Large Amplitude pH Oscillations in the Hydrogen Peroxide−Dithionite Reaction in a Flow Reactor

Abstract

The kinetics of the reaction between S2O4 2- and H2O2 is found to be very complex both in a closed reactor and in a continuous-flow stirred tank reactor (CSTR) in an unbuffered aqueous solution. The main products of the oxidation are SO4 2- and S2O6 2- in excess H2O2. The reaction is accompanied by acidification of the initially alkaline mixture. The measured pH-time traces show multiple inflection points in a closed reactor, suggesting that the reaction takes place in several distinct steps. Transient formation of SO 3 2- /HSO3 - buffer system is observed in the early stage of the reaction. Next, the sulfite ions are oxidized further to sulfate ions in an autocatalytic reaction. Simultaneously a small amount of dithionate ions is also formed. The pH exhibits very large amplitude relaxation oscillations in a CSTR in a narrow range of input concentrations, flow rate, and temperature. A simple empirical rate law model consisting of three redox reactions and three protonation equilibria is proposed. Numerical simulations based on this model agree well with the experimental results. The key to the pH-regulated oscillation is the H + -autocatalysis that results from the more rapid oxidation of the protonated than of the unprotonated sulfite intermediate.