Radical chain mechanism for the S2O82−–S2O32−–Cu(ii) flow system explains high-amplitude pH oscillations in the NH4OH-modified version

TL;DR

A chemical system involving peroxodisulfate, thiosulfate, and copper shows large pH oscillations when ammonium hydroxide is added, which could be useful in chemical oscillators.

Contribution

A radical chain mechanism is proposed to explain high-amplitude pH oscillations in the NH4OH-modified flow system.

Findings

The proposed mechanism includes 13 reactions and radical ions that explain oscillatory behavior.

NH4OH amplifies pH oscillations without affecting redox processes.

The model successfully simulates both batch and flow reactor dynamics.

Abstract

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.

Autocatalytic reaction of sulfate radical ions explains the oscillatory phenomena observed in flow reactors.

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