Decay Mechanism of NO₃^• Radical in Highly Concentrated Nitrate and Nitric Acidic Solutions in the Absence and Presence of Hydrazine

The decay mechanism of NO₃^• has been determined through a combination of experiment and calculation for 7 mol dm^-3 solutions of deaerated aqueous LiNO₃ and HNO₃, in the absence and presence of hydrazine (N₂H₄, N₂H₅⁺, and N₂H₆²⁺). In the absence of hydrazine, the predominant NO₃^• decay pathways are strongly dependent upon the pH of the solution. For neat, neutral pH LiNO₃ solutions (7 mol dm^-3), NO₃^• produced by the pulse is fully consumed within 160 μs by OH^• (37%), H₂O (29%), NO₂^- (17%), and NO₂ (17%). For acidic HNO₃ solutions (7 mol dm^-3), radiolytically produced NO₃^• is predominantly consumed within 1 ms by HNO₂ (15%) and NO₂ (80%). Intervening formulations exhibit the mechanistic transition from neat LiNO₃ to neat HNO₃. In highly acidic nitric acid solution, hydrazine exists mainly as N₂H₅⁺ and N₂H₆²⁺, both of which rapidly consume NO₃^• in addition to other decay mechanisms, with rate constants of 2.9 (±0.9) × 10⁷ and 1.3 (±0.3) × 10⁶ dm³ mol^-1 s^-1, respectively.