The photolysis of aqueous nitrate solutions

Abstract

Data have been obtained relevant to the mechanism of formation of nitrite, oxygen and peroxynitrite by the photolysis of aqueous sodium nitrate solutions. In alkaline solutions, the origin of O₂ as determined by the use of ¹⁸O-enriched water, and the lowering of nitrite yields by various solutes, are consistent with the dissociative process                                              (NO₃^-)*→ NO₂^-+Orather than with                               (NO₃^-)*→ NO₂+O^- The effect of these solutes is attributed to reduction of (NO₃^-)*. It is proposed that peroxynitrite (OONO^-) is formed by intramolecular rearrangement of the pyramidal (NO₃^-)* in the ³ππ state. Peroxynitrite is not a precursor to NO₂^-. The quantum yield depends on both wavelength and temperature, and the apparent energy of activation decreases with decreasing wavelength. These results are interpreted in terms of the different reactivities of (NO₃^-)* produced in the nπ* and the ππ* absorption bands.     Low initial yields at low pH can be explained by protonation of (NO₃^-)* followed by                                   (HOONO)*→ OH+NO₂and                                    OH+NO₂→ H⁺+NO₃ The self-inhibition process evident in acidic conditions is consistent with the formation of NO₃ (from NO₂ and O) which re-oxidizes NO₂-. Sharp yield increases on the addition of some reagents are due to scavenging of the OH intermediate in the (HOONO)* → NO₃^- reversion.