Kinetics and mechanism of oxidation of hydrazine by tri-iodide ion in aqueous acidic media

Abstract

The kinetics of oxidation of hydrazine (L) by I₃^– follows a pseudo-first-order rate law in aqueous acetic acid–sulphuric acid media in the range [H⁺] 2.0–1.58 × 10^–3 mol dm^–3. The pseudo-firstorder rate constants (k_obs.) exhibit: (1) at [H⁺]= 0.9 mol dm^–3 a linear dependence on [L]_o at low [L]_o, with a tendency to a limiting value at high relative [L]_o; (2) a decreasing and complex trend in [I^–] and in [H⁺]; (3) a decreasing trend with decreasing dielectric constant of the medium; and (4) negligible dependences on the ionic strength of the medium, on added Cu²⁺(aq), and on added ethylenediaminetetra-acetate. The results are interpreted in terms of a mechanism which envisages (a) negligible reactivities of I₃^– and of HL⁺ predominant in the medium, (b) an inner-complex mechanism of electron transfer (i.e. a 1 : 1 σ-charge-transfer co-ordination prior to the electron transfer) involving I₂ and L, and (c) an encounter reaction between IOH and L. Mechanistic ambiguities of some earlier reports of the same reaction are explained.