Determination of Monochloramine Formation Rate Constants with Stopped-flow Spectrophotometry

Authors

Z Qiang
Craig D. Adams, Utah State UniversityFollow

Document Type

Article

Journal/Book Title/Conference

Environmental Science and Technology

Volume

38

Publication Date

2004

First Page

1435

Last Page

1444

Abstract

The production of monochloramine by the reaction of aqueous ammonia and free chlorine is important in both drinking water and wastewater treatment systems. Accurate prediction of the rate of monochloramine formation is a prerequisite for any modeling work related to this fundamental reaction. There are significant discrepancies between rate constants reported in the literature. Furthermore, little information is available on the temperature dependence of the reaction rate constant. The purposes of this study were to kinetically examine the potential reaction pathways, accurately determine the specific rate constants, and establish the Arrhenius equation for the reaction of monochloramine formation using the stopped-flow technique. Results indicate that the rate constants are highly pH dependent due to the speciation of both free chlorine and ammonia. From a strictly kinetic point of view, monochloramine formation could be explained by either the nonionic pathway between HOCl and NH₃ or the ionic pathway between OCl^- and NH₄⁺. However, because the ionic pathway is mechanistically implausible the reaction is shown to be between the nonionic species (HOCl and NH₃). The specific rate constant for the nonionic pathway at 25 °C was determined to be 3.07 × 10⁶ (M^-1·s^-1). The Arrhenius equation was obtained as kHOCl,NH₃ = 5.40 × 10⁹ exp(− 2237/T), which provided an activation energy of 18.6 kJ·mol^-1.

Recommended Citation

Qiang, Z., Adams, C. (2004) “Determination of Monochloramine Formation Rate Constants with Stopped-flow Spectrophotometry,” Environmental Science and Technology, 38, 1435-1444.