Monochloramine (NH2Cl) is a widely used drinking water disinfectant.  When introduced into drinking water distribution systems, it degrades slowly through a series of reactions. Combined chlorine residuals (NH2Cl + NHCl2) decay mostly through reactions involving dichloramine (NHCl2), and two processes are responsible for forming dichloramine from monochloramine.  The first is the monochloramine disproportionation reaction (two NH2Cl forming NHCl2 + NH3), and the second process is initiated by monochloramine hydrolysis, forming HOCl and ammonia (NH3).  Once formed, HOCl can react with another NH2Cl to form dichloramine (NHCl2), or recombine with NH3.  To understand these reactions quantitatively, the rate constant for NH2Clhydrolysis, therefore, must be known; yet it has never been measured directly.  In this research, we have directly measured the rate of NH2Cl hydrolysis by eliminating the �back reaction� between HOCl and NH3 by adding sodium cyanide (NaCN) to the system, as CN- reacts much more rapidly, than NH3, with HOCl.  A kinetic model that calculates the changes in concentration of 10 species over time (including H+) and the decay of NH2Cl in the presence of CN- was formulated. Experimental data were collected at different pH values (11-12) and initial NH2Cl concentrations (0.5-1.5 mg/L at Cl2).  By comparing the experimental data with the model, the rate constant for NH2Cl hydrolysis was determined to be 2.1�10-5 s-1 at 25�C, which is in good agreement with reported values obtained by measuring the (forward) rate of reaction between HOCl and NH3, and the equilibrium constant.

Rate constant for monochloramine hydrolysis (NH<sub>2</sub>Cl)

248th National Meeting (2014)

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Rate constant for monochloramine hydrolysis (NH2Cl)

Rate constant for monochloramine hydrolysis (NH₂Cl)