This paper is a study of the composition dynamics of Liesegang band strata of Co(OH)2 and Ni(OH)2 from NH4OH, with redissolution by complex formation with ammonia. At a fixed time, the cobalt hydroxide composition was found to exhibit a random variation with band number, yet within a general overall decrease. The decrease with band number becomes more pronounced as the initial concentrations of Co2+ and Ni2+ get closer to each other. At equal concentrations, periodic oscillations in Co(OH)2 composition appear over consecutive bands. The time evolution of the total Co(OH)2 mass percent (over the entire pattern of strata) passes through a maximum. The dynamics of this complex system has been simulated by theoretical calculations using the model of Müller and Polezhaev, modified by Al-Ghoul and Sultan in a series of two papers in J. Phys. Chem. A; the present paper is the third in the series. The simulations capture the essential features of the experimentally observed dynamics.

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.