Thiocyanogen as an intermediate in the oxidation of thiocyanate by hydrogen peroxide in acidic aqueous solution

The kinetics of the reaction of H2O2 with excess SCN- in acidic media was studied by use of Ti(IV) as an indicator for the concentration of H2O2. Pseudo-first-order behavior was realized by this method, and these data confirm the acid-catalyzed rate law and rate constant reported some 40 years ago for this reaction under conditions of excess H2O2. Under the same conditions except without Ti(IV), repetitive-scan spectra reveal the formation and decay of an intermediate that absorbs in the UV. In the proposed mechanism, HOSCN is produced in the first step and it is converted rapidly to (SCN)2 through its equilibrium reaction with SCN-. The observed intermediate is believed to be (SCN)2, which decays on a longer time scale. Excellent global fits of this mechanism to the repetitive-scan data are obtained with rate constants constrained by the Ti(IV) data and published previously in our study of the ClO2/SCN- reaction. These fits yield a spectrum for (SCN)2 that is characterized by lambda(max) = 297 nm and epsilon297 = 147 M(-1) cm(-1), in fine agreement with our prior report.