Kinetics and mechanism of nitrite oxidation by hypochlorous acid in the aqueous phase

Abstract

The rate coefficient for the reaction of nitrite with hypochlorite and hypochlorous acid has been studied using spectrophotometric measurements. The reaction rate has been determined in a wide range of H+ concentration (5 ≤ - log [H+] ≤ 11). The kinetics were carried out as a function of NO2-, H+ and total hypochlorite ([HOCl]total = [HOCl] + [ClO-] + [ClNO2]) concentrations. The observed overall rate law is described by: d[HClO]T/dt = {aNO2-]2 + b[NO2-]}[H+]2/c + d[H+ + e[NO2-][H+2 At T = 298 K and in Na2SO4 at an ionic strength (I = 1.00 M), we obtained using a nonlinear fitting procedure: a = (1.83 ± 0.36) × 107 s-1, b = (1.14 ± 0.23) × 105 Ms-1, c = (1.12 ± 0.17) × 10-13 M, d = (1.43 ± 0.29) × 10-6 M2 and e = (1.41 ± 0.28) × 103 M where the errors represent 2σ. According to the overall rate law, a/b = k1/k3, b/e = k3, c = Kw, d/c = Ka, d = KaKw and e = KlKa. In Na2SO4 at an ionic strength (I = 1.00 M), the values of Kl and Ka are (1.1 ± 0.1) ± 10-4 and 1.28 × 107 M-1, respectively. A mechanism is proposed for the NO2 oxidation which involves the reversible initial step: NO2- + HOCl ⇋ ClNO2 + OH- (Kl), while ClNO2 undergoes the two parallel reactions: attack by NO2- (kl) and hydrolysis (k3). ClNO2 and N2O4 are proposed as important intermediates as they control the mechanism. The rate coefficients k1 and k3 have been determined at different ionic strengths in NaCl and Na2SO4. The influence of the ionic strength and ionic environment has been studied in this work. © 2003 Elsevier Science Ltd. All rights reserved.