Electrochemical and Chemical Mechanistic Aspects of Temporal and Spatiotemporal Instabilities in the Hydrogen Peroxide – Thiocyanate – Copper(II) Homogeneous Oscillator

The H₂O₂ - SCN^-- OH^- - Cu²⁺ system [1-2] belongs to the most complex and interesting chemical oscillators, due to rather untypical lack of connection between the oscillatory and bistable behaviors. Our systematic studies have revealed further specific dynamic properties of this system which are caused by its complicated kinetic mechanism. Our new observations comprise the following phenomena. First, significantly different shapes and phases of oscillatory potentiometric responses were reported for various inert electrodes monitoring reaction course (including even ca. 180^o phase shift between the Au and Pt electrodes) [3]. Second, although the occurrence of periodic bursts of luminescence in this system upon addition of luminol was known before, we discovered the formation of spatiotemporal patterns in this process, emerging as travelling fronts of luminescence [4]. In order to explain the above phenomena, with special emphasis on the mechanism of  generation of spatiotemporal patterns which is by no means trivial, we performed appropriate theoretical (numerical) and experimental studies. For that purpose we simplified the original complicated kinetic reaction mechanism to the 9-variable core more suitable for such analysis [5]. Based on this mechanism we explained the anomalous potentiometric responses in terms of the mixed potential of the inert electrode, identifiying the crucial redox couples as Cu(OH)₃^-/Cu(OH)₂^- and HO₂^·/HO₂^- as involved in unique, sophisticated mechanism [5]. In order to explain spatiotemporal phenomena, we analyzed thermochemical characteristics of the system, involving monitoring both the bulk and surface temperature variations, in the latter case using, presumably for the first time, the infrared camera for detection of surface temperature gradients [6]. In conclusion we developed a novel model for the wave generation and propagation in the H₂O₂-SCN^--OH^--Cu²⁺ system, showing the essential role of the non-uniform temperature distribution in the solution. In other words, we suggest the thermokinetic coupling between the oscillatory kinetics and spatial progress of the warmed zone of the solutions [7] as a source of wave phenomena. Luminescent patterns observed are thus the phase waves, caused by variation of the oscillatory kinetics along the reactor, due to the non-homogeneous temperature distribution. We consider this phenomenon a rather unique example of instabilities of thermokinetic origin developing in aqueous media.

Concluding, the H₂O₂-SCN^--OH^--Cu²⁺ oscillator is a unique, very complex system, the dynamics of which can be understood only if appropriate concepts of: (i)  homogeneous nonlinear chemical kinetics of redox processes, (ii) interfacial electrochemistry and (iii) physical processes of heat conduction are combined in the mechanisms underlying the observed phenomena.

References:

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[2] Y. Luo, M. Orbán, K. Kustin, I. R. Epstein, J. Am. Chem. Soc., 111 (1989) 4541

[3] K. Pekala, R. Jurczakowski, M. Orlik, J. Solid State Electrochem., 14 (2010) 27

[4] K. Pekala, R. Jurczakowski, A. Lewera, M. Orlik, J. Phys. Chem. A 111 (2007) 3439

[5] A. Wiśniewski, K. Pekala, M. Orlik,  J. Phys. Chem. A 114 (2010) 183

[6] K. Pekala, A. Wiśniewski, R. Jurczakowski, T. Wiśniewski, M. Wojdyga, M. Orlik, J. Phys. Chem. A, 114 (2010) 7903

[7] A. Wiśniewski, M. T. Gorzkowski, K. Pekala, M. Orlik, J. Phys. Chem. A, 117 (2013) 11155