The Formation and Spatiotemporal Progress of the pH Wave Induced by the Temperature Gradient in the Thin-Layer H2O2-Na2S2O3-H2SO4-CuSO4 Dynamical System.

The H2O2-S2O3(2-)-H(+)-Cu(2+) dynamical system exhibits sustained oscillations under flow conditions but reveals only a single initial peak of the indicator electrode potential and pH variation under batch isothermal conditions. Thus, in the latter case, there is no possibility of the coupling of the oscillations and diffusion which could lead to formation of sustained spatiotemporal patterns in this process. However, in the inhomogeneous temperature field, due to dependence of the local reaction kinetics on temperature, spatial inhomogeneities of pH distribution can develop which, in the presence of an appropriate indicator, thymol blue, manifest themselves as the color front traveling along the quasi-one-dimensional reactor. In this work, we describe the experimental conditions under which the above-mentioned phenomena can be observed and present their numerical model based on thermokinetic coupling and spatial coordinate introduced to earlier isothermal homogeneous kinetic mechanism.

Thermokinetic origin of luminescent traveling fronts in the H2O2-NaOH-SCN(-)-Cu2+ homogeneous oscillator: experiments and model.

According to our original discovery, the oscillatory course of the Cu(2+)-catalyzed oxidation of thiocyanate ions with hydrogen peroxide, in nonstirred medium and upon the addition of luminol as an indicator, can be a source of a novel type of dissipative patterns--luminescent traveling waves. The formation of these fronts, contrary to the patterns associated with the Belousov-Zhabotinsky reaction, cannot be explained in terms of coupled homogeneous kinetics and diffusion, and under isothermal conditions. Both experimental studies and numerical simulations of the kinetic mechanism suggest that the spatial progress of these waves requires mainly the temperature gradient in the solution, which affects the local chemical reaction rate (and thus the oscillation period), with practically negligible contribution from diffusion of reagents. As a consequence of this thermokinetic coupling, the observed traveling patterns are thus essentially the phase (or kinematic) waves, formed due to the spatial phase shift of the oscillations caused by differences in chemical reaction rates. The temperature gradient, caused by the significant heat effect of exothermic oxidation of thiocyanate by hydrogen peroxide, can emerge spontaneously as a local fluctuation or can be forced externally, if the control of progress of the luminescent waves is to be achieved.

Monitoring of spatiotemporal patterns in the oscillatory chemical reactions with the infrared camera: experiments and model interpretation.

An infrared camera was used for the first time to monitor the progress of traveling fronts in oscillatory chemical reactions, taking the Belousov-Zhabotinsky (BZ) reaction as the test system. The experiments involved comparative visual imaging and infrared thermography measurements for the thin-layer of the BZ solution in the Petri dish, including both aqueous and gel media, the latter one hindering the convection. Infrared thermography experiments that supply information on the temperature distribution at the solution surface were compared with the bulk temperature variations of the stirred solution with BZ reaction, measured simultaneously with the oscillatory variations of the Pt electrode potential. The experimentally observed correlation between the ferroin catalyst concentration and the temperature distribution was compared with the results of numerical modeling of these distributions in 2-D reactor space, based on the classical Oregonator. Analogous experiments were performed for the oscillatory oxidation of thiocyanates with hydrogen peroxide, catalyzed with Cu(2+) ions, in search of factors causing the development of traveling fronts, previously reported. The inhomogeneous distribution of the free surface temperature that could contribute to surface instabilities was found. Also, periodical increase and decrease in temperature of solution surface was reported. This was interpreted as periodically predominating cooling of the surface in contact with the surroundings because for the bulk, thermally isolated stirred solution, the temperature monotonically increases. In terms of our nine-variable kinetic model of this system, it was possible to identify the reaction steps responsible for the experimentally observed temperature dynamics and ascribe the appropriate heat effects to them. Our results constitute the first contribution to the thermochemical characteristics of the H(2)O(2)-SCN(-)-OH(-)-Cu(2+) oscillator.

Kinetic model of the H(2)O(2)-SCN(-)-OH(-)-Cu(2+) oscillator and its application to the interpretation of the potentiometric responses of various inert electrodes monitoring the reaction course.

The high complexity of the kinetic mechanism of the H(2)O(2)-SCN(-)-OH(-)-Cu(2+) oscillator, involving numerous intermediates, causes the oscillations monitored potentiometrically with gold or glassy carbon electrodes to exhibit opposite phases compared with the oscillations recorded with palladium or platinum electrodes. Following our previous work on the outline explanation of these phenomena, involving the concept of the mixed potential, in this paper, we present their more detailed and advanced study. For that purpose, we built up a simplified but realistic kinetic model of the studied oscillator, involving nine intermediates. Of those nine species, Cu(OH)(3)(-), Cu(OH)(2)(-), and HO(2)(*) were found to be crucial for the explanation of the potentiometric responses of various electrodes, under an additional assumption that the interfacial exchange current density of the HO(2)(*)/HO(2)(-) couple increases in the series GC < Au < Pt. Calculated oscillatory variations of the mixed potential for various model electrodes, compared with experimental results, allowed us to conclude that the potentiometric oscillations are caused largely by the oscillations of the [Cu(OH)(3)(-)]/[Cu(OH)(2)(-)] concentration ratio, irrespective of the electrode material used as a potentiometric sensor. For the Au electrode, the increase of the potential within every oscillatory peak largely reflects the increase in the [Cu(OH)(3)(-)]/[Cu(OH)(2)(-)] ratio. The simultaneous shift of the relatively high Pt electrode potential toward more negative equilibrium potential of the Cu(OH)(3)(-)/Cu(OH)(2)(-) couple is caused by the increase of the exchange current density of the latter couple. Thus, even the opposite phases of the potentiometric oscillations are explainable in terms of the oscillatory behavior of the same redox couple. Understanding of such phenomena is crucial for the proper interpretation of potentiometric data in complex chemical systems.

Luminescent chemical waves in the Cu(II)-catalyzed oscillatory oxidation of SCN- ions with hydrogen peroxide.

The oscillatory oxidation of thiocyanate ions with hydrogen peroxide, catalyzed by Cu2+ ions in alkaline media, was so far observed as occurring simultaneously in the entire space of the batch or flow reactor. We performed this reaction for the first time in the thin-layer reactor and observed the spatiotemporal course of the above process, in the presence of luminol as the chemiluminescent indicator. A series of luminescent patterns periodically starting from the random reaction center and spreading throughout the entire solution layer was reported. For a batch-stirred system, the bursts of luminescence were found to correlate with the steep decreases of the oscillating Pt electrode potential. These novel results open possibilities for further experimental and theoretical investigations of those spatiotemporal patterns, including studies of the mechanism of this chemically complex process.