Kinetics and mechanism of the iodine oxidation of hydroxylamine

Studies of the stoichiometry and kinetics of the reaction between hydroxylamine and iodine, previously studied in media below pH 3, have been extended to pH 5.5. The stoichiometry over the pH range 3.4–5.5 is 2NH₂OH + 2I₂ = N₂O + 4I^? + H₂O + 4H⁺. Since the reaction is first-order in [I₂] + [I₃^?], the specific rate law, k₀, is k₀ = (k₁ + k₂/[H⁺]) {[NH₃OH⁺]₀/(1 + K_p[H⁺])} {1/(1 + K_I[I^?])}, where [NH₃OH⁺]₀ is total initial hydroxylamine concentration, and k₁, k₂, K_p, and K_I are (6.5 ± 0.6) × 10⁵ M^?1 s^?1, (5.0 ± 0.5) s^?1, 1 × 10⁶ M^?1, and 725 M^?1, respectively. A mechanism taking into account unprotonated hydroxylamine (NH₂OH) and molecular iodine (I₂) as reactive species, with intermediates NH₂OI₂^?, HNO, NH₂O, and I₂^?, is proposed.