Hydrolysis Equilibrium of Dinitrogen Trioxide in Dilute Acid Solution

Solutions of nitrous acid in dilute (0.1 M HCI) or more concentrated ( 5 M HCIOJ acid exhibit a contribution to the ultraviolet absorbance that is quadratic in [HN02]. In dilute acid this excess absorbance is attributed to N2O3 formed through equilibrium 1, 2HN02 = N2O3 + H20. From the measured spectrum of eNf13KI vs. wavelength (245-260 nm) and by comparison with tNS, obtained in pulse radiolysis by Grfitzel et al., KI is determined as (3.03 f 0.23) X lo-' M-', in agreement with the value calculated thermochemically. The coefficient of physical solubility of N2O3 in water is computed as 0.70 f 0.05 M atm-I. The extinction quadratic in [HNO,] is independent of acid concentration in dilute acid but increases strongly in more concentrated acid, suggesting a possible second absorbing species of stoichiometry HN203+.