Spectroscopic and optimization modeling study of nitrous acid in aqueous solution

Nitrous acid (HONO) and the nitrite ion represent a particularly important conjugate pair of trace species with regard to heterogeneous behavior within the bulk, and on the surface, of aqueous atmospheric dispersions: this role results from their chemical reactivity, photolysis pathways, solubility, and ambient concentration levels. The actual ratio of NO₂^-: HONO in solution is determined by the pH and the nitrous acid dissociation constant (pK_a) which is generally quoted in the literature as 3.27 at 298 K. However there is much disagreement in published works as to the exact value, which should be used in model calculations relevant to the atmosphere. Furthermore even though the nitrite ion is known to absorb solar radiation in the 300-400 nm region and represents a dominant source of OH radicals in surface seawater, large variations in the measured molar decadic absorption coefficients, ε, for nitrite ions (and aqueous HONO) are evident in the literature. In the current study, using a UV - vis spectrometric approach with careful baseline subtraction, the relevant ε values for the nitrite ion were determined to be 8.16 ± 0.08 M^-1 cm^-1 for the nπ* transitions at 290 nm and 22.1 ± 0.22 M^-1 cm ^-1 at 354 nm. For HONO, the wavelength maximum for the strongest vibronic band in solution was found at 372 nm with an e value of 60.52 ± 0.6 M^-1 cm^-1. Using the Henderson-Hasselbalch equation and the above data, a value of 2.8 ± 0.1 is therefore reported here for the pK_a, of nitrous acid. A Newton-Gauss method was then employed to solve a set of nonlinear equations defining the chemical speciation model for HONO in solution using an algorithm written in FORTRAN 90. A model based on a simple one-step protonation worked well for intermediate pHs (6-3) but departed from the experimental observations in highly acidic media. A two-step equilibrium model involving the nitroacidium ion, H₂ONO⁺, gave a much closer fit in the very acidic region, while having little or no effect on the pH 6-3 section of the profile.