Pattern Selection in Three-Precipitate Liesegang Systems.

Liesegang patterns present a display of parallel stripes of precipitate that arise from the interdiffusion of coprecipitate ions in a gel medium. The bands observed in rocks are typical analogies of this phenomenon, and their composition is not restricted to the banded deposition of a single mineral. We here extend the study to a three-precipitate system, wherein Co, Ni, and Cd cations are precipitated by the same anion (OH from NHOH) to form Co(OH), Ni(OH), and Cd(OH), respectively. The resulting pattern exhibits an alternation of compact mixed Co(OH) and Ni(OH) bands with granules of pure Cd(OH) between them. The obtained pattern confirms the generic type of ( + )//( + )//( + )/ alternation conjectured analytically using the competitive particle growth (CPG) model and stability analysis.