Experimental Study of the Dynamics of Front Propagation in the Co(OH)₂/NH₄OH Liesegang System Using Spectrophotometry

Abstract

In this paper we study the temporal dynamics of the Co(OH) 2/NH 4OH Liesegang system with redissolution by complex formation with ammonia using UV-vis spectrophotometry with a special setup. The formation of precipitate bands is accompanied with band redissolution at the top, and because of such precipitation-redissoultion dynamics, the bands appear as a propagating wave. The spectrophotometric technique developed in this study allows us to study at the kinetics of formation of the bands and their redissolution in great details. The formation, growth, and dissolution of multiple bands are monitored by the time evolution of the absorbance. It was found that the individual band formation is sudden and takes between 15 min to half an hour to form before the next band appears. The speed of formation of bands was different for different bands and the maxima of these speeds fit a Gaussian curve. The content of cobalt hydroxide in these bands was calculated and is shown to increase to a maximum away from the interface and then decreases. The bands later grow by further precipitation. This growth was demonstrated to be nonlinear in time. On the other hand, the dissolution of bands was shown to take place simultaneously and collectively among the multiple bands under study. The effect of the concentration of Co (+2) ions on the dynamics of band formation and dissolution was studied. A time law for this Liesegang system was also determined. The system was also found to be very sensitive to temperature fluctuations.

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

“…In specific mineral − or hybrid systems, , the nonlinear coupling of precipitation reactions with crystal nucleation and growth leads to the formation of a pattern consisting of a series of alternative precipitate bands separated by precipitate-free zones, which is known as the Liesegang banding phenomenon. , In previous works, , we revealed a polymorphic transformation between α- and β-nickel hydroxide Ni­(OH) 2 during the transition from a propagating pulse to a static Liesegang band regime in the precipitation of nickel hydroxide in agar with redissolution in excess ammonia. This polymorphic transition was also accompanied by a dynamic Ostwald ripening (OR) process whereby the nanospheres of α-Ni­(OH) 2 in the propagating pulse dissolved to transform to larger platelets of β-Ni­(OH) 2 in the bands as clearly revealed by the mass oscillations of the static bands and by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) .…”

Modification of the Matalon–Packter Law for Self-Organized Periodic Precipitation Patterns by Incorporating Time-Dependent Diffusion Flux

“…Many kinds of chemical LPs have been reported so far, and they typically consist of periodic spatial patterns of inorganic salt precipitates (e.g. copper chromate, − silver chromate, − and cobalt hydroxide − ). LP is usually formed when an aqueous solution (called the outer electrolyte) containing metal cations comes into contact with an anion-containing hydrogel (the inner electrolyte) .…”

Programmable Design of Self-Organized Patterns through a Precipitation Reaction

“…The outer electrolyte diffuses into the gel and salt formation reaction occurs to yield sparingly soluble salt species if the concentration reaches the solubility product ( K 1 ), which finally precipitates through nucleation and growth, resulting in periodic precipitation bands in the gel. So far, LPs have been observed with sparingly soluble salts − and various other functional materials (metal nanoparticles, − polymers, and metal–organic frameworks − ). Therefore, LP formation has great potential as a novel patterning technique for bottom-up processes, which are developed as alternatives to the conventional top-down processes .…”