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 co mpeting 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 xylen ol 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 th e competing species. Under the parameter values studied, which allowed clear spatiotemporal separation of the two competing reactions, we fi nd that the behavior of the reaction front at early times follows a pe rturbation theory developed for a simple elementary A + B --> C reacti on with initially separated reactants. The global reaction rate, obser ved over the entire time scale of the experiments, is highly non-monot onic. Overall, with no free parameters, our theoretical model is quant itatively consistent with the experimental observations of the spatiot emporal patterns, the unusual scaling laws, and the crossover behavior s. The geometrical constraints and nonclassical behavior of the reacti on rate allow a quantitative determination of the reaction probability of the chromium ion monomer relative to that of the higher order olig omers.