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 Co²⁺, Ni²⁺, and Cd²⁺ cations are precipitated by the same anion (OH^- from NH₄OH) to form Co(OH)₂, Ni(OH)₂, and Cd(OH)₂, respectively. The resulting pattern exhibits an alternation of compact mixed Co(OH)₂ and Ni(OH)₂ bands with granules of pure Cd(OH)₂ between them. The obtained pattern confirms the generic type of (<i>A</i> + <i>B</i>)/<i>C</i>/(<i>A</i> + <i>B</i>)/<i>C</i>/(<i>A</i> + <i>B</i>)/<i>C</i> alternation conjectured analytically using the competitive particle growth (CPG) model and stability analysis.