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 that K_sp(solubility product), as envisaged in recent theories, plays a significant role in initiating bifurcation depending on K, the magnitude of the product of concentration of A and B, which determines the degree of supersaturation. The value of K at 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 relation d²= m't + C' at any time t, where d is the distance between the lower front of the precipitate and the junction and m' and C' 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 time t and k is 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.