Chlorine dioxide-induced and Congo red-inhibited Marangoni effect on the chlorite-trithionate reaction front.

Hydrodynamic flows can exert multiple effects on an exothermal autocatalytic reaction, such as buoyancy and the Marangoni convection, which can change the structure and velocity of chemical waves. Here we report that in the chlorite-trithionate reaction, the production and consumption of chlorine dioxide can induce and inhibit Marangoni flow, respectively, leading to different chemo-hydrodynamic patterns. The horizontal propagation of a reaction-diffusion-convection front was investigated with the upper surface open to the air. The Marangoni convection, induced by gaseous chlorine dioxide on the surface, produced from chlorite disproportionation after the proton autocatalysis, has the same effect as the heat convection. When the Marangoni effect is removed by the reaction of chlorine dioxide with the Congo red (CR) indicator, an oscillatory propagation of the front tip is observed under suitable conditions. Replacing CR with bromophenol blue (BPB) distinctly enhanced the floating, resulting in multiple vortexes, owing to the coexistence between BPB and chlorine dioxide. Using the incompressible Navier-Stokes equations coupled with reaction-diffusion and heat conduction equations, we numerically obtain various experimental scenarios of front instability for the exothermic autocatalytic reaction coupled with buoyancy-driven convection and Marangoni convection.

pH oscillations and mechanistic analysis in the hydrogen peroxide-sulfite-thiourea reaction system.

A new pH oscillator has been constructed by combining the pH clock reaction H2O2-SO3(2-)-H(+) with thiourea (Tu, (NH2)2CS) as a proton-consuming species. The system exhibited oligo-oscillatory behavior in a closed system, and large amplitude oscillations in a continuous-flow stirred tank reactor (CSTR) were observed in a narrow range of input concentrations, flow rate, and temperature. For the purpose of constructing the kinetic model, a reversed-phase ion-pair high-performance liquid chromatography (HPLC) and mass spectrometer (MS) were used to track and determine intermediate species during the oxidation of thiourea by hydrogen peroxide. Experimental results illustrated that the four species: thiourea monoxide (TuO), formamidine disulfide (Tu2(2+)), thiourea dioxide (TuO2), and thiourea trioxide (TuO3) were formed during the oxidation process. A ten-step mechanistic model was proposed, where TuO was another key species participating in two proton feedback loops in addition to bisulfite. Numerical simulations based on this model agreed well with the experimental results.

Pattern formation in the iodate-sulfite-thiosulfate reaction-diffusion system.

Sodium polyacrylate-induced pH pattern formation and starch-induced iodine pattern formation were investigated in the iodate-sulfite-thiosulfate (IST) reaction in a one-side fed disc gel reactor (OSFR). As binding agents of the autocatalyst of hydrogen ions or iodide ions, different content of sodium polyacrylate or starch has induced various types of pattern formation. We observed pH pulses, striped patterns, mixed spots and stripes, and hexagonal spots upon increasing the content of sodium polyacrylate and observed iodine pulses, branched patterns, and labyrinthine patterns upon increasing the starch content in the system. Coexistence of a pH front and an iodine front was also studied in a batch IST reaction-diffusion system. Both pH and iodine front instabilities were observed in the presence of sodium polyacrylate, i.e., cellular fronts and transient Turing structures resulting from the decrease in diffusion coefficients of activators. The mechanism of multiple feedback may explain the different patterns in the IST reaction-diffusion system.

Investigations of different chemiluminescent peaks in H2O2-SCN(-)-Cu(2+)-OH(-)-luminol flow system.

In the H2O2-SCN(-) -Cu(2+)-OH(-)-luminol oscillatory system of chemiluminescence, the effects of the ingredient concentrations, temperature, flow rate and complexing agent on the oscillatory dynamics were investigated in a continuous-flow stirred tank reactor (CSTR). The dynamical structure of two peaks during a period was discussed in detail. By addition of EDTA to the oscillating system, the peak I height decreased sharply while the peak II height was little affected, and the period kept constant. This may be due to the fast reaction between Cu(II) and EDTA and the highly stable complex Cu(II)-EDTA. From the experimental study and mechanism analysis, the chemiluminescent peak I corresponds to Cu(II) → Cu(I) transformation and the peak II corresponds to the Cu(I) → Cu(II) transformation process. The key species involving in the two-transformation process are inferred to be superoxide radical and hydroxyl radical.

Oscillations and mechanistic analysis of the chlorite-sulfide reaction in a continuous-flow stirred tank reactor.

Sustained oscillations in pH and redox potential are found in the chlorite-sulfide reaction in a continuous-flow stirred tank reactor (CSTR). Autocatalytic oxidation of HSO(3)(-) by ClO(2)(-) is the major source of positive feedback of hydrogen ions. The reaction between H(2)S and ClO(2)(-) to form S(8), which consumes H(+), is an important source of negative feedback. A model consisting of five protonation-deprotonation equilibria and nine redox reactions is proposed for the oscillatory reaction between S(2-) and ClO(2)(-). The 10 species included are HS(-), H(2)S, S(2)O(3)(2-), SO(3)(2-), HSO(3)(-), OCl(-), HOCl, ClO(2)(-), H(+), and OH(-). In contrast to the H(2)O(2)-S(2-) oscillatory reaction, S(2)O(3)(2-) is shown here by capillary electrophoresis to be an important intermediate. Simulations give qualitative agreement with the pH oscillatory behavior observed in the CSTR.