Radical chain mechanism for the S₂O₈ ^2--S₂O₃ ^2--Cu(ii) flow system explains high-amplitude pH oscillations in the NH₄OH-modified version

Peroxodisulfate is well known as an important reagent in analytical, environmental and other branches of chemistry, as well as in industrial processes. One of the most studied oxidative reactions peroxodisulfate participates in as an oxidizer is the Cu(ii)-catalyzed peroxodisulfate-thiosulfate reaction. When carried out in a flow reactor, this system shows oscillatory dynamics characterized by periodic changes in the Pt-potential and [O₂] while it only displays variation in the pH with a few tenths of unit magnitude. Our recent experiments unveiled an increase of the amplitude of the pH oscillations that exceeds 4 units when NH₄OH was introduced into the oscillatory flow system. The dynamics of Cu(ii)-catalyzed peroxodisulfate-thiosulfate reaction has been described in detail but the chemical mechanism explaining the oscillatory behavior has not been established. Based on what is known about the uncatalyzed reaction between peroxodisulfate and thiosulfate in the literature, we have developed a mechanism that includes radical chain reactions which can explain the oscillatory phenomena. The proposed mechanism includes 13 reactions with the radical ions SO₄˙^-, S₂O₃˙^-, S₂O₈˙^-, OH˙ and two acid-base equilibria, including the dissociation equilibrium of NH₄OH accounting for its effect on the amplitude of pH oscillations. Using this model, we successfully simulated the behavior of this system: (1) the evolution of the concentrations of the initial reagents, radicals, and catalyst over time in batch configuration, (2) the periodic changes in the concentrations of radicals and the oxidized and reduced forms of the catalyst, pH and [O₂] in flow conditions. Our model also explains the amplification of the pH cycles without impacting the redox processes when NH₄OH is added, which is a novel phenomenon observed in nonlinear chemical reactions. The high amplitude pH oscillations we report in the peroxodisulfate-thiosulfate-Cu(ii)-NH₄OH flow reaction may enable future applications where this system may serve (a) as a core oscillator in coupled chemical systems, or (b) as a pH oscillator capable of running in a closed reactor configuration.