Revisiting the Decay of Monochloramine
An aqueous solution containing predominantly monochloramine can be very complex chemically, as once it is formed, monochloramine can undergo a number of reactions. The loss of monochloramine primarily occurs through monochloramine hydrolysis or through a disproportionation reaction. However, there are only a few research papers that have investigated the kinetics of these two reactions. Some of the findings of these previous studies show inconsistencies with respect to the rate constant values of the reactions. Hence, both of these reactions were studied in depth in this work. In the case of the disproportionation reaction, previous studies have indicated that the presence of a buffer enhances the reaction rate, thought to occur through a general acid catalysed pathway. In this study, the assumption that the reaction proceeds through a general acid catalysed mechanism was re-evaluated, as it can be shown that this mechanism, in some cases, is mathematically identical to a specific acid (H+), general base catalysed mechanism. To test these mechanisms, the rate of the reaction was measured in different buffer systems (carbonate, acetate, phosphate) over a range of pH values from 3 to 5. The change of monochloramine concentration was monitored by measuring the change in light absorbance at wavelengths in the ultraviolet region where monochloramine and dichloramine have absorbance maxima. Whereas the presence of phosphate and acetate buffers increased the reaction rate, there was no rate enhancement in the presence of carbonate buffer. The results of the study showed that the reaction rate is dependent on the total concentration of the buffer, rather than the concentration of either the acid or base form alone. The results also suggest that the reaction may proceed between protonated and neutral monochloramine molecules, consistent with the specific acid catalysed mechanism. The buffer likely catalyses the reaction either by participating in the initial protonation process of monochloramine, or by assisting in the abstraction of a proton, allowing for the transfer of the chlorine atom to the monochloramine from which the proton is abstracted, forming dichloramine. In the case of the hydrolysis reaction, no previous study has been performed in which this reaction has been satisfactorily isolated from all other interfering reactions. To this end, a suitable hypochlorous acid (HOCl) scavenger was identified that allowed the hydrolysis reaction to be isolated from all competing reactions. Indeed, cyanide (the scavenger) reactions with (HOCl) at a rate much faster than HOCl reacts with ammonia (NH3). Hence, in a series of experiments, cyanide was added to monochloramine solutions to quench HOCl, formed during the hydrolysis reaction, preventing the rapid reverse reaction of monochloramine reformation from occurring. The result of this study showed that without interference from the back reaction, monochloramine decayed quite rapidly with a first order rate constant of k obs = 2.156 × 10-5 s-1 at a room temperature. A significant increase in the reaction rate was observed when the experiments were conducted at higher temperature. In addition to investigating these two reactions, a critical review of the experimental methods used to measure chlorine in water is provided.
Jafvert, Purdue University.
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