Spatiotemporal patterns and nonclassical kinetics of competing elementary reactions: Chromium complex formation with xylenol orange in a capillary

An experimental investigation of chemical reaction fronts, created by an initial separation of reactants, is reported for a system of two competing reactions. Spatiotemporal patterns are observed experimentally for the competing reaction front and are accounted for quantitatively by a reaction-diffusion model. We use the reaction of xylenol orange with Cr³⁺ in aqueous solution. Different oligomers of Cr³⁺ provide the two kinetically different species that react competitively with xylenol orange. The parameters that determine whether pattern formation is observable at the front are the ratios of (1) the microscopic reaction constants of the competing reactions and (2) the concentrations of the competing species. Under the parameter values studied, which allowed clear spatiotemporal separation of the two competing reactions, we find that the behavior of the reaction front at early times follows a perturbation theory developed for a simple elementary A + B → C reaction with initially separated reactants. The global reaction rate, observed over the entire time scale of the experiments, is highly non-monotonic. Overall, with no free parameters, our theoretical model is quantitatively consistent with the experimental observations of the spatiotemporal patterns, the unusual scaling laws, and the crossover behaviors. The geometrical constraints and nonclassical behavior of the reaction rate allow a quantitative determination of the reaction probability of the chromium ion monomer relative to that of the higher order oligomers.