Model Calculations and Experimental Studies as a Route Toward Simplification of the Kinetic Mechanism of the H₂O₂–NaSCN–NaOH–CuSO₄ Homogeneous Oscillator

Abstract

Oscillatory chemical reactions are usually characterized by complicated kinetic mechanisms in which the source of instability is combined with the parallel dissipative process. One of such systems is the H2O2–NaSCN–NaOH–CuSO4 homogeneous oscillator, in which dynamic instabilities are associated with the irreversible oxidation of thiocyanate ions with hydrogen peroxide. Following our previous studies on this system, we now propose further intuitive and substantial simplification of its kinetic mechanism toward the scheme involving only five intermediates. Its compatibility with our previous, nine‐variable model is verified in terms of model calculations, compared with experimental potentiometric and spectrophotometric data. In particular, supercritical nature of a Hopf bifurcation as a route toward oscillations born out of a single steady state upon increasing catalyst (copper species) concentration was observed in the model and an analogous type of bifurcation is suggested by available experimental data. Our work is a step toward final reduction of the mechanism of the studied system to the “minimum oscillator,” the concept used earlier, e.g., for the Belousov–Zhabotinsky reaction, of however remarkably different kinetic mechanism.

Chemical Oscillations With Sodium Perborate as Oxidant

“…Adding ligand which forms complex with copper ions influences this catalytic cycle. EDTA prevents to bring about oscillations in a mixture of oscillatory composition due to its (Wiśniewski et al, 2015).…”

“…The extension of this model with the assumed reactions of the perborate species would result in a poorly defined mechanism, since we have no direct kinetic data about these reactions. To obtain information about the effect of the replacement of H 2 O 2 by perborate, we decided to use a recently published simple five variables core model (Table 3), which was deduced from the original one by Wiśniewski et al (2015). In this model the concentration of the oxidant, that is H 2 O 2 , is a constant parameter.…”

The Model of “Minimal Oscillator” Derived from the Kinetic Mechanism of the H2O2-NaSCN-NaOH-CuSO4Dynamical System

“…As in the case of our previously studied five-variable model [9], all diagonal elements are negative, indicating direct autoinhibitive dynamics of all intermediates, so the autocatalysis is again of indirect type, which requires the products a ij a ji > 0 [16]. In comparison to the five-variable model, when the elements a 12 and a 21 were positive, the elimination of two intermediates modifies the indirect then autocatalysis into its "competitive" variant, indicated by the negative signs of the elements a 12 and a 21 .…”

“…The original mechanism of Luo et al [3] engaged 26 dynamical variables, from which we extracted simplified nine- [5] and five-variable [9] schemes as the models of the oscillations. In particular, the nine-variable model, being a reasonable compromise between its relative simplicity and complexity of a real system, allowed us to reproduce the untypical potentiometric oscillatory response, exhibiting a change both in its amplitude and phase [4,5], as well as to explain the thermokinetic origin of luminescent phase waves in the system [8].…”

“…Another complex oscillatory and bistable system, discovered and studied by M. Orbán et al as well as later in our group , involved the oxidation of thiocyanate ions with hydrogen peroxide in an alkaline medium (pH ≥ 9), in the presence of CuSO 4 as an initial catalytic species. The original mechanism of Luo et al engaged 26 dynamical variables, from which we extracted simplified nine– and five–variable schemes as the models of the oscillations. In particular, the nine‐variable model, being a reasonable compromise between its relative simplicity and complexity of a real system, allowed us to reproduce the untypical potentiometric oscillatory response, exhibiting a change both in its amplitude and phase , as well as to explain the thermokinetic origin of luminescent phase waves in the system .…”