Reaction Dynamics and Chemical Pattern Formation in Capillary Tubes Resulting from the Competition Between Two Elementary Complex Formation Reactions

A system of competing elementary reactions is investigated experimentally usingthe reaction of xylenol orange with Cr3+ in aqueous solution. The two reagents areinitially separated in a long, thin capillary tube and meet in the center, forming a reactionfront (s). The geometry of the reactor and the initial separation of the reagents makes the system effectively one-dimensional. Aqueous Cr3+ solution has a very rich chemistryand provides two different chemical Cr3+ reactants which compete to react with xylenolorange. Rich spatio-temporal patterns are observed experimentally and are explained by a reaction-diffusion model. Results from exact enumeration simulations predict that when the concentrations of the competing species are very different and the microscopic rate constants of the competing species are such that the majority species reaction rate is much faster than the reaction rate of the minority species, the reaction front splits into two distinct regions. The spatio-temporal patterns generated by theory and experiment agree quantitatively. Also in agreement with the theory are the experimental early time and asymptotic time global rate behaviors, which exhibit multiple crossovers.