Kinetics of oxidation of nitrite by aqueous chlorine

Abstract

The kinetics of oxidation of nitrite to nitrate by chlorine in aqueous acid solution (pH 0-1) have been studied using a spectrophotometric stopped flow technique. The rate law is                  -d[Cl2]_a/dt =([Cl₂][NO₂^-]/K_a[Cl-]²)(f+g[HNO₂]) where [Cl₂]_a = [Cl₂]+[Cl₃^-] and where K_a is the ionization constant for nitrous acid. At 298.2 K and ionic strength 2.75M, f = 60.8±0.5 mol² l^-2 s^-1 and g = (2.35±0.05)x10⁵ mol l^-1 s^-1: the associated activation energies are 68±3 and 44±2 kJ mol^-1 respectively. A mechanism is proposed involving the reversible initial step:                            NO₂^-+Cl₂ ↔ NO₂Cl+Cl^- with the NO₂Cl undergoing two parallel subsequent reactions, one a unimolecular decomposition and the other an attack by HNO₂ upon NO₂Cl.     Oxidation of nitrite by the three halogens, Cl₂, Br₂, I₂, is discussed.