Kinetics and mechanism of the reduction of monochloramine by hydroxylamine and hydroxylammonium ion

✍ Scribed by David M. Robinson; Todd J. Hoppe; Timothy J. Paslay; Gordon H. Purser

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: English
• Weight: 235 KB
• Volume: 38
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.20141

✦ Synopsis

Abstract

The kinetics and mechanism by which monochloramine is reduced by hydroxylamine in aqueous solution over the pH range of 5–8 are reported. The reaction proceeds via two different mechanisms depending upon whether the hydroxylamine is protonated or unprotonated. When the hydroxylamine is protonated, the reaction stoichiometry is 1:1. The reaction stoichiometry becomes 3:1 (hydroxylamine:monochloramine) when the hydroxylamine is unprotonated. The principle products under both conditions are Cl^–^, NH^+^~4~, and N~2~O. The rate law is given by −[d[NH~2~Cl]/d__t__] = k~+~[NH~3~OH^+^][NH~2~Cl] + k~0~[NH~2~OH][NH~2~Cl]. At an ionic strength of 1.2 M, at 25°C, and under pseudo‐first‐order conditions, k~+~= (1.03 ± 0.06) ×10^3^ L · mol^−1^ · s^−1^ and k~0~=91 ± 15 L · mol^−1^ · s^−1^. Isotopic studies demonstrate that both nitrogen atoms in the N~2~O come from the NH~2~OH/NH~3~OH^+^. Activation parameters for the reaction determined at pH 5.1 and 8.0 at an ionic strength of 1.2 M were found to be Δ__H__‡ = 36 ± 3 kJ · mol^–1^ and Δ S‡ = −66 ± 9 J · K^−1^ · mol^−1^, and Δ H‡ = 12 ± 2 kJ · mol^−1^ and Δ S‡ = −168 ± 6 J · K^−1^ · mol^−1^, respectively, and confirm that the transition states are significantly different for the two reaction pathways. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 124–135, 2006