Competing elementary reactions in a capillary - Two reaction fronts moving in opposite directions

The behavior of two reaction fronts created in a system of two competing reactions has been investigated using the reaction between xylenol orange (XO) and chromium(HI) ion in aqueous solution. Two reaction fronts have been shown to move in opposite directions, which is explained in terms of relative concentrations among three reactant species. This explanation was verified by the observation of only one reaction front for selected different relative concentrations, where both reaction fronts move in unison. As predicted by theory and confirmed by computer simulation, the position (x_j) of each reaction front scales as t^1/2, irrespective of the direction of motion.