Reduction of iodine by hydroxylamine within the pH range 0.7–3.5

The reduction of iodine by hydroxylamine within the H⁺] range 3×10^?1–3×10^?4 mol.L^?1 was first studied until completion of the reaction. In most cases, the concentration of iodine decreased monotonically. However, within a narrow range of reagent concentrations (NH₃OH⁺]₀/I₂]₀ ratio below 15, H⁺] around 0.1 mol.L^?1, and ionic strength around 0.1 mol.L^?1), the I₂] and I₃^?] vs. time curves showed 2 and 3 extrema, respectively. This peculiar phenomenon is discussed using a 4 reaction scheme (I₂+I^???I₃^?, 2 I₂+NH₃OH⁺+H₂O→HNO₂+4 I^?+5 H⁺, NH₃OH⁺+HNO₂→N₂O+2 H₂O+H⁺, and 2 HNO₂+2 I^?+2 H⁺→2 NO+I₂+2 H₂O). In a flow reactor, sustained oscillations in redox potential were recorded with an extremely long period (around 24 h). The kinetics of the reaction was then investigated in the starting conditions. The proposed rate equation points out a reinforcement of the inhibition by hydrogen ions when H⁺] is above 4×10^?2 mol.L^?1 at 25°C. A mechanism based on ion-transfer reactions is postulated. It involves both NH₂OH and NH₃OH⁺ as the reducing reactive species. The additional rate suppression by H⁺ at low pH would be connected to the existence of H₂OI⁺ in the reactive medium. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 785–797, 1998