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Ind. Eng. Chem. Res. 2009, 48, 6280–6286
The Kinetics and Stoichiometry of the Reaction between Hypochlorous Acid and Chlorous Acid in Mildly Acidic Solutions Ville Tarvo,*,† Tuula Lehtimaa,‡ Susanna Kuitunen,† Ville Alopaeus,† Tapani Vuorinen,‡ and Juhani Aittamaa† TKK Helsinki UniVersity of Technology, Department of Biotechnology and Chemical Technology and Department of Forest Products Technology, P.O. Box 6100, FI-02015 TKK, Finland
The kinetics and stoichiometry of the reaction between hypochlorous acid and chlorous acid was studied under mildly acidic conditions (pH ) 2.5-4.7). The experiments were conducted in dilute solutions (10-4-10-3 M) at T ) 5-25 °C. External buffer compounds were not used. Chlorous acid, chlorine dioxide, and chlorate concentrations were monitored with titrimetric methods. Rate coefficients and activation energies were determined for the five main steps of the kinetic model (others were adopted from literature). Both hypochlorous acid and chlorine were found to react at an appreciable rate with chlorous acid/chlorite. Chloride ion was found to promote both the overall reaction rate and chlorine dioxide/chlorate ratio. The results may be exploited in optimizing the red-ox efficiency of technical chlorine dioxide bleaching processes. Introduction The chemistry of aqueous oxychlorine species is encountered in technical processes, where chlorine dioxide (ClO2) is either consumed or produced. Chlorine dioxide is applied for instance in industrial water disinfection,1,2 pulp bleaching,3-5 and textile bleaching processes. In pulp bleaching chlorine dioxide is used to chemically modify (oxidize) residual lignin so that pulp brightness is increased. Chlorine dioxide is consumed primarily in one and three electron red-ox reactions that produce chlorite (ClO2-) and hypochlorous acid (HOCl), respectively.6-10 Both chlorite and hypochlorous acid react further in a complex reaction system, which involves various oxy-chlorine and organic species.9 Some of the reactions regenerate chlorine dioxide, while some produce chlorate (ClO3-). Chlorine dioxide regeneration is highly desirable, whereas chlorate formation is not. Chlorate ion possesses a high apparent oxidation power (valence V+); yet it is unable to facilitate bleaching reactions.11,12 Chlorate is also highly toxic (acute toxicity
