Nitrous acid from nitrogen dioxide reaction

Jenkin, M. E., R. A. Cox, and D. J. Williams, Laboratory Studies of the Kinetics of Formation of Nitrous Acid from the Thermal Reaction of Nitrogen Dioxide and Water Vapor, Atmos. Enriron., 22, 487-498 (1988). 

The gas may be made by the reaction of sodium chlorate with hydrochloric acid with the production of coproduct chlorine (56, 106, 197, 206) or by the reaction of chlorate with concentrated sulfuric acid (225) or a reducing agent such as sulfur (205), sulfur dioxide (225), oxalic acid (225), nitrous acid (5), nitrogen dioxide (169, 221), methanol (54), or organic peroxides (124). Chlorine dioxide may be generated electrolytically from chlorite (51, 188). 

At low acid concentrations, nitric oxide tends to form. This evidently may attack nitrosophenol to form diazonium compounds directly. The diazonium salts, in turn, may couple with unreacted phenol to give colored products. Nitrous acid may also produce nitrophenols from phenols. The mechanism of this reaction may involve oxidation of initially formed nitrosophenols, homolytic attack by nitrogen dioxide, or nucleophilic attack by nitrite ions [1]. 

When oxides of nitrogen come in contact with water, both nitrous and nitric acids are formed (18) (Table IV). Toxic reactions may result from pH decrease. Other toxic reactions may be a consequence of deamination reactions with amino acids and nucleic acid bases. Another consideration is the reactions of oxides of nitrogen with double bonds (Table IV). The cis-trans isomerization of oleic acid exposed to nitrous acid has been reported (19). Furthermore, the reaction of nitrogen dioxide with unsaturated compounds has resulted in the formation of both transient and stable free radical products (20, 21) (Table V). A further possibility has been raised in that nitrite can react with secondary amines to form nitrosamines which have carcinogenic properties (22). Thus, the possible modes of toxicity for oxides of nitrogen are numerous and are not exhausted by this short list. 

---