Kinetics and a revised mechanism for the autocatalytic oxidation of SCN- by ClO2

The aqueous reaction of chlorine dioxide with excess thiocyanate is known to be autocatalytic at pH 2. We find that under strongly acidic conditions (>0.1 M HClO4) it is pseudo-first-order with respect to [ClO2]. Under these conditions, the rate law is -d[ClO2]/dt = (2k(1) + 2k(2)[H+])[ClO2][SCN-], with 2k(1) = 0.42 M-1 s(-1) and 2k(2) = 0.52 M-2 s(-1) at 25 degrees C and mu = 1 M (LiClO4). Effective activation parameters in 1 M HClO4 are Delta H double dagger = 66.5 kJ/mol and Delta S double dagger = -26.3 J/mol for the temperature range 13.4-52.2 degrees C. Reaction products were identified by ion chromatography and ion-selective electrodes, which allowed a verification of the stoichiometry, 6ClO(2) + 5SCN(-) + 8H(2)O --> 6Cl(-) + 5CN(-) + 5SO(4)(2-) + 16H(+). Direct evidence for a reaction intermediate, thiocyanogen, was obtained through careful analysis of multiwavelength rapid scanning data. This intermediate provides a reactive autocatalytic species OSCN- via hydrolysis equilibrium, and this equilibrium was found to be important to simulations and fits to raw data. A 14-step mechanism is employed for these fits and simulations, allowing a more thorough description of the reaction system; this mechanism successfully reproduces both the full rate law under pseudo-first-order conditions and the autocatalytic behavior at pH 2.