Characteristics of aqueous bromine and hypobromous acid and the kinetics and mechanism of the reaction of bromine and hydroxylamine

Richard Clare Beckwith, Purdue University

Abstract

Hypobromous acid is formed in wastewater systems from chlorine or ozone disinfection of water containing bromide ion. Hypobromous acid disproportionates to form bromate ion, a known carcinogen. Analysis by UV spectrophotometry reveals that HOBr(aq) undergoes slow disproportionation by a second-order reaction ($k\sb{\rm 1a}$ = 2 x 10$\sp{-3}$ M$\sp{-1}$ s$\sp{-1}$, 25$\sp\circ$C, pH 3-8). This process is further suppressed below pH 2. Decomposition of HOBr is strongly assisted by carbonate and phosphate buffer, and weakly assisted by bromide ion. The much slower parallel reaction of OBr$\sp{-}$ + OBr$\sp{-}$ becomes important above pH 10. Equilibrium constants for bromine hydrolysis, $K\sb1$ = (HOBr) (H$\sp+$) (Br$\sp-$) / (Br$\sb2$(aq)), are determined as a function of ionic strength ($\mu$) at 25.0$\sp\circ$C and as a function of temperature at $\mu\ \simeq$ 0 M. At $\mu$ = 0.50 M and 25.0$\sp\circ$C, $K\sb1$ = (6.1 $\pm$ 0.1) $\times$ 10$\sp{-9}$ M$\sp2$, and the rate constant for the reverse reaction of HOBr + H$\sp+$ + Br$\sp-$ equals (1.6 $\pm$ 0.2) $\times$ 10$\sp{10}$ M$\sp{-2}$ s$\sp{-1}$. The overall stoichiometry for the reaction of bromine at pH 0-2 with excess hydroxylamine is 2NH$\sb2$OH + 2Br$\sb2\ \to$ 4Br$\sp-$ + 4H$\sp+$ + N$\sb2$O + 2H$\sb2$O. The proposed first step in bromine oxidation of NH$\sb2$OH is Br$\sp+$ transfer to form the intermediate species BrNHOH. A second equivalent of Br$\sb2$ reacts with BrNHOH to give HONO. On a slower time scale, NH$\sb2$OH subsequently reacts with HONO to account for the 2 Br$\sb2$: 2 NH$\sb2$OH stoichiometry. Stopped-flow spectrophotometry was utilized to obtain a second-order rate constant of 1.8 $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$ at $\mu$ = 0.50 M and 25.0$\sp\circ$C; a Br$\sb3\sp-$ path is 9 times slower than the Br$\sb2$ path. The activation parameters at 25.0$\sp\circ$C are $\Delta H\sp{\ddagger}$ = 15 kJ mol$\sp{-1}$ and $\Delta S\sp{\ddagger}$ = $-$15 J mol$\sp{-1}$ K$\sp{-1}$, but these values are temperature dependent because $\Delta C\sb{\rm p}\sp{\ddagger}$ = $-$440 J mol$\sp{-1}$ K$\sp{-1}$. At pH 5-6 the (NH$\sb2$OH) $\sb{\rm T}$ reaction with (Br$\sb2\rbrack\sb{\rm T}$ is much faster and is measured by pulsed-accelerated-flow spectrophotometry. The initial step appears to consume all the Br$\sb2$, but BrNHOH disproportionates to form NH$\sb2$OH and Br$\sb2$NOH; further stepwise hydrolysis gives NO$\sb2\sp-$ and Br$\sp-$.

Degree

Ph.D.

Advisors

Margerum, Purdue University.

Subject Area

Chemistry|Analytical chemistry|Chemistry

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