Revert Banding in One-Dimensional Periodic Precipitation of the (AgNO₃ + KBr) System in Agar Gel

Abstract

A periodically precipitating system wherein interband distance successively decreases is known as revert Liesegang banding. The phenomenon is rare, and the underlying mechanism is implicit. In the present paper, the Liesegang system comprising of AgNO 3 and KBr as the outer and inner electrolyte pair showing revert banding in agar gel by employing a 1D experimental setup is studied under varying concentrations of participating species. Revert banding was observed under all the experimental conditions. The concentrations of inner and outer electrolytes were found to play a major role in reverting since they build the ionic strength inside Liesegang tubes. We hypothesize that the band reverting is the interplay of van der Waals and electrical double-layer interactions, and hence classical DLVO (Derjaguin–Landau–Verwey–Overbeek) theory can be applied to interpret reverting. We propose that revert deposition of precipitates is the outcome of flocculation and peptization of sols, which is the manifestation of balancing attractive and repulsive interactions acting on colloidal particles responsible for band formation.

Regular-Type Liesegang Pattern of AgCl in a One-Dimensional System

“…Because the concentration gradient of silver ions forms from x = 0, the barrier to the aggregation and precipitation decreases from x = 0, leading to the precipitated bands at increasingly closer distances, that is, revert Liesegang geometry. A similar revert geometry was observed for AgI [ 22 , 28 , 29 ] and AgBr [ 30 ] systems, whereas it has been reported that AgCl exhibited a regular geometry [ 31 ]. There appear to be two possibilities that could change the pattern geometry from the revert-type of AgI and AgBr to the regular-type of AgCl: (i) the difference in the concentration gradient of silver ions and (ii) differences in the preferential adsorption to the colloidal particles.…”

“…Materials that exhibit the revert pattern are mainly Pb [ 22 , 23 , 24 , 25 , 26 , 27 ] and Ag [ 22 , 28 , 29 , 30 ] salts. One of the proposed mechanisms to form the revert pattern in Pb and Ag systems is the preferential adsorption theory, in which the key factor that switches the pattern between regular and revert is the surface charge of colloidal particles, which are precursors of the precipitates [ 22 , 29 ].…”

Precise Evaluation of Spatial Characteristics of Periodically Precipitating Systems via Measurement of RGB (Red, Green, and Blue) Values of Pattern Images

“…The decreasing trend confirmed revert periodic precipitation. 32,33 RGB and Absorbance Values for Periodically Precipitating AgCl Systems. Figure 4A shows the image of banding of AgCl (formed via the chemical reaction AgNO 3 + KCl → AgCl ↓ + KNO 3 ) in the 1D Liesegang system when inner, outer, and gel concentrations were 0.05 M, 0.50 M, and 1% respectively.…”

“…In PART-A, no specific trend is seen, whereas in PART-B, however, U values decrease with the band number, indicating explicit evidence for the revert banding in the present system. 32,33 It should be noted that the confirmation of revert banding based on U and Δx values may be erroneous as determination of the exact bandwidth is often tricky due to diffused band boundaries by the previous methods of distance analysis. Nevertheless, an evaluation of interband distance (Δx) and W can be done precisely using the present method.…”

“…A total of nine experiments on two systems in triplicate were performed (four on the AgCl system by varying the concentration of the outer electrolyte and five on Co(OH) 2 by increasing the ionic strength inside the diffusion tube. The images for the AgBr system were taken from our previous report 32 ). In a typical experiment, an agar gel was prepared by step-wise addition of the required mass of agar powder into 200 mL of distilled water maintained in a water bath at 80.0 °C.…”