at variance with the latest values recommended by the National Bureau of Standards but are essentially identical with the previously selected set of values. The rather limited solubility of HNO2, in combination with its low atmospheric concentrations, suggests that HNO2 by itself contributes insignificantly to the acidification of atmospheric water, e.g., cloudwater. However, aqueous-phase reactions that produce HNO2 followed by degassing remain a plausible route for the production of atmospheric HNO2. © 1988 American Chemical Society.