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 HClO 4 ) it is pseudo-first-order with respect to [ClO 2 ]. Under these conditions, the rate law is −d[ClO 2 ]/d t = (2 k 1 + 2 k 2 [H + ])[ClO 2 ][SCN - ], with 2 k 1 = 0.42 M -1 s -1 and 2 k 2 = 0.52 M -2 s -1 at 25 °C and μ = 1 M (LiClO 4 ). Effective activation parameters in 1 M HClO 4 are Δ H ⧧ = 66.5 kJ/mol and Δ S ⧧ = −26.3 J/mol for the temperature range 13.4−52.2 °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.