The determination of the kinetics and the products from the reactions between intermediate chlorine and bromine compounds in water treatment
Abstract
Multiwavelength spectrophotometric analysis of 32 wavelengths, over the range of 240-400 nm, has resolved the decay of HOCl from a multicomponent solution. It has been found that HOBr catalyzes the decomposition of HOCl under identical experimental conditions, where the decay of HOCl by itself is otherwise slow $\rm({\ll}10\sp{-5}\ s\sp{-1}).$ To a first approximation, the rate-determining step is the 1:1 reaction of HOCl with HOBr. However, HOBr is regenerated in a subsequent reaction between $\rm Br\sp-$ and HOCl. The rate of loss of HOCl is first-order in HOCl and HOBr, in the pH range of 5.49-7.57, with a rate constant of $\rm 2.44\times 10\sp{-1}M\sp{-1}s\sp{-1}$ at $\rm 25\sp\circ C$ in 0.32 M phosphate buffer $\rm(\mu = 1.0\ M\ NaClO\sb4).$ Product recoveries from the reaction between Br(I) and excess Cl(I) have shown that initially the relative yield of chlorate ion, Cl(V), predominates over bromate ion, Br(V), in the pH range of 5.49-7.57. Reaction mixtures ranging from 2:1 to 10:1 $\rm\lbrack Cl(I)\rbrack\sb{T}{:}\lbrack Br(I)\rbrack\sb{T}$ were studied and the final yields of $\rm\lbrack ClO\sb3\sp-\rbrack$ and $\rm\lbrack BrO\sb3\sp-\rbrack$ are one-third of the respective, hypohalous acids. The chlorate and bromate ion levels were determined from these solutions via capillary zone electrophoresis (CZE) after initial treatment of the samples with ethylenediamine, a reagent which removes the Br(I) and Cl(I) from solution. The reaction of $\rm ClO\sb2\sp-$ with HOBr is first-order in each reactant and is general-acid assisted (with a Bronsted-Pedersen $\alpha$ value of 0.59) in the presence of phosphate or carbonate buffers at $\rm p\lbrack H\sp+\rbrack$ 5.00-9.01. The products are $\rm ClO\sb2$ and $\rm ClO\sb3\sp-,$ where the relative yield depends on $\rm \lbrack ClO\sb2\sp-\rbrack/\lbrack OH\sp-\rbrack.$ The kinetic dependence indicates the presence of a steady-state species, $\rm HOBrOClO\sp-$ or $\rm HOBrClO\sb2\sp-,$ that generates a metastable intermediate of BrOClO or $\rm BrClO\sb2.$ This intermediate reacts rapidly via two competing pathways, where (1) $\rm ClO\sb2\sp-$ reacts to form $\rm 2ClO\sb2$ and $\rm Br\sp-$ or (2) $\rm H\sb2O/OH\sp-$ can react to form $\rm ClO\sb3\sp-$ and $\rm Br\sp-.$ Competition between these pathways determines the yield of $\rm ClO\sb2\sp-,$ but does not affect the rate of HOBr loss.
Degree
Ph.D.
Advisors
Margerum, Purdue University.
Subject Area
Analytical chemistry|Chemistry|Sanitation
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