Dynamical profiles of the reactive components in direct and revert Liesegang patterns

In periodic precipitation (Liesegang banding), the overwhelmingly common trend is the increase in spacing between consecutive bands as we move away from the interface. Revert spacing is a rare trend observed in a few systems, wherein the spacing decreases with distance from the interface. In this Letter, we compare the spatial profiles of the aqueous components in two chromate Liesegang precipitates: CuCrO4 and PbCrO4, displaying direct and revert spacing, respectively. The profiles are measured using UV–Vis spectrophotometry. The formation of a Pb2+–PbCrO4 complex in equilibrium with the free Pb2+, seems to play a key role in the revert spacing scenario.