Liesegang Ring Type Structures and Bifurcation in Solid–Vapor and Liquid Phase Reactions between Cobalt Nitrate and Ammonium Hydroxide

Abstract

New results on the reaction between (i) cobalt nitrate (A) and ammonium hydroxide (B) in gelatin and (ii) cobalt nitrate (solid) and ammonia vapor are reported. Spatial bifurcation of the blue wave into a number of normal Liesegang type bands is observed in a one-dimensional tube in gel media depending on the concentration of electrolytes and gel. Similar bifurcation is also observed when the reaction was carried out in gel media between two microslides. Results show thatKsp(solubility product), as envisaged in recent theories, plays a significant role in initiating bifurcation depending onK, the magnitude of the product of concentration ofAandB, which determines the degree of supersaturation. The value ofKat the instant of bifurcation, when (i) concentration of Co(NO3)2is alone changed or when (ii) concentration of NH4OH alone is changed, is similar when uncertainties are taken into account. The bifurcation behavior when gel concentration is changed suggests that in case of high gel concentration pore size is affected, thereby influencing the diffusion of ions. The results satisfy the spacing law. The results further satisfy the relationd2= m′t + C′ at any timet, wheredis the distance between the lower front of the precipitate and the junction andm′ andC′ are constants, thereby suggesting that the role of diffusion is vital, as envisaged in various theories of the phenomenon. A colored banded structure separated by air gaps (vacant spaces) is formed when the solid cobalt nitrate reacts with ammonia vapor. This type of behavior has not been reported earlier. Initially, a single band is formed in a one-dimensional experiment, but in the course of time (∼1 month) several bands are formed simultaneously. The diffusion kinetics in the initial stages of the solid-vapor reaction has been studied in a one-dimensional tube. The data obey the relation ξ2=kt, where ξ is the thickness of the product layer at any timetandkis some constant. Similar studies were made on microslides which showed that during the reaction, a colored product is formed which undergoes change in the sequence pink → blue → brown. Results in the one-dimensional case support recent theories which postulate nucleation followed by aggregation.