Chemical Waves and Periodic Precipitation Processes

Abstract

A nonlinear chemical reaction with sufficiently complex reaction mechanism, maintained far from equilibrium, may show the phenomenon of chemical waves or fronts which are traveling concentration gradients 1,2,3,4 Chemical waves may occur in a variety of systems, including oscillatory reactions or reactions in a stable stationary state with so-called excitatory properties in which, upon a perturbation of appropriate direction and magnitude, the system responds with a large variation of concentrations prior to return to the stationary state. The subject of chemical waves includes single fronts, pulses and wave trains. Three types of waves have been discussed in some detail. in kinematic waves5 an apparent wave travels through the system due to an initial gradient in period or phase; hence these waves in the literature are also sometimes called phase waves. Kinematic waves involve only reaction but not diffusion. Relaxation oscillation waves6,7,8 occur in both oscillatory and excititory systems with multiple time scales, systems in which variations of concentrations occur on both a long and short time scale. Hence in relaxation-oscillation systems there occur sharp variations in concentration and there exist steep gradients. Phase waves9 occur in systems with shallow gradients and are distinguished from relaxation-oscillation waves (also called trigger waves) by the shape of the wave. the steepness of the gradient. the velocity and the range of constant velocity10. in both trigger waves and phase waves, diffusion and reaction both play an important role.