Determination of monochloramine formation rate constants with stopped-flow spectrophotometry.

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 degrees C was determined to be 3.07 x 10(6) (M(-1) x s(-1)). The Arrhenius equation was obtained as k(HOCl,NH3) = 5.40 x 10(9) exp(- 2237/T), which provided an activation energy of 18.6 kJ x mol(-1).