Photodecomposition of Nitrite and Undissociated Nitrous Acid in Aqueous Solution

Quantum yields of phenol and nitrate, produced by photodecomposition in aqueous solutions of NO2- and HNO2 in the presence of benzene as scavenger for OH radicals, have been determined as a function of wavelength between 280 and 390 nm. The production of phenol was used to calculate primary OH quantum yields. For NO2- photolysis at pH 6 Φ1(OH) was found to decrease with increasing wavelength from 0.069 ± 0.008 at 280 nm to 0.022 ± 0.004 at 390 nm, in agreement with previous data. The OH quantum yield Φ2(OH) for the photolysis of HNO2 at pH 2 was essentially constant over the entire wavelength range with Φ2 = 0.35 ± 0.02 (2σ). Quantum yields for NO3- are comparable in magnitude to those of phenol, indicating that NO as primary product is largely oxidized to nitrate. The most likely conversion processes are reactions of NO with O2- (pH 6), the latter resulting from the oxidation of benzene, to form peroxynitrous acid, which undergoes thermal decomposition, and of NO2 with HO2 (pH 2) to form peroxynitric acid, which reacts further with HNO2. The rate of NO3- production decreases with time in the photolysis of NO2-, whereas it increases in the photolysis of HNO2, and these features remain unexplained.