Riordan Eoin, Minogue Nicholas, Healy David, O'Driscoll Paul, Sodeau John R
Department of Chemistry, University College Cork, Ireland.
J Phys Chem A. 2005 Feb 10;109(5):779-86. doi: 10.1021/jp040269v.
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 NO(2)(-): 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, epsilon, 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 epsilon values for the nitrite ion were determined to be 8.16 +/- 0.08 M(-1) cm(-1) for the npi 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 epsilon 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, H(2)ONO(+), 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.
亚硝酸（HONO）和亚硝酸根离子是一类特别重要的痕量物质共轭对，在大气水相分散体的主体及表面的非均相行为方面具有重要意义：这一作用源于它们的化学反应性、光解途径、溶解度及环境浓度水平。溶液中NO₂⁻:HONO的实际比例由pH值和亚硝酸离解常数（pKₐ）决定，在文献中通常引用298K时的pKₐ值为3.27。然而，已发表的研究对于在与大气相关的模型计算中应采用的确切值存在很大分歧。此外，尽管已知亚硝酸根离子在300 - 400nm区域吸收太阳辐射，并且是表层海水中·OH自由基的主要来源，但文献中亚硝酸根离子（以及水相HONO）的摩尔吸收系数ε的测量值存在很大差异。在本研究中，采用紫外可见光谱法并仔细扣除基线，确定亚硝酸根离子在290nm处nπ跃迁的相关ε值为8.16±0.08 M⁻¹ cm⁻¹，在354nm处为22.1±0.22 M⁻¹ cm⁻¹。对于HONO，溶液中最强振动带的波长最大值在372nm处，ε值为60.52±0.6 M⁻¹ cm⁻¹。利用亨德森 - 哈塞尔巴尔赫方程及上述数据，此处报道亚硝酸的pKₐ值为2.8±0.1。然后采用牛顿 - 高斯方法，使用用FORTRAN 90编写的算法求解一组定义溶液中HONO化学形态模型 的非线性方程。基于简单一步质子化的模型在中等pH值（6 - 3）时效果良好，但在高酸性介质中偏离了实验观测结果。涉及硝鎓离子H₂ONO⁺的两步平衡模型在极酸性区域拟合得更好，而对pH值6 - 3区间几乎没有影响。
