Propagating fronts in periodic precipitation systems with redissolution

Abstract

When co-precipitate ions interdiffuse in a gel medium, the sparingly soluble salt may precipitate in a stratification of parallel bands, traditionally known as Liesegang bands. In some salt systems, the precipitate can redissolve in excess diffusing electrolyte due to complex formation. As a result, the whole pattern propagates via band formation due to precipitation and band disappearance due to redissolution driven by complex formation. The properties of such unusual migrating Liesegang patterns were studied in our laboratory with an emphasis on the Co(OH)₂ system. The latter is soluble in excess NH₄OH because of the formation of the Co(NH₃)₆²⁺ complex ion. We review the main experiments and results on that system and discuss potential directions for the extension of this research. In the Co(OH)₂ propagation, the number of bands exhibits chaotic oscillations with time. The variation of the velocity of propagation with the concentrations of both inner and outer electrolytes was investigated, and the correlation between dissolution and precipitation was found to approach linearity at long times. An electric field of variable strength was applied across the propagation medium and a profound effect on the pattern properties was demonstrated. The introduction of Ni²⁺ ions, which compete with Co²⁺ for complex formation, induced oscillations in the band locations (at a given time) with concentration of diffusing electrolyte. It was shown that the intermediate species NH₄⁺ is a precursor of such oscillations. The system was modeled using the theory of Polezhaev and Müller, incorporating ion diffusion, nucleation and kinetics of particle growth augmented by the dissolution process in a special kinetic scheme. The calculations agree qualitatively with the experimental results.