Chlorination of Ammonium Ion under Acidic Conditions

The kinetics and mechanism of the reaction between NH₄⁺ and hypochlorous acid was studied under acidic conditions (25 °C, I = 1.0 M NaClO₄). The rate-determining step is the second order formation of monochloramine (MCA): v = k₁c_NH4⁺c_HOCl. MCA is converted into trichloramine (TCA) via dichloramine (DCA) in fast consecutive chlorination steps. The reactions of MCA and DCA with hypochlorous acid are second order processes; the active forms of the chlorinating agent are HOCl and Cl₂O, respectively. The rates of both reactions increase by decreasing the pH. This implies that MCA and DCA are involved in fast acid-base equilibria and the protonated species have considerably higher reactivity compared to the corresponding base forms. In contrast to earlier assumptions, the chlorination steps are irreversible for all practical purposes. The evaporation of TCA from the reaction mixture may significantly contribute to its decay. The chemical decomposition of TCA is a slow process yielding N₂ and nitrate ion. The formation of NO₃^-, was monitored by ion chromatography. A coherent mechanism is proposed that postulates the rate determining hydrolysis of TCA into dichloro-hydroxylamine (Cl₂NOH), which is followed by the elimination of HCl and HOCl in subsequent reaction steps.