Band Propagation, Scaling Laws, and Phase Transition in a Precipitate System III: Effect of the Anions of Precursors

Abstract

In this third part of this work, we study the effect of the nature of the anion (nitrate, chloride, bromide, and sulfate) of the nickel salt precursor on the emerging Liesegang pattern in the nickel hydroxide/ammonia system in agar gel. We show that multiple Liesegang patterns, such as direct, irregular, revert, and pulse, can thereby result from such an effect. The microscopic analysis of the solids in the bands reveals that the different anions, present in the gel and previously thought of as spectator ions, interact chemically and variably with the crystalline solid in the leading band, thus affecting drastically the resulting pattern exhibited by the trailing bands. Moreover, we demonstrate that, in these four systems, the hydrotalcite-like α-Ni(OH)2 in the leading band redissolves to form the brucite-like polymorph β-Ni(OH)2 in the trailing bands via an Ostwald ripening mechanism. The macroscopic features of the various Liesegang patterns in the four anion systems as portrayed by the spacing and width laws are determined for various initial concentrations. © 2019 American Chemical Society.