In this study we explored nonlinear spatial–temporal behavior in the oxidation of sulfite by hydrogen peroxide,in which thiosulfate was added to implement a negative feedback,which consequently led to a transition from propagating fronts to target patterns.Our investigation in the homogeneous system showed that increasing the concentration of hydrogen peroxide or decreasing the concentration of sulfite accelerated both the positive and negative feedbacks and therefore could facilitate the transition from front waves to target patterns.On the other hand,increasing the initial concentration of hydrogen ions or decreasing the concentration of thiosulfate mainly accelerated the autocatalytic production of hydrogen ions.The experimental observations were qualitatively reproduced with a model developed recently by Rabai and Hanazaki.In reaction–diffusion media,spontaneous formation of propagating pH fronts was obtained within a narrow range of sulfite and thiosulfate concentrations.The velocity of these traveling fronts was found to decrease with respect to increasing concentrations of thiosulfate and sulfite or decreasing concentrations of hydrogen ion and hydrogen peroxide.

The concentration of hydrogen ions is found to evolve periodically in the non-buffered chlorite-thiourea reaction.Perioddoubling,homoclinic and quasiperiodic bifurcations have been uncovered in this study.Experiments further illustrate that under the reaction conditions employed in this investigation reducing the reaction temperature favors the occurrence of complex pHo scillations.These experimental results provide new insight into the underlying mechanism of the chlorite–thiourea reaction,in particular regarding the significance of the nonlinear oxidations of sulfur(-II) compounds.

Pattern formation in glycolysis is studied with a classical reaction-diffusion allosteric enzyme model.It is found that,similar to recent experimental reports in the yeast extracts,a small magnitude local perturbation can induce transient target waves in a two dimensional oscillatory medium. An above threshold stimulation generates target waves which eventually evolve into spatiotemporal chaos upon collisions with the boundary or other wave activities.Detailed simulation studies show that the studied simple glycolytic reaction-diffusion model can support three types of spatiotemporal behaviors which are independent of the boundary conditions:(1) a spatially uniform stable steady state,(2) periodic global oscillations and (3) spatiotemporal chaos.

An approach leading to the development of spiral turbulence is reported here in an oscillatory reactiondiffusion medium,which is through the spontaneous formation of targetlike waves near the core of a spiral wave.The newly formed target wave emerges with its own characteristic frequency and propagates on top of the original spiral wave,which eventually leads to the breakup of the spiral at a location far from the spiral center.The radius of the surviving spiral segment decreases rapidly with the bifurcation control parameter.Calculation of power spectra suggests that the meandering of the spiral tip is responsible for the onset of the superimposed target and the phase desynchronization of the superimposed target waves.

In this report the decomposition and oxidation kinetics of thiourea dioxide and thiourea trioxide are investigated with a reversed-phase ion-pair high-performance liquid chromatography (HPLC) method.The HPLC method allows us to simultaneously determine and quantify several sulfur-containing reagents such as (NH2)2CSO2,(NH2)2CSO3,(NH2)2CO,and SO32- (HSO3－). Experiments illustrate that the decomposition of thiourea oxides is a first-order reaction,in which the rate constants increase with the pH of the solution.Oxidations of thiourea oxides by hydrogen peroxide are first order with respect to both reagents within the studied pH range between 4.0 and 8.0.Oxidation rate constants are measured under different pH conditions,which show that increasing the pH of the reaction solution significantly accelerates the oxidation process

Thiourea has exhibited extremely rich dynamical behavior when being oxidized either through a chemical approach or via an electrochemical method.In this study,thermodynamic properties of thiourea and its oxides are investigated by measuring their thermogravimetry (TG),differential thermogravimetry (DTG),and differential scanning calorimetry (DSC) simultaneously.Online FT-IR measurements show that products of the thermal decomposition vary significantly with the reaction temperature.In addition to the determination of their apparent activation energy (E),preexponential factor (A),and entropy (△S≠),enthalpy (△H≠),and Gibbs energy (△G≠) of thermal decomposition,our investigation further illustrates that the decomposition kinetics of thiourea and thiourea oxides follows the Johnson-Mehl-Avrami Equation,f(α)=n(1-α)-[-ln(1-α)]1-1/n and G(α)=[-ln(1-α)]1/n with n equal to 2,3.43,and 3,respectively.

The ferroin-catalyzed Belousov-Zhabotinsky (BZ) reaction was studied in a batch reactor under anaerobic conditions and was found to evolve through two separated regimes of complex oscillations.Significantly,the two bifurcation regimes exhibited qualitatively different dependence on compositions of the reaction mixture,i.e.,initial concentrations of bromate, sulfuric acid,malonic acid,and ferroin.The reaction temperature also showed opposite effects on the two bifurcation regimes,in which complexities of the first bifurcation regime were enhanced while oscillations in the second bifurcation regime became simpler as a result of decreasing temperature.Numerical simulations with a 12-variable model developed specifically for the ferroin-BZ system were able to reproduce transient complex oscillations observed in experiments.These calculations further illustrated that reactions such as ferroin and HOBr,ferroin and HBrO2,and ferriin and Br- were not essential in describing complex dynamics of the ferroin-BZ reaction.

The electrochemical oxidation of thiosulfate is revealed to have two distinct oscillatory regimes in both linear potential and galvanic voltammograms,where various nonlinear behaviors such as period-2,mixed-mode and quasi-periodic oscillations,and chaos are observed under potentiostatic or galvanostatic conditions.Electrochemical impedance spectroscopy and iR compensation characterization indicate that,depending on the operating conditions,the system could be either a strictly potentiostatic oscillator or an S-shaped negative differential resistance oscillator.Chronoamperometry measurements reveal that the first oscillatory process involves a single-electron transfer,whereas within the second oscillatory regime the average number of electrons transferred is around 3.8.Measurements with capillary electrophoresis and chemical methods illustrate that the oxidation products include S2O62-,S4O62-,S5O62-,S3O62-,and SO42-.

The electro-oxidation of sulfide on a Pt electrode is found to exhibit both N-NDR (negative differential resistance) and HN-NDR (hidden negative differential resistance) types of oscillations,making it the first system known to be capable of supporting both types of nonlinear instabilities without changing electrolyte compositions.Six distinct oscillation windows are observed when the external current is adjusted as the control parameter.In addition,temperature also exhibits subtle influences on the observed reaction behavior,where varying the temperature from 30.0℃ results in more complicated oscillatory phenomena.

The electrochemical oxidation of thiourea on a Pt electrode was found to exhibit both simple and period-doubled oscillations.Measurements of the potentiostatic I/